Method for producing polyamide powder for 3D printing

Abstract

The invention relates to a method for producing a 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 said solvent, the introduction of at least one catalyst into said solvent and the introduction of at least one activator into said solvent at a given temperature; b) polymerizing the lactam monomer to give polyamide in the reaction medium; c) precipitating the polyamide in powder form in the reaction medium; and d) heating, after step a), preferably after step c), the reaction medium to a temperature higher than the temperature at which the at least one activator was introduced and which ranges from 140 to 200°C The invention also relates to a powder obtained by such a method and to uses of such a powder.

Description

[0001] Description

[0002] Title: Manufacturing process of polyamide powder for 3D printing

[0003] Field of invention

[0004] The present invention relates to a method for manufacturing a polyamide powder by anionic polymerization in a solvent medium. The invention also relates to a powder obtained by such a method, as well as its use in a method for constructing a three-dimensional article.

[0005] Technical background

[0006] The construction of three-dimensional (3D) articles can be used to produce prototypes or various parts, for example in the automotive, nautical, aeronautical, aerospace, medical (notably for the manufacture of prostheses, hearing systems, cellular tissues, etc.), textile, clothing, fashion, decoration, housings for electronics, telephony, home automation, IT, lighting, sports and industrial tooling sectors.

[0007] Among the manufacturing techniques for 3D articles, the manufacturing process by sintering polyamide powder (also called powder agglomeration by fusion) is particularly interesting. This technology makes it possible, for example, to obtain fine and complex geometries, impossible to achieve by conventional molding techniques. According to this process, a layer of polyamide powder is, in a conventional manner, selectively and briefly irradiated in a chamber by radiation, generally electromagnetic (e.g., laser beam, infrared radiation, UV radiation), the result being that the powder particles impacted by the radiation melt. The molten particles coalesce and solidify to lead to the formation of a solid mass. This process can produce 3D articles by the repeated irradiation of a succession of freshly applied powder layers.

[0008] In the case of selective laser sintering (SLS), the procedure is traditionally carried out as follows. A thin layer of polyamide powder is deposited on a horizontal plate held in an enclosure heated to a temperature between the crystallization temperature Te and the melting temperature Tf of the polyamide powder. The laser agglomerates powder particles at different points of the powder layer according to a geometry corresponding to the object, for example using a computer that stores the shape of the object and reproduces it in the form of slices. Then, the horizontal plate is lowered by a value corresponding to the thickness of a layer of powder (for example between 0.05 and 2 mm and generally of the order of 0.1 mm) then a new layer of powder is deposited and the laser agglomerates powder particles according to a geometry corresponding to this new slice of the object.The procedure is repeated until the entire object has been produced. Then the whole thing is gently cooled and the object solidifies as soon as its temperature drops below the crystallization temperature Te. The parts that have not been agglomerated therefore remain in the powder state. Inside the enclosure, we obtain an object surrounded by powder. After cooling, the object is separated from the powder, which can be reused for another operation.

[0009] Immediately after the action of the laser beam, the temperature of the sample is higher than the crystallization temperature Te of the powder. But it happens that the addition of a new layer of colder powder causes the temperature of the part to drop rapidly which, when it falls below said temperature Te, causes deformations (the phenomenon of "curling" in English) in the manufactured object. Similarly, when the temperature of the powder in the machine gets too close to the melting temperature (Tf) of the powder, this causes a caking around the parts (the phenomenon of "caking" in English), which is manifested by the presence of lumps or clumps of powder in certain places on the surface of the object, instead of having a good definition of the final object. When a caking phenomenon occurs, it is then necessary to undertake cleaning of the parts to remove the powder remaining attached to the parts, before their use.This cleaning is generally done by sandblasting, which can lead to the degradation of certain fine and / or fragile elements of the constructed 3D parts.

[0010] To avoid these phenomena, it is therefore important to keep the Te as far away as possible from the Tf of the powder. The Tf - Te difference of the powder limits the working temperature window of the device used to agglomerate the powder particles by fusion caused by radiation. In this text, the expression "working window" refers to the build temperature range applicable to a powder when used in 3D printing according to the definition below. The working window is defined by its upper temperature limit and its lower temperature limit. The upper limit of the working window corresponds to the temperature of the build chamber above which agglomeration or "caking" occurs. The lower limit of the working window corresponds to the temperature of the build chamber below which distortion or deformation or "curling" occurs.Having a wide working window allows for greater flexibility in terms of the objects built. In addition, it helps neutralize the large temperature variations typically observed within 3D printing machines, which are usually around + / -3°C.

[0011] If, in order to widen the working window, it is desirable to have a Tf-Tc gap that is wide (since it includes the working window), other parameters also come into play in defining the working window. Thus, a widening of the working window does not necessarily result from an increase in the Tf-Tc gap.

[0012] The widening of a powder's working window is determined for a certain 3D printing system; however, this widening of a powder's working window will occur on any device, although not necessarily to the same extent.

[0013] Different processes for preparing or treating polyamide powder have been described to obtain powders suitable for use in 3D printing.

[0014] WO 2013 / 090174 relates to a method for treating a powder of a thermoplastic polymer in which a polymerized and isolated powder is subjected to a heat treatment by heating for one or more hours at a temperature relatively close to its melting temperature as determined before this operation. This heat treatment method aims to change the melting temperature, the recrystallization temperature and / or the enthalpy of fusion of the polymer.

[0015] Document EP 1571173 describes a process for preparing a polyamide 12 powder by anionic polymerization of lauryllactam in a solvent for said lauryllactam, in the presence of an organic or mineral filler and an amide in particular proportions relative to the lauryllactam. In the examples of this document, the polymerization process comprises a step in which the polymerization ingredients in the solvent are heated for 2 hours at 120°C after the end of the introduction of the activator, to complete the polymerization.

[0016] Document FR 3095205 relates to a process for preparing a polyamide powder by anionic polymerization in a solvent to obtain a powder in which the particles comprise a polyamide core and a polyamide shell, the shell having an inherent viscosity in solution and a melting temperature higher than those of the core, respectively. In the examples of this document, the polymerization process comprises a step of heating the polymerization medium to 130°C for 3 hours after the end of the introduction of the activator.

[0017] There is a real need to provide a process for preparing polyamide powder for use in 3D printing, in particular by sintering, making it possible to obtain a working window for the construction temperature of 3D printing devices, enlarged, while remaining simple to implement.

[0018] Summary of the invention

[0019] The invention relates firstly to a process for manufacturing a polyamide powder by anionic polymerization in a solvent, comprising the following steps: a) the formation of a reaction medium comprising:

[0020] - the introduction of at least one lactam monomer into said solvent;

[0021] - the introduction of at least one catalyst into said solvent; and

[0022] - the introduction of at least one activator into said solvent at a given temperature; b) the polymerization of the lactam monomer into polyamide in the reaction medium; c) the precipitation of the polyamide in powder form in the reaction medium; and d) the heating, after step a) and preferably after step c), of the reaction medium to a temperature higher than the temperature of the introduction of the at least one activator and ranging from 140 to 200°C.

[0023] Alternatively, the process for manufacturing a polyamide powder by anionic polymerization in a solvent comprises the following steps:

[0024] (a) the formation of a reaction medium comprising:

[0025] - the introduction of at least one lactam monomer into said solvent;

[0026] - the introduction of at least one catalyst into said solvent; and

[0027] - the introduction of at least one activator into said solvent at a given temperature, so as to polymerize the lactam monomer into polyamide, which precipitates in powder form in the reaction medium; and (b) heating, after step a), the reaction medium to a temperature higher than the temperature of the introduction of the at least one activator and ranging from 140 to 200°C.

[0028] In embodiments, the heating of step d) is carried out for a duration greater than or equal to 2 hours, preferably greater than or equal to 3 hours, more preferably greater than or equal to 5 hours, more preferably for a duration of 8 to 15 hours, more preferably 10 to 12 hours. In embodiments, step a) further comprises the introduction of at least one filler into the solvent, the at least one filler preferably being a mineral filler, preferably silica, and / or an organic filler, preferably a polyamide powder.

[0029] In embodiments, step a) further comprises introducing at least one amide into the solvent, the at least one amide preferably being an N,N'-alkylene bisamide, more preferably N,N'-Ethylene bis-stearamide and / or N,N'-Ethylene bis-oleamide.

[0030] In embodiments, the at least one lactam monomer is selected from the group consisting of 2-pyrrolidone, caprolactam, 2-azacyclononanone, lauryllactam, and mixtures thereof.

[0031] In embodiments, the at least one catalyst is selected from the group consisting of sodium, potassium, alkali metal hydrides and hydroxides, alkali metal alcoholates, and mixtures thereof, preferably from the group consisting of sodium hydride, potassium hydride, sodium, sodium methoxide, sodium ethoxide, and mixtures thereof.

[0032] In embodiments, the at least one activator is selected from the group consisting of lactam-N-carboxyanilides, (mono)isocyanates, polyisocyanates, carbodiimides, cyanamides, acyllactams and acylcarbamates, triazines, ureas, N-substituted imides, esters, phosphorus trichloride, and mixtures thereof.

[0033] In embodiments, the solvent is a paraffinic hydrocarbon fraction having a boiling temperature range of 120 to 170°C.

[0034] In embodiments, the heating temperature in step d) is from 140 to 170°, preferably from 145 to 160°C, more preferably from 145 to 155°C.

[0035] In embodiments, the temperature of the introduction of the at least one activator is from 50 to 150°C, preferably from 60 to 135°C. In embodiments, the method further comprises a step of introducing into the solvent one or more additives chosen from the group consisting of pigments, dyes, carbon black, carbon nanotubes, antioxidants, anti-UV agents and plasticizers.

[0036] The invention also relates to a polyamide powder obtained by a manufacturing process as described above.

[0037] The invention also relates to the use of a powder as described above, for the construction of a three-dimensional article, preferably layer by layer, more preferably by sintering, even more preferably by sintering caused by electromagnetic radiation.

[0038] The invention also relates to the use of a powder as described above, for the manufacture of 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.

[0039] The invention also relates to a method for manufacturing a three-dimensional article comprising the following steps: manufacturing a powder by a method as described above; depositing, preferably in the form of a layer, said powder; and sintering the powder, preferably by means of electromagnetic radiation.

[0040] The present invention makes it possible to meet the need expressed above. More particularly, it 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 parts of better quality and having better definition and / or to facilitate, limit or even avoid mechanical cleaning which could damage the manufactured parts, in particular those which are thin. Consequently, the powders prepared according to the invention allow the manufacture by 3D printing of a wider spectrum of part geometries.

[0041] This is accomplished by carrying out a so-called "baking" step of the anionic polymerization reaction medium comprising heating said reaction medium, after the introduction of the reagents and other compounds involved in the polymerization, to a certain temperature. Without wishing to be bound by a theory, the inventors believe that the baking step results in a physical and chemical modification of the surface of the powder particles, for example a crystalline improvement on the surface, which allows the widening of the working window.

[0042] The heat treatment described in document WO 2013 / 090174 is an additional step, which takes place after the synthesis of the powder particles, on a powder already polymerized and isolated from its synthesis medium. On the other hand, the cooking step according to the present invention, carried out on the reaction medium, in particular at the end of the main part of the polymerization, is an integral part of the anionic polymerization phase in a solvent medium, and it allows the crystalline improvement of the precipitated powder particles.

[0043] Brief description of the figures

[0044] [Fig. 1] represents a snapshot showing an illustrative example of a Type 1 B tensile dumbbell (15 cm long and 4 mm thick) undergoing the deformation measurement test as described in the “Examples” section below.

[0045] [Fig. 2] represents a snapshot 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 deformed 5 mm between these two ends.

[0046] [Fig. 3] represents a snapshot showing four examples of parts having 10 holes of different sizes manufactured by selective laser sintering with polyamide powders as described in the “Examples” section below. These parts illustrate different caking sensitivities and the scores that can be assigned to evaluate said sensitivities depending on the number of holes opened, as described in the “Examples” section below. Part A has a score of 0 / 10, part B has a score of 6 / 10, part C has a score of 8 / 10 and part D has a score of 10 / 10.

[0047] Detailed description

[0048] The invention is now described in more detail and in a non-limiting manner in the following description.

[0049] Unless otherwise stated, all percentages relating to quantities are percentages by mass. In this text, quantities indicated for a given species may apply to that species according to all its definitions (as mentioned in this text), including the more restricted definitions.

[0050] Powder manufacturing

[0051] The invention relates firstly to a process for manufacturing a polyamide powder by anionic polymerization in a solvent medium. Unless otherwise indicated, the steps of the process, and in particular steps a), b) and c) as defined above, can take place at least partly simultaneously. In particular, the polymerization generally begins as soon as all the reactants are introduced into the reaction mixture and therefore as soon as step a) is completed.

[0052] Anionic polymerization occurs by opening the lactam ring. It generally involves three steps: an initiation step to form the lactamate anion, then an activation reaction that leads to the formation of an acyllactam, and finally the propagation (or polymerization) step.

[0053] By “polyamide powder” is meant a powder comprising at least particles comprising at least one polyamide, this term including homopolyamides and copolyamides resulting from the polymerization of several different comonomers; for the purposes of the present invention, the “polyamide powder” may thus comprise components other than polyamide (in the particles comprising the polyamide or in particles devoid of polyamide).

[0054] The anionic polymerization can be carried out continuously or discontinuously (also called "batch"). Preferably, it is carried out discontinuously. 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. In the present text, the solvent comprising at least one of the different compounds involved in the polymerization is called "reaction medium" or "reaction medium". The compounds mentioned above can be introduced all simultaneously, all successively or partly simultaneously and partly successively.

[0055] Preferably, the solvent is introduced into any suitable device, for example a reactor, then the lactam monomer(s), the amide(s) (when present), the filler(s) (when present), the catalyst(s) and the activator(s), simultaneously or successively. Advantageously, the solvent, the lactam monomer(s), the amide(s) (when present) and the filler(s) (when present) are first introduced into the polymerization device, then the water present in the reaction medium is removed, for example using azeotropic distillation, before the catalyst(s) are added to the anhydrous medium. Preferably, the activator(s) are not added all at once to the reaction medium. This makes it possible to avoid caking or loss of control of the polymerization.Thus, the activator(s) are preferably added incrementally, or injected continuously over a period of time, at one or more introduction rates. Preferably, the activator(s) are introduced into the reaction medium after the introduction of the lactam monomers, amides (when present), fillers (when present) and catalyst(s).

[0056] Preferably, the solvent used dissolves (at least in part) the lactam monomer(s) and the amide(s). However, the solvent does not dissolve the polyamide that forms during the polymerization (i.e., the polyamide is insoluble in said solvent under the polymerization conditions). Thus, the polymerization of the polyamide leads to its precipitation directly in the form of particles (and therefore powder) in the solvent.

[0057] The solvent may be supersaturated with lactam monomer at the activator introduction temperature. Various means are used to supersaturate the solvent with monomer. One such means may include saturating the solvent with monomer at a temperature higher than the activator introduction temperature and then lowering the temperature to that temperature.

[0058] Alternatively, the polymerization may be undertaken in a solvent not supersaturated with lactam monomer. In this case, the reaction medium preferably contains the monomer(s) dissolved in the solvent at a concentration far from saturation at the temperature of introduction of the activator.

[0059] Any solvent for the lactam monomers that is inert to the polymerization reaction may be used. The solvent is preferably a paraffinic hydrocarbon fraction (preferably a mixture of isoparaffin, N-paraffin and cycloparaffin). Advantageously, its boiling range is between 120 and 200°C, preferably between 140 and 170°C. The lactam monomers are preferably selected from the group consisting of lauryllactam (lactam 12), caprolactam (lactam 6), 2-pyrrolidone (lactam 4), 2-azacyclononanone (lactam 8) and mixtures thereof. More preferably, the lactam monomers are lauryllactam and / or caprolactam. In a particularly preferred manner, a mixture of lactams comprising mainly lauryllactam and a minor quantity of shorter chain lactam, in particular caprolactam or 2-pyrrolidone (lactam 4) may be used.In this case, it is particularly preferred to use these two lactams in a mass ratio of 90 to 99.999: 0.001 to 10, preferably 92 to 99.99: 0.01 to 8, and in particular 94 to 99.9: 0.1 to 6. Indeed, it has been observed that a very small amount of caprolactam makes it possible to maintain the melting temperature of the majority polymer but affects its crystallization. A lowering of the crystallization temperature and / or a delay in crystallization is thus observed, which in both cases reduces the tendency to curl, and thus makes it possible to widen the working window.

[0060] Most preferably, the amide which can be introduced into the reaction medium comprises, or is, one (or more) N,N'-alkylene bisamide(s). Even more advantageously, the amide is chosen from N,N'-alkylene bisamides of fatty acids, and is more preferably chosen from the group consisting of N,N'-ethylene bis-stearamide (of formula C17H35-C(=O)-NH-CH2-CH2-NH-C(=O)-C17H35 and abbreviated EBS), N,N'-ethylene bis-oleamide (of formula C17H33-C(=O)-NH-CH2-CH2-NH-C(=O)-C17H33 and abbreviated EBO), N,N'-alkylene bis-palmitamide (in particular N,N'-ethylene bis-palmitamide), N,N'-alkylene bis-gadoleamide (in particular N,N'-ethylene bis-gadoleamide), N,N'-alkylene bis-ketoleamide (in particular N,N'-ethylene bis-ketoleamide), N,N'-alkylene bis-erucamide (especially N,N'-ethylene bis-erucamide), and mixtures thereof. More preferably, the amide is chosen from EBS, EBO and mixtures thereof.The amide may comprise a primary amide preferably containing from 12 to 22 carbon atoms, preferably in combination with an N,N'-alkylene bisamide as described above. This primary amide is preferably selected from the group consisting of oleamide, N-stearamide, isostearamide, erucamide and mixtures thereof.

[0061] The amide, for example the N,N'-alkylene bisamide(s), may be introduced into the reaction medium in an amount of 0.001 to 4 moles, preferably 0.075 to 2 moles, per 100 moles of lactam monomer; in particular the amount of amide (for example N,N'-alkylene bisamide) may be 0.001 to 0.05 moles, or 0.05 to 0.1 moles, or 0.1 to 0.5 moles, or 0.5 to 1 mole, or 1 to 1.5 moles, or 1.5 to 2 moles, or 2 to 3 moles, or 3 to 4 moles, per 100 moles of lactam monomer. The addition of an amide as described above helps to adjust the apparent specific surface area (measurable by the BET method) of the polyamide powder particles. The greater the amount of amide added, the higher the apparent specific surface area will be.

[0062] The filler possibly introduced into the reaction medium is intended to serve as a crystallization seed. It may be mineral or organic, or comprise one (or more) mineral fillers and one (or more) organic fillers. As mineral fillers suitable for the invention, mention may be made of silicas, carbon black and / or talc. As suitable organic fillers, mention may be made of polymer powders (thermoplastic or thermosetting) insoluble in the synthesis solvent, and more particularly polyamide powders, in particular PA 4, PA 6, PA 8, PA 11, PA 12, PA 6 / 12, PA 6.12, PA 6.13, PA 6.10, PA 6.6 and / or PA 10.10. Examples of such polyamide powders include Arkema's Orgasol® powders, Arkema's Rilsan® fine powders, Evonik's Vestosint® powders, and Chemopharma's MICROPAN® powders. In advantageous embodiments, the filler is a mineral filler and is more particularly a silica.In other advantageous embodiments, the filler is an organic filler and is more particularly a polyamide powder such as a PA 12 powder. Preferably, the filler according to the invention is a filler of finely divided particles, in particular having a volume average diameter of 0.01 to 40 μm, preferably of 10 to 30 μm. Such average diameter ranges make it possible to obtain powder particles having a volume average diameter suitable in particular for use in a method of constructing a 3D article. The value of the volume average diameter of the particles corresponds to the arithmetic mean of the particle diameters weighted by the volume of said particles. It can be determined according to the ISO 13319:2007 standard, for example using a Multisizer 3 Coulter Counter granulometer from Beckman Coulter.The weight ratio of the feedstock introduced into the reaction medium relative to the lactam monomers introduced into the reaction medium, expressed in %, may be from 0.001 to 65%, preferably from 0.005 to 45%, more preferably from 0.01 to 30%, even more preferably from 0.05 to 20%. In embodiments, this weight ratio may be from 0.001 to 0.1%, or from 0.01 to 0.05%, or from 0.05 to 0.1%, or from 0.1 to 0.3%, or from 0.3 to 0.5%, or from 0.5 to 1%, or from 1 to 2%, or from 2 to 5%, or from 5 to 10%, or from 10 to 20%, or from 20 to 30%, or from 30 to 45%, or from 45 to 65%. The proportion of filler to the amount of lactam monomer and the average diameter of the filler influence the average diameter of the resulting polyamide particles. The lower the proportion of filler to the amount of lactam monomer, the higher the volume average diameter of the powder particles will be.The larger the volume average diameter of the charge, the larger the volume average diameter of the powder particles will be.

[0063] The catalyst may be any catalyst that can be used in a process for the anionic polymerization of lactams. More particularly, the catalyst is a base that is sufficiently strong to lead to the formation of a lactamate after reaction with the lactam. The catalyst may be chosen from alkali metals (especially sodium and potassium), alkali metal hydrides and hydroxides, alkali metal alcoholates and mixtures thereof. Non-limiting examples of suitable catalysts include sodium hydride, potassium hydride, sodium, sodium methoxide and / or sodium ethoxide. The catalyst according to the invention may be a mixture of several catalysts, in particular as described above.Advantageously, the amount of catalyst is from 0.1 to 5 moles, preferably between 0.3 and 3 moles, per 100 moles of lactam monomer, for example from 0.1 to 0.3 moles, or from 0.3 to 0.5 moles, or from 0.5 to 0.7 moles, or from 0.7 to 1 mole, or from 1 to 1.5 moles, or from 1.5 to 2 moles, or from 2 to 3 moles, or from 3 to 5 moles, per 100 moles of lactam monomer.

[0064] The activator has the role of causing the formation of the acyllactam and controlling the polymerization. The activator is advantageously chosen from lactam-N-carboxyanilides, (mono)isocyanates, polyisocyanates, carbodiimides, cyanamides, acyllactams and acylcarbamates, triazines, ureas, N-substituted imides, esters, phosphorus trichloride, carbon dioxide and mixtures thereof. Preferably, the molar ratio of catalyst to activator is 0.2 to 2, preferably 0.8 to 1.2, for example 0.2 to 0.5, or 0.5 to 0.8, or 0.8 to 0.9, or 0.9 to 1, or 1 to 1.1, or 1.1 to 1.2, or 1.2 to 1.5, or 1.5 to 2. In embodiments, other additives may be added to the reaction medium, such as pigments, dyes, carbon black, carbon nanotubes, antioxidants, UV-blocking agents, and / or plasticizers.

[0065] Polymerization is preferably carried out at atmospheric pressure or at a slightly higher pressure (partial pressure of the hot solvent). It can be carried out under an inert gas atmosphere, for example under nitrogen. Polymerization begins as soon as the catalyst and activator are added to the reaction mixture.

[0066] Preferably, the temperature applied to the reaction medium during the introduction of the activator (called "activator introduction temperature") is between 50°C and 150°C, and is more preferably 60 to 135°C, more preferably 75 to 125°C. It may in particular be 50 to 65°C, or 65 to 75°C, or 75 to 85°C, or 85 to 95°C, or 95 to 105°C, or 105 to 115°C, or 115 to 125°C, or 125 to 135°C, or 135 to 150°C.

[0067] The activator introduction temperature may be applied to the reaction medium at any time before the addition of the activator or at the time of the addition of the activator. Advantageously, this temperature is applied to the reaction medium after the addition to the solvent of the lactam monomers, the amide (when present) and the feedstock (when present), and before the addition of the catalyst and the injection of the activator. Before the application of the activator introduction temperature, the reaction medium may for example be at room temperature (i.e. for example at a temperature of 15 to 30°C).

[0068] The activator introduction temperature may be maintained for a certain period of time after the start of the activator introduction. Preferably, it is maintained until the end of the activator introduction. The activator introduction temperature may be maintained for a period of at least 1 hour, preferably 1 to 12 hours, preferably 3 to 10 hours, for example 1 to 2 hours, or 2 to 3 hours, or 3 to 4 hours, or 4 to 5 hours, or 5 to 6 hours, or 6 to 7 hours, or 7 to 8 hours, or 8 to 9 hours, or 9 to 10 hours, or 10 to 11 hours, or 11 to 12 hours. Advantageously, the activator introduction temperature is applied for a period of time equal to or substantially equal to the activator introduction time.

[0069] The process according to the invention further comprises a step of heating the reaction medium to a temperature of 140 to 200°C, which is carried out once the introductions of the lactam monomer, the catalyst and the activator into the reaction medium are complete and the majority of the polymerization has taken place. This step is also called the "cooking step" in the present text. According to the present invention, this cooking step is carried out on the reaction medium, that is to say while the ingredients and polymerization products are still in the solvent. The cooking step allows the polymerization to continue by chain extension reactions, once the activator has been completely added to the reaction medium.Preferably, the temperature of this cooking step (called "cooking temperature" in the present text) is different from the introduction temperature of the activator, and more preferably higher than the introduction temperature of the activator. Preferably, the cooking step as defined in the present text is carried out after the lactam monomer, the amide when present, the filler when present, the catalyst and the activator have been added (or brought into contact) in the solvent.

[0070] The baking step is preferably carried out in the same device (e.g., reactor) as the steps described above. The baking step is preferably carried out once all of the catalyst and activator have been added, and therefore most of the polymerization has taken place. Advantageously, the reaction medium is heated to the baking temperature, preferably gradually, for example over a period of 15 to 60 min, preferably 20 to 40 min, in particular approximately 30 min.

[0071] The cooking temperature is from 140 to 200°C, preferably from 145 to 160°C, more preferably from 145 to 155°C. In particular, the cooking temperature may be from 140 to 145°C, or from 145 to 150°C, or from 150 to 155°C, or from 155 to 160°C, or from 160 to 165°C, or from 165 to 170°C, or from 170 to 175°C, or from 175 to 180°C, or from 180 to 185°C, or from 185 to 190°C, or from 190 to 195°C, or from 195 to 200°C.

[0072] Advantageously, the cooking temperature is higher than the activator introduction temperature by at least 20°C, preferably by at least 40°C. In particular, the activator introduction temperature may be higher by at least 20°C, or by at least 30°C, or by at least 40°C, or by at least 50°C, or by at least 60°C, or by at least 70°C, or by at least 80°C, or by at least 90°C.

[0073] The duration of the cooking step (also called "cooking time" in this text) is preferably greater than or equal to 2 hours, more preferably greater than or equal to 3 hours and even more preferably greater than or equal to 5 hours. Even more advantageously, it is 8 to 15 hours, more preferably 10 to 12 hours. For example, the cooking time may be 2 to 3 hours, or 3 to 4 hours, or 4 to 5 hours, or 5 to 6 hours, or 6 to 7 hours, or 7 to 8 hours, or 8 to 9 hours, or 9 to 10 hours, or 9 to 10 hours, or 10 to 11 hours, or 11 to 12 hours, or 12 to 13 hours, or 13 to 14 hours, or 14 to 15 hours.

[0074] After the cooking step, the reaction medium may be cooled, preferably to a temperature ranging from room temperature to 110°C, more preferably to a temperature of 60 to 90°C.

[0075] The polyamide obtained by polymerization precipitates in the solvent in powder form. 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 decantation, spinning, etc.

[0076] The polyamide powder can be subjected to a drying step, in particular vacuum drying, to remove any remaining solvent, for example in an oven.

[0077] The powder obtained by polymerization is preferably a polyamide powder chosen from the group consisting of PA 4, PA 6, PA 12, PA 12 / 6, PA 12 / 4, PA 4 / 6, PA 4 / 6 / 12 and mixtures thereof.

[0078] The invention also relates to a powder obtained by, or capable of being obtained by, a process as described above.

[0079] 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, more preferably 30 to 50 μm. In 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.

[0080] Preferably, the temperature difference Tf-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 ISO 11357-3:2018 Plastics — Differential Scanning Calorimetry (DSC) — Part 3.

[0081] Polyamide powder can have an inherent viscosity of 0.8 to 1.7 (g / 100 g) -1 , preferably from 1.0 to 1.5 (g / 100 g) -1 . Inherent viscosity is measured according to ISO 307:2019, preferably in an Ubbelohde viscometer, except that m-cresol is used as the solvent and a temperature of 20°C. Inherent viscosity has the dimension of the inverse of a concentration and is equal to the natural logarithm of the relative viscosity, divided by the concentration of polymer dissolved in the solvent.

[0082] Use of powder

[0083] The invention also relates to the use of a polyamide powder as described above for the manufacture (or construction) of a three-dimensional article.

[0084] The polyamide powder according to the invention can be used in a 3D printing process. For the purposes of the invention, "3D printing" or "additive manufacturing" means any process for manufacturing parts in volume by adding or agglomerating powder, layer by layer. For the purposes of the invention, "3D printing" or "additive manufacturing" also means selective sintering technologies using an absorber, in particular the technologies known as "High Speed ​​Sintering" (HSS) and "Multi-Jet Fusion" (MJF).

[0085] Preferably, the polyamide powder according to the invention is used in a method for manufacturing a three-dimensional object by agglomeration of the powder by fusion using radiation or a selective sintering method. The term "sintering" in the present text includes all 3D printing methods by agglomeration of the powder by fusion, regardless of the type of radiation.

[0086] More particularly, the method for manufacturing a 3D object according to the invention comprises: a) the deposition, preferably in the form of a layer, of the powder according to the invention; and b) the sintering of the powder, preferably by means of an electromagnetic radiation beam.

[0087] Preferably, steps a) and b) are repeated, so as to form the three-dimensional article.

[0088] The layer deposited in step a) is preferably heated to a temperature called the build temperature. The term "build temperature" (also called "powder bed temperature") refers to the temperature to which the powder bed, of a constituent layer of a three-dimensional object under construction, is heated during the layer-by-layer sintering process 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 in the working window of the polyamide powder.

[0089] The method for manufacturing a 3D object according to the invention may comprise a step of manufacturing the powder used in step a) according to a method as described above.

[0090] The radiation may be chosen from any radiation well known to those skilled in the art. Examples of radiation include a laser beam (laser sintering), infrared radiation, UV radiation, or any source of electromagnetic radiation that allows the powder to be melted layer by layer to manufacture three-dimensional objects.

[0091] The device used may be any sintering device well known to those skilled in the art. For example, sintering devices marketed by EOS, 3D Systems, Aspect, Trump Precision Machinery, Hewlett Packard, Sinterit, Sintratec, Sharebot, FormLabs, Sonda Sys, Farsoon, Prodways, Ricoh, Wematter3D, Voxel Jet, Xaar, etc. Examples of sintering devices include EOSINT P396 and Formiga P100 from EOS GmbH.

[0092] According to an advantageous sintering process, a thin layer of powder is deposited on a horizontal plate maintained in an enclosure heated to the construction temperature. Advantageously, electromagnetic radiation subsequently provides the energy necessary to sinter the powder particles at different points of the powder layer according to a geometry corresponding to an object (for example using a computer having in memory the shape of an object and restoring this shape in the form of slices). Then, the horizontal plate is lowered by a value corresponding to the thickness of a layer of powder, and a new layer is deposited. The electromagnetic radiation provides the energy necessary to sinter the powder particles according to a geometry corresponding to this new slice of the object, and so on. The procedure is repeated until the object has been manufactured.

[0093] The layer of powder deposited on a horizontal plate (before sintering) may have a thickness of 20 to 200 pm, preferably 50 to 150 pm. The layer of agglomerated material after sintering may have a thickness of 10 to 150 pm, preferably 30 to 100 pm.

[0094] Preferably, the composition of the invention is used in a selective laser sintering process. The composition can also be used in a sintering process of the MJF type and HSS high-speed sintering. Preferably, the three-dimensional object manufactured by a process according to the invention is chosen from a prototype, a part model ("rapid prototyping"), a part finished in small series ("rapid manufacturing"), in particular for the automotive, nautical, aeronautical, aerospace, medical (prostheses, hearing systems, etc.) fields, textiles, clothing, fashion, decoration, the field of housings for electronics, telephony, home automation, IT, lighting, sport, and industrial tools.

[0095] The use of the powder according to the invention in additive manufacturing is advantageous because the polyamide powder is recyclable in several successive constructions. The polyamide powder can thus be used several times, alone or in a mixture with other recycled or non-recycled powders. Indeed, the powder that has not been agglomerated can be recovered by sieving, the sieve retaining the 3D objects and allowing the powder to flow. Preferably, the powder according to the invention is recyclable at least 3 times, preferably at least 5 times, more preferably at least 10 times.

[0096] Preferably, in each build cycle, or "run", the recycled powder content is at least 50%, preferably at least 60%, more preferably at least 70% by weight, of the total weight of powder used in the machine in each run. In other words, apart from the first run which uses 100% fresh powder, each subsequent run reuses at least 50%, preferably at least 60%, more preferably at least 70%, by weight of powder from the previous run which has not been sintered, of the total weight of powder used in the machine in each run.

[0097] Before use, the manufactured 3D object can be easily cleaned using any cleaning technique well known to those skilled in the art. For example, the object can be cleaned using a sandblaster.

[0098] The invention also relates to the use of a polyamide powder as described above for the manufacture of a composite, a substrate coating, in particular a metal substrate (coil-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.

[0099] Examples

[0100] The following examples illustrate the invention without limiting it.

[0101] In the examples below, the working windows of different polyamide powders were evaluated. The crystallization and melting temperatures and the enthalpy of fusion of the powders were measured according to ISO 11357-3:2018 (DSC), the volume average diameter of the powders was determined according to ISO 13319:2007 and the inherent viscosity of the powders was determined according to ISO 307:2019 but at a temperature of 20°C and using m-cresol as solvent.

[0102] Procedure for evaluating the working window

[0103] The working temperature window was determined in a P100 laser sintering machine (EOS) by operating at constant laser energy and constant shrinkage chamber temperature so that only the build chamber temperature varied. The laser conditions used for this test were as follows:

[0104] - Withdrawal chamber temperature: 140°C

[0105] Outline

[0106] ■ Laser power: 16 W

[0107] ■ Speed: 1500 mm / s

[0108] Hatching

[0109] ■ Power: 18 W

[0110] ■ Speed: 3000 mm / s

[0111] ■ Spoke displacement: 0.20 mm

[0112] ■ Energy: 0.3 mJ / mm 3

[0113] The temperature of the build chamber (T cc ) corresponds to the temperature to which the upper layer of the powder bed is heated before the laser passes through it. A 3D printing model construction of different parts has been developed, according to which:

[0114] 1) 40 layers of powder are deposited successively and they are each heated up to Tcc before the deposition of the next layer (for these 40 layers, the laser is not used to selectively melt the powder); then

[0115] 2) from 41 ème layer, 3D construction by selective fusion of the powder by the laser starts as soon as the temperature of the powder layer reaches Tcc, and this for each layer, so as to build a defined part; then

[0116] 3) 40 layers of powder are deposited successively and each is heated up to Tcc before the deposition of the next layer (for these 40 layers the laser is not used to selectively melt the powder). The part defined in step 2) above is either a type 1 B traction dumbbell according to ISO 527-2 (15 cm long and 4 mm thick) as shown in Figure 1, particularly sensitive to curling, or a part with 10 holes of different sizes (4 mm thick) as shown in Figure 2, particularly sensitive to caking. For each of the powders tested, 5 type 1 B traction dumbbells and 2 parts with 10 holes were manufactured according to the model construction above.

[0117] A two-step protocol, based on a temperature scan of the build chamber, was then implemented to determine the limits of the powder working window. These two steps are as follows:

[0118] 1) The model construction described above is first carried out at Tcc = 168°C. It is then reproduced several times, each time increasing the temperature of the construction chamber by 2°C until reaching Tcc = 176°C. The machine is then cooled and the 3D objects are recovered.

[0119] 2) The model construction described above is again carried out at Tcc = 168°C. It is then reproduced several times, each time decreasing the temperature of the construction chamber by 2°C until reaching Tcc = 160°C. The machine is then cooled and the 3D objects are recovered.

[0120] For each build chamber temperature (Tcc) tested, the following is determined:

[0121] - the deformation of the dumbbells (reflecting sensitivity to curling), and the ease of cleaning of the 10-hole parts (representing sensitivity to caking).

[0122] The deformation of the dumbbells was determined by measuring their flatness. The flatter the specimen (dumbbell), the less marked the deformation, and therefore the manufactured 3D object will correspond to the intended geometry. To determine their flatness, each specimen was placed on a flat surface, a 1 kg weight was placed on the left jaw of the dumbbell, and the distance between the end of the right jaw and the flat surface was measured. This measurement was repeated on each of the four longitudinal faces of the dumbbells and the greatest distance measured corresponds to the retained deformation.

[0123] For each build temperature tested, the deformation was measured for each of the 5 parts built at said build temperature, and the final deformation is the average of the five measurements retained (this allows us to take into account the temperature variations at the surface of the powder bed).

[0124] It was considered that a distance less than or equal to 2 mm meets the geometric needs of 3D objects and therefore corresponds to a less marked curling phenomenon.

[0125] The lower limit of the working window corresponds to the lowest temperature in the build chamber for which a distance less than or equal to 2 mm was obtained as the final deformation. The ease or difficulty of cleaning parts with 10 holes of different sizes was assessed using a compressed air blower without sandblasting. The more holes the part was cleared with the blower, the easier the cleaning would be. A score out of 10 was assigned based on the number of holes cleared. For example, a score of 10 / 10 is assigned to powders for which all 10 holes were cleared, and which would therefore be easiest to clean.Conversely, a score of 0 / 10 is given to powders for which no holes have been unblocked, and which therefore clump strongly and will be the most difficult to clean (inducing a longer cleaning time and potentially leading to the degradation of certain fine and / or fragile elements of the 3D parts constructed).

[0126] For each build temperature tested, a score was assigned to each of the two parts built at that build temperature, and the final score is the average of the two scores obtained (this takes into account temperature variations at the surface of the powder bed).

[0127] It was considered that a score greater than or equal to 8 / 10 allows satisfactory ease of cleaning to be maintained and therefore corresponds to a mild caking phenomenon.

[0128] The upper limit of the working window corresponds to the highest temperature in the build chamber for which a score greater than or equal to 8 / 10 was obtained.

[0129] Preparation of polyamide powder

[0130] The powder preparation processes described below were repeated until sufficient powder was obtained to determine the working window.

[0131] Example 1 (comparative): Preparation of PA 12 powder This comparative example is similar to example 4 of the document

[0132] EP 1571173.

[0133] 2800 mL of solvent (paraffinic hydrocarbon fraction with a boiling range of 145 to 160°C) are introduced into the 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 is gradually heated to 110°C, then 290 mL of solvent is distilled under vacuum in order to remove any traces of water that may be present by azeotrope formation.

[0134] After returning to atmospheric pressure, the anionic catalyst, 1.79 g of sodium hydride at 60% purity by weight in oil, is then rapidly introduced under nitrogen, and stirring is increased to 400 rpm, under nitrogen at 110°C for 30 minutes. Then, the temperature is brought back to 100.5°C and, using a small metering pump, a continuous injection of the stearyl isocyanate activator is carried out into the reaction medium, according to the following program:

[0135] 3.6 g of stearyl isocyanate for 60 minutes, then

[0136] - 5.9 g of stearyl isocyanate for 132 minutes.

[0137] At the same time, the temperature is maintained at 100.5°C for the first 60 minutes then increased to 120°C in 30 minutes and maintained at 120°C until 2 hours after the end of the introduction of the stearyl isocyanate. The polymerization is then complete, the reaction medium is cooled to 80°C, then the powder is separated by decantation and dried.

[0138] The obtained PA 12 powder particles have a volume average diameter of 55 pm, an inherent viscosity of 1.48 (g / 100 g)' 1 , a melting temperature of 183°C and a crystallization temperature of 135°C.

[0139] The working window was evaluated as indicated above. It is 168°C, as model construction was only possible at this temperature.

[0140] Example 2 (according to the invention): Preparation of PA 12 powder

[0141] 2800 mL of solvent (paraffinic hydrocarbon fraction with a boiling range of 145 to 160°C) are introduced into the reactor, maintained under nitrogen, followed successively by 919 g of lauryllactam (lactam 12), 15.0 g of EBS and 3.2 g of silica (SIPERNAT 320DS). After starting stirring at 300 rpm, the mixture is gradually heated to 105°C, then 360 mL of solvent is vacuum distilled in order to azeotropically remove any traces of water that may be present. After returning to atmospheric pressure, the anionic catalyst, 2.4 g of sodium hydride at 60% by weight of purity in oil, are then rapidly introduced under nitrogen, and stirring is increased to 550 rpm, under nitrogen at 105°C for 30 minutes.

[0142] Using a small metering pump, a continuous injection of the chosen activator, namely stearyl isocyanate (27.3 g in 119.7 g of solvent) is carried out into the reaction medium according to the following program:

[0143] - 12 g / h of stearyl isocyanate solution for 180 minutes, then

[0144] - 50 g / h of stearyl isocyanate solution for 133 minutes. At the same time, the temperature is maintained at 105°C for 313 minutes during the injection, then after the end of the introduction of the isocyanate the reaction medium is heated to 150°C in 30 minutes and maintained at this temperature for 12 hours.

[0145] At the end of polymerization, the polyamide powder is dispersed in the synthesis solvent. The reaction medium is cooled to 80°C in order to drain the reactor; after solid / liquid separation, the polyamide powder is placed in an oven at 75°C in order to dry it from the solvent.

[0146] The obtained PA 12 powder particles have a volume average diameter of 38 pm, an inherent viscosity of 1.32 (g / 100 g)' 1, a melting temperature of 184°C associated with an enthalpy of fusion of 112 J / g, and a crystallization temperature of 146°C.

[0147] The working window was evaluated as indicated above. It is 166 to 172°C.

[0148] Example 3 (according to the invention): Preparation of PA 12 / 6 powder

[0149] 2800 mL of solvent (paraffinic hydrocarbon fraction with a boiling range of 145 to 160°C) were introduced into the reactor, maintained under nitrogen, followed successively by 919 g of lauryllactam (lactam 12), 4.6 g of caprolactam (lactam 6), 14.0 g of EBS and 4.2 g of Orgasol® 2001 UD Nat 1 (PA 12 powder). After starting stirring at 300 rpm, the mixture was gradually heated to 105°C, and 360 mL of solvent was then distilled under vacuum to azeotropically remove any traces of water that might be present.

[0150] After returning to atmospheric pressure, the anionic catalyst, 3.9 g of sodium hydride at 60% by weight purity in oil, is then rapidly introduced under nitrogen and stirring is increased to 550 rpm, under nitrogen at 105°C for 30 minutes. Using a small metering pump, a continuous injection of the chosen activator, namely stearyl isocyanate (25.3 g in 185.1 g of solvent) is carried out into the reaction medium according to the following program:

[0151] - 12 g / h of stearyl isocyanate solution for 180 minutes, then

[0152] - 50 g / h of stearyl isocyanate solution for 210 minutes. At the same time, the temperature is maintained at 105°C for 390 minutes during the injection, then increased to 150°C in 30 minutes and maintained at this temperature for 12 hours after the end of the introduction of the isocyanate.

[0153] At the end of polymerization, the polyamide powder is dispersed in the synthesis solvent. The reaction medium is cooled to 80°C in order to drain the reactor; after solid / liquid separation, the polyamide powder is placed in an oven at 75°C in order to dry it from the solvent.

[0154] The obtained PA 12 / 6 powder particles have a volume average diameter of 39 pm, an inherent viscosity of 1.34 (g / 100 g) -1 , a melting temperature of 183°C associated with an enthalpy of fusion of 109 J / g, and a crystallization temperature of 145°C.

[0155] The working window was evaluated as indicated above. It is 164 to 172°C.

[0156] Example 4 (comparative): Preparation of PA 12 powder with subsequent heat treatment

[0157] Orgasol® 2002 ES4 Nat 3 PA 12 powder particles have a volume average diameter of 41 pm, an inherent viscosity of 1.02 (g / 100 g)-1 , a melting temperature of 177°C associated with an enthalpy of fusion of 112 J / g, and a crystallization temperature of 150°C.

[0158] The working window was evaluated as stated above. The powder has no working window.

[0159] A heat treatment of 88h at 167°C was undertaken on this PA12 powder in a stirred reactor under nitrogen flushing, as described in document WO 2013 / 090174. The powder particles after heat treatment have a volume average diameter of 41 pm, an inherent viscosity of 1.02 (g / 100 g)' 1 , a melting temperature of 182°C (with a marked shoulder at 180°C on the DSC curve) associated with a fusion enthalpy of 118 J / g, and a crystallization temperature of 150°C.

[0160] The working window was evaluated as indicated above. It is 168°C, as model construction was only possible at this temperature. Example 5 (according to the invention): Preparation of PA 12 / 6 powder

[0161] 2800 mL of solvent (paraffinic hydrocarbon fraction with a boiling range of 145 to 160°C) were introduced into the reactor, maintained under nitrogen, followed successively by 919 g of lauryllactam (lactam 12), 4.6 g of caprolactam (lactam 6), 14.0 g of EBS and 3.0 g of Orgasol® 2001 UD Nat 1 (PA 12 powder). After starting stirring at 300 rpm, the mixture was gradually heated to 105°C, and 360 mL of solvent were then distilled under vacuum to azeotropically remove any traces of water that might be present.

[0162] After returning to atmospheric pressure, the anionic catalyst, 3.9 g of sodium hydride at 60% purity by weight in oil, is then rapidly introduced under nitrogen and stirring is increased to 550 rpm, under nitrogen at 105°C for 30 minutes.

[0163] Using a small metering pump, a continuous injection of the chosen activator, namely stearyl isocyanate (25.3 g in 185.1 g of solvent) is carried out into the reaction medium according to the following program:

[0164] - 50 g / h of stearyl isocyanate solution for 253 minutes.

[0165] At the same time, the temperature is maintained at 105°C for 253 minutes during the injection, then increased to 150°C in 30 minutes and maintained at this temperature for 6 hours after the end of the introduction of the isocyanate.

[0166] At the end of polymerization, the polyamide powder is dispersed in the synthesis solvent. The reaction medium is cooled to 80°C in order to drain the reactor; after solid / liquid separation, the polyamide powder is placed in an oven at 75°C in order to dry it from the solvent.

[0167] The obtained PA12 / 6 powder particles have a volume average diameter of 42 pm, an inherent viscosity of 1.16 (g / 100 g)' 1 , a melting temperature of 182°C associated with an enthalpy of fusion of 122 J / g, and a crystallization temperature of 149°C.

[0168] The working window was evaluated as indicated above. It is 168 to 172°C.

[0169] Results

[0170] It is noted that the powders according to the invention have a wide working temperature window for 3D printing, of at least 4°C, when the baking step was carried out for a period of 6 hours at 150°C and of at least 6°C when it was carried out for a period of 12 hours at 150°C. In addition, it is noted that the PA 12 powder according to the invention of example 2 has a very wide working temperature window compared to the PA 12 powder of comparative example 1, in which the baking step is carried out for a period of 2 hours at 120°C. In addition, when a heat treatment is undertaken on the powder a posteriori (comparative example 4), the widening of the working window is not achieved even if the Tf - Te gap has increased. According to the inherent viscosity results before and after heat treatment, no change in molar mass is observed.These results suggest that it is necessary for the curing step to take place on the compounds under polymerization conditions in order to obtain a widening of the working window.

Claims

Claims 1. Process for manufacturing a polyamide powder by anionic polymerization in a solvent, comprising the following steps: a) the formation of a reaction medium comprising: - the introduction of at least one lactam monomer into said solvent; - the introduction of at least one catalyst into said solvent; and - the introduction of at least one activator into said solvent at a given temperature; b) the polymerization of the lactam monomer into polyamide in the reaction medium; c) the precipitation of the polyamide in powder form in the reaction medium; and d) the heating, after step a), and preferably after step c), of the reaction medium to a temperature higher than the temperature of the introduction of the at least one activator and ranging from 140 to 200°C.

2. Method according to claim 1, in which the heating of step d) is carried out for a duration greater than or equal to 2 hours, preferably greater than or equal to 3 hours, more preferably greater than or equal to 5 hours, more preferably for a duration of 8 to 15 hours, more preferably of 10 to 12 hours.

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

4. Method according to one of claims 1 to 3, in which step a) further comprises the introduction of at least one amide into the solvent, the at least one amide preferably being an N,N'-alkylene bisamide, more preferably N,N'-Ethylene bis-stearamide and / or N,N'-Ethylene bis-oleamide.

5. Method according to one of claims 1 to 4, in which the at least one lactam monomer is chosen from the group consisting of 2- pyrrolidone, caprolactam, 2-azacyclononanone, lauryllactam and mixtures thereof.

6. A method according to one of claims 1 to 5, wherein the at least one catalyst is selected from the group consisting of sodium, potassium, alkali metal hydrides and hydroxides, alkali metal alcoholates, and mixtures thereof, preferably from the group consisting of sodium hydride, potassium hydride, sodium, sodium methoxide, sodium ethylate and mixtures thereof.

7. Method according to one of claims 1 to 6, in which the at least one activator is chosen from the group consisting of lactam-N-carboxyanilides, (mono)isocyanates, polyisocyanates, carbodiimides, cyanamides, acyllactams and acylcarbamates, triazines, ureas, N-substituted imides, esters, phosphorus trichloride and mixtures thereof.

8. Process according to one of claims 1 to 7, in which the solvent is a paraffinic hydrocarbon fraction having a boiling temperature range of between 120 and 170°C.

9. Method according to one of claims 1 to 8, in which the heating temperature in step d) is from 140 to 170°C, preferably from 145 to 160°C, more preferably from 145 to 155°C.

10. Method according to one of claims 1 to 9, in which the temperature of the introduction of the at least one activator is from 50 to 150°C, preferably from 60 to 135°C.

11. Method according to one of claims 1 to 10, further comprising a step of introducing into the solvent one or more additives chosen from the group consisting of pigments, dyes, carbon black, carbon nanotubes, antioxidants, anti-UV agents and plasticizers.

12. Polyamide powder obtained by a manufacturing process according to one of claims 1 to 11.

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

14. Use of a powder according to claim 12, for the manufacture of 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 of manufacturing a three-dimensional article comprising the following steps: manufacturing a powder by a method according to one of claims 1 to 11; depositing, preferably in the form of a layer, said powder; and - sintering the powder, preferably by means of electromagnetic radiation.

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