Recycling process for a used polyamide composition

Abstract

The invention relates to a process for recycling a used polyamide composition into a recycled polyamide powder having a monomodal melting endotherm and a single melting temperature (Tf1max), said process comprising the steps of: contacting a used polyamide composition with a solvent to obtain a mixture; heating the mixture to solubilize the polyamide in the solvent; cooling the mixture to the precipitation temperature (Tp) of the polyamide in said solvent, thereby obtaining a precipitated polyamide powder characterized by a non-monomodal melting endotherm and more than one melting temperature, (Tf1max) being the highest melting temperature;and maintaining the mixture temperature at a temperature not exceeding Tp, specifically within the range of Tp - 0.1°C to Tp - 15°C, until the precipitated polyamide powder is characterized by a monomodal melting endotherm and a melting temperature (Tf1max); and recovery of the resulting recycled polyamide powder. Figure for the abstract: none;

Description

Description Title of the invention: Method for recycling a composition of used polyamide Field of invention

[0001] — The present invention relates to a method for recycling a composition of used polyamide into a polyamide powder having an increased gap between the melting temperature and crystallization temperature (T, — T,) of the powder polyamide.

[0002] A large gap between the T, and the T. of a polyamide-based powder is useful in many uses, and in particular in powder agglomeration technology by melting or sintering caused by radiation such as a laser beam (laser sintering), infrared radiation or UV radiation or any source of electromagnetic radiation to melt the powder to manufacture objects.

[0003] — The present invention also relates to the polyamide powders obtained according to this process.

[0004] — Finally, it concerns the use of this powder and the articles manufactured from it the latter. Prior art

[0005] — The technology of agglomeration of polyamide powders under a laser beam is used to to make three-dimensional objects such as prototypes and models, in various fields.

[0006] — A thin layer of polyamide powder is deposited on a horizontal plate maintained in a heated enclosure at a temperature between the temperature of crystallization temperature T, and the melting temperature T, of the polyamide powder. The laser agglomerates powder particles at different points in the powder layer according to a geometry corresponding to the object, for example using a computer having in memory the shape of the object and restoring the latter in the form of slices. The areas of powder exposed to the laser solidify as soon as their temperature drops below the crystallization temperature T,Then the horizontal plate is lowered from a distance corresponding to the thickness of a layer of powder then a layer is deposited new layer of powder and the laser agglomerates powder particles according to a geometry corresponding to this new slice of the object and so on. The The procedure is repeated until the entire object has been manufactured. We obtain inside the enclosure an object surrounded by non-agglomerated powder. Then we gently cools the whole thing. After complete cooling, the object is separated from the powder which can be reused for another operation. Immediately after the action of the laser beam, the temperature of the exposed area is higher than the crystallization temperature (T) of the powder. But when the temperature drops too quickly below this temperature, for example by adding a new layer of colder powder, this leads to deformations of the part being printed (the "curling" phenomenon). Similarly, when the temperature of the powder in the machine gets too close to the melting temperature (T), this leads to caking around the parts, which is manifested by the formation of lumps of powder affecting the quality of the print. To avoid these phenomena, it is therefore important to have powders with a temperature Tc as far as possible from the T, of the powder. The difference T,, - T, of the powder determines the working temperature window of the device used to agglomerate the powder particles by fusion caused by radiation. This 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 at which agglomeration or "caking" occurs. The lower limit of the working window corresponds to the temperature at which distortion or deformation or "curling" occurs. It is desirable that this working window be greater than the temperature variation within 3D printing machines, which is generally of the order of + / -3°C. Furthermore, a high enthalpy of fusion (AH) is advantageous in order to optimize the geometric definition of the manufactured parts. Indeed, if the latter is too low, the energy provided by the laser risks sintering, by thermal conduction, the powder particles surrounding the part under construction, which limits the geometric precision of the part obtained. It is clear that everything that has just been explained for the agglomeration of polyamide powders under a laser beam is valid regardless of the electromagnetic radiation that causes the fusion, whether the fusion process is selective or non-selective. Document US 5,932,687 discloses a process for preparing a precipitated polyamide powder having a narrow particle distribution and low porosity. This process comprises a first step of cooling the polyamide previously dissolved in an alcohol solvent to a temperature T, (higher than the precipitation temperature of the polyamide in the solvent) so as to obtain nucleation of the polyamide, followed by a second cooling step so as to obtain supersaturation of the medium and thus precipitation of the polyamide at a temperature T». The suspension obtained is directly cooled and dried to recover polyamide powder. Document US 2008 / 0166496 discloses a polyamide 11 powder that can be used in a powder agglomeration process, in particular for preparing three-dimensional objects. These powders are prepared according to a process comprising a step of cooling the polyamide previously dissolved in ethanol to a temperature at which the polyamide precipitates. The heat generation induced by the precipitation maintains the medium at this temperature for 25 minutes, then the temperature decreases slightly and a 35-minute isotherm is achieved. At the end of this stage, the mixture is cooled to isolate the precipitated polyamide powder. However, the inventors were able to observe that the processes of the state of the art made it possible to obtain polyamide powders whose analysis by differential scanning calorimetry, in 1 heating, shows the presence of two temperature peaks, associated with two relatively close but distinct melting points, revealing the presence of at least two distinct crystalline phases. However, for the reasons mentioned above, this heterogeneity of the thermal characteristics of the powder reduces the working window and is therefore likely to harm the quality of the objects manufactured according to the powder agglomeration process by fusion using electromagnetic radiation, in particular their definition. These processes concern the preparation of virgin polyamide powders. As mentioned, the technology described produces a significant quantity of non-agglomerated powder but altered due to having undergone a temperature close to T; for a substantial period. It is interesting to recycle these powders in order to limit energy and resource consumption. Processes for recycling polyamides contained in used compositions, in particular from 3D printing waste, have been described. Thus, document CN110483986 describes a process for recycling residual polyamide 12 powder after selective laser sintering. This process involves solubilizing the powder to be recycled in an acid solution and then neutralizing and atomizing this solution to obtain a recycled polyamide powder. This process, specific to the treatment of waste in 3D printing, therefore involves treatment in an acidic medium, which is very aggressive towards the polyamide. In addition, it does not allow the polyamide to be separated from the other products present in the composition, and does not allow the physicochemical characteristics of the recycled polyamide powder to be controlled, in particular its viscosity, its particle size or its thermal characteristics. In addition, it requires specific equipment and the management of acid solution flows makes it particularly cumbersome to implement. Document CN109810284 describes a process for dissolving polyamide 12 waste from 3D printing, using a composite solvent system comprising a mixture of hydrochloric, formic and acetic acid. A solid / liquid separation step is carried out hot before precipitating the polyamide by adding water as a non-solvent. However, this dissolution / precipitation process in an acidic medium is very aggressive towards the polyamide and also does not allow control of the physicochemical characteristics of the recycled powder. Furthermore, the management of acid solution flows also makes this process cumbersome to implement. There is therefore a real need for a process for recycling used polyamide compositions into recycled polyamide powder, particularly useful for powder agglomeration technologies by fusion caused by electromagnetic radiation, making it possible to overcome these drawbacks. Summary of the invention The inventors have now developed a dissolution / precipitation process which makes it possible to both recycle used polyamide compositions and effectively increase the difference T,—T, of the recycled polyamides, by obtaining a monomodal melting endotherm. More particularly, it was discovered that by introducing, at the end of the polyamide precipitation phase, a temperature plateau of sufficient duration, it was possible to convert a precipitated polyamide powder characterized by a non-monomodal melting endotherm and more than one melting temperature (T4), (T,1"**) being the highest melting temperature, into a polyamide powder characterized by a monomodal melting endotherm, and a single melting temperature (Tf; ) equal to (T,"*) and thus to increase the temperature difference (T, — T.). The inventors were notably able to demonstrate that this temperature plateau made it possible to carry out crystalline improvement, and thus to obtain a single crystalline phase. The polyamide powders obtained are thus particularly advantageous for use in a powder agglomeration process by fusion using electromagnetic radiation, in particular in that they make it possible to widen the working window and therefore to improve the quality and / or definition of the objects manufactured from these powders. According to yet other advantages, the recycling process according to the invention is easy to implement and does not require the use of acidic conditions. It also makes it possible to control the particle size of the powder, in particular its span factor, and to separate, at least partially, the polyamide from the other compounds present in the used composition such as additives and fillers. The recycling process thus makes it possible to obtain a recycled polyamide powder with a high degree of purity, and whose thermal characteristics are improved compared to those of the used polyamide. Thus, according to a first aspect, the invention thus aims to provide a method for recycling a used polyamide composition, into a recycled polyamide powder having a monomodal melting endotherm and a single melting temperature (T,"**), said method comprising the steps of: 1 bringing a used polyamide composition into contact with a solvent in order to to obtain a mixture; ii. heating the mixture in order to solubilize the polyamide in the solvent; iii. cooling the mixture to the precipitation temperature (T,) of the polyamide in said solvent, whereby a polyamide powder is obtained precipitate characterized by a non-monomodal melting endotherm and more of a melting temperature, (T,"**) being the highest melting temperature high: and iv. maintaining the temperature of the mixture at a temperature at most equal to T,, in particular included in the range from T,- 0.1°C to T,-15°C, until that the precipitated polyamide powder is characterized by an endotherm of monomodal melting and a melting temperature (T,"**); and v. recovery of the recycled polyamide powder obtained. Advantageously, the method further has one or more of the following characteristics. Thus, in embodiments, the method according to the invention is a method: in which the solvent which is brought into contact with the polyamide is an alcohol, in particular a C1-C4 aliphatic alcohol, preferably ethanol; in which the heating of the mixture is carried out at a temperature of 100°C at 200°C, and preferably from 120°C to 160°C; and / or in which the heating of the mixture has a duration of 1 to 6 hours, and preferably 1 to 3 hours; wherein the cooling of the mixture in step iii) is carried out at a speed of 1°C to 100°C per hour and preferably of 10°C to 60°C per hour; wherein the polyamide is polyamide 11, polyamide 6, or polyamide 10.10, or polyamide 10.12, or polyamide 6.10; in which the precipitation temperature T of the polyamide is between 80°C and 130°C, especially between 100 and 120°C; wherein in step iv), the mixture is maintained at a temperature close to the precipitation temperature for a period of at least 2 hours, in particular between 3 and 12 hours, from the start of the preci- polyamide pitation; further comprising a step vi) of drying the polyamide powder precipitate recovered in step v) or obtained at the end of step iv), in particular at a temperature between 10°C and 150°C, more particularly between 50 and 100°C: wherein the drying of the precipitated polyamide powder is carried out at a pressure ranging from 10 mbar to atmospheric pressure; wherein the composition further comprises organic compounds volatile (VOCs); in which the composition comprises mineral fillers, in particular fibers, in particular glass and / or carbon fibers; and / or further comprising a step vii) of separation and recovery of the mineral fillers possibly present in the polyamide powder precipitate, especially after step iv), v) or vi). According to a second aspect, the present invention also aims to provide a polyamide powder having a monomodal melting endotherm and a single melting temperature (T;,*) capable of being obtained by the recycling process according to the invention. Advantageously, the powder has one or more of the following characteristics. Thus, in embodiments, the powder according to the invention is a polyamide powder: characterized in that it has a mean diameter in volume included between 10 and 200 um, in particular between 20 and 100 um, and preferably- typically between 40 and 80 um; characterized in that it has a diameter Dv10 greater than 5 um, in particular between 10 and 70 um, and preferably between 20 and 60 um; characterized in that it has a diameter Dv90 of less than 350 um, in particular between 30 and 200 um, and preferably between 50 and 150 um; characterized in that it has a median diameter Dv50 of between 10 and 200 um, in particular between 20 and 100 um, and preferably between 30 and 90 um; characterized in that it has a span factor of between 0.1 and 1.5; preferably between 0.1 and 1.0 and more and preferably between 0.5 and 1.0: in which the polyamide is polyamide 11; characterized in that it has a melting temperature (T,"*) of between 195 and 205°C; and / or in which the difference between the melting temperature (T,"**) and the temperature of crystallization (T.) is between 35 and 45°C. According to a third aspect, the invention relates to a polyamide 11 powder having a monomodal melting endotherm and a single melting temperature (Ta"#*) between 195°C and 205°C, also having at least one of the following characteristics: a volume average diameter of between 10 and 200 um, in particular between 20 and 100 um, and preferably between 40 and 80 um; a diameter Dv10 greater than 5 um, in particular between 10 and 70 um, and preferably between 20 and 60 um; a median diameter in volume Dv50 between 10 and 200 um, in particular between 20 and 100 um, and preferably between 30 and 90 um; a diameter Dv90 less than 350 um, in particular between 30 and 200 um, and preferably between 50 and 150 um; a span factor between 0.1 and 1.5; preferably between 0.1 and | and more preferably between 0.5 and 1.0; an enthalpy of fusion greater than 100 J / g; and preferably included between 110 and 160 J / g; and / or an inherent viscosity between 0.8 and 1.8, and preferably between 1.0 and 1.5. According to a fourth aspect, the invention relates to a composition in powder form for 3D printing, in particular by laser sintering, comprising: - a polyamide powder according to the invention; and - at least one filler or additive. According to a fifth aspect, the invention relates to a method for manufacturing polyamide objects by agglomeration of powder by fusion using electromagnetic radiation, the powder being as defined above. According to a sixth aspect, the invention relates to a manufactured article obtained by melting using electromagnetic radiation of a powder or a composition according to the invention. According to a seventh aspect, the invention relates to the use of a method according to the invention for increasing the difference (Ty -T,) between the melting temperature (T;) and the crystallization temperature (T.) of a polyamide. According to an eighth aspect, the invention relates to recycled mineral fillers capable of being obtained according to the recycling method according to the invention. Figures |Fig.1] represents an image obtained by scanning electron microscopy (SEM) (magnification x 120) of the glass fibers, pre-coated with PAI1, obtained at the end of the process according to inventive example 1. [Fig.2] represents an image obtained by scanning electron microscopy (SEM) (magnification x 240) of carbon fibers, pre-coated with PA11, obtained at the end of the process according to inventive example 2. Detailed description The invention is now described in more detail and in a non-limiting manner in the following description. Definitions It is specified that the expressions “from to…” and “between… and…” used in this description must be understood as including each of the limits mentioned. The term “used polyamide powder composition” means a composition in powder form containing a polyamide, possibly in association with other constituents, including in particular additives or fillers, resulting from an industrial transformation of a polyamide-based composition, for example by extrusion, molding, typically by injection, or even by 3D printing. It may in particular be a composition resulting from used finished products, or from scrap or production waste generated during the process of transforming the polyamide-based composition. These used compositions are generally characterized by a partial degradation of the macromolecular chain of the polyamide which may be in a partially oxidized form and therefore contain imide, and / or alcohol and / or primary amide functions which did not exist on the virgin polyamide (before transformation and possibly, use). In addition, the polyamides are associated with other constituents such as stabilizers which may themselves have undergone degradation. The recycling process according to the invention advantageously makes it possible to separate the polyamide from the used compositions, from the other constituents and to obtain a virtually pure polyamide powder. The term "powder" means a solid material in finely divided form, generally in the form of very small particles, generally of the order of a few hundred micrometers or less. Powders are generally characterized by thermograms obtained by differential scanning calorimetry (DSC) according to: a first heating, allowing to characterize the phenomenon of fusion of the polyamide powder; cooling to characterize the crystallization phenomenon polyamide material; a 2nd heating allowing to characterize the phenomenon of fusion of the polyamide material itself. The following terms are understood to mean in relation to thermal properties, as defined in ISO 11357-1:2016: a "peak" refers to the part of the thermogram obtained by differential calorimetry differential scanning ca- lorimetry”) which deviates from the baseline to reach a local maximum or a local minimum, then returning to the baseline. Such a peak can indicate a first-order transition (crystallization exotherm or en- fusion heat); a melting peak, within the meaning of this description, may include several peaks or shoulders before returning from signal to baseline. a "baseline" means the part of the recorded thermogram without no transition, especially here without any first-order transition of melting or crystallization type. At a transition zone, a line of virtual base can be determined: it is an imaginary line drawn through the transition region, assuming that the heat due to the transition is zero. The virtual baseline can be drawn by interpolating the baseline of the test piece by means of a straight line; a "peak area" means the area bounded by the peak and the line of interpolated virtual base. It is assimilated to a transition enthalpy, expressed in J / g. The term "enthalpy of fusion" is understood to mean the heat required to melt the composition, corresponding to the area below the melting peak(s) on the thermogram, measured according to the standard ISO 11357-3:2018; The term "melting temperature" is understood to mean the temperature representative of the melting phenomenon during which the polyamide powder or the at least partially crystalline polyamide material passes into the viscous liquid state as measured according to standard ISO 11357-3:2018. Unless otherwise indicated, this is more particularly the temperature corresponding to the maximum intensity of the melting peak measured by DSC. Thus, within the meaning of the present description, a melting peak, which would comprise several peaks or shoulders, would be associated with several melting temperatures, namely a melting temperature for each peak or shoulder. By "melting temperatures in 1st and 2nd heating" we mean melting temperatures, noted respectively T; for 1st heating and Tp, for 2nd heating, measured by DSC, according to the ISO11357-3: 2018 standard, and corresponding respectively to the maximum intensity of the signal of the melting peak in first heating and in second heating, both carried out with a temperature ramp of 20°C / min. Thus, at meaning of the present description, if several melting temperatures (Tf,) are detected in the first heating, then the one which must be used in the calculation of the difference (T, — T.) is the temperature T, corresponding to the lowest melting temperature, namely Tf; min, Tf 0x designates the highest melting temperature (Tf,) and corresponds to the single melting temperature (Tf,) obtained at the end of step iv). By "crystallization temperature", hereinafter referred to as T,, is meant the temperature at which the at least partially crystalline compound passes from the viscous liquid state to the semi-crystalline state as measured according to ISO 11357-3:2018, with a temperature ramp of -20°C / min. The crystallization temperature corresponds more particularly to that measured during cooling after the first melting of the compound (1st heating) and before the second melting (2nd heating), the first melting making it possible to erase the thermal history of the compound. Unless otherwise indicated, this is the temperature of the crystallization peak, corresponding to the maximum intensity of the signal in DSC. Thus, within the meaning of this description, if several crystallization temperatures are detected during cooling, then T. corresponds to the highest crystallization temperature and it is this value which must be used in the calculation of the difference (T, — T,). By "monomodal melting endotherm" of the polyamide powder is meant the part of the thermogram obtained by differential scanning calorimetry (DSC) corresponding to the first melting of the polyamide powder, and which is characterized by a single melting temperature Tf,. In other words, the melting peak corresponding to the first heating comprises only a single peak. Conversely, a multimodal melting endotherm is characterized by a melting peak in the first heating having several peaks, i.e. several melting peak temperatures. Similarly, a melting endotherm whose melting peak in the first heating has a shoulder would also not be considered a monomodal endotherm within the meaning of the present description. By "precipitation temperature", hereinafter referred to as T,, is meant the temperature at which the mixture, formed by the polyamide and the solvent used in the process, passes from a homogeneous state to a heterogeneous state. The precipitation temperature is detected by using a temperature probe (PT100 type) coupled with a dynamic thermoregulation system (for example, a "small flower" system sold by the Huber company). At the time of precipitation, there is a strong spontaneous input of thermal energy (exothermicity) that the thermoregulation system cannot compensate for instantaneously. Computer-wise, it is possible to precisely detect the precipitation temperature by plotting the derivative of the temperature of the reaction medium as a function of time. The value of this derivative is equal to the cooling rate programmed using the thermoregulation system. regulation before and after the precipitation phenomenon: the exothermicity induces a perturbation of the derivative which allows it to be detected. The temperature corresponding to the start of the perturbation of the derivative is assimilated to the precipitation temperature (T, > The term "Dv50" is understood to mean the value of the volume median diameter of the powder particles so that the cumulative function of the particle diameter distribution, weighted by their volume, is equal to 50%. Similarly, "Dv10" and "Dv90" are respectively the corresponding diameters so that the cumulative function of the particle diameters, weighted by their volume, is equal to 10%, and respectively, to 90%. These values ​​are measured according to ISO 13319-1: 2021, for example on a Coulter counter multisizer 3 granulometer. The rules for representing results of a particle size distribution are given by ISO 9276 - parts 1 to 6. By "span" factor, we mean a factor characterizing the width of the particle size distribution, defined by: span = (Dv90-Dv10) / Dv50, the diameters "Dv 10", "Dv50" and "Dv90" being as defined previously. The term "average diameter" is understood to mean the value of the volume-average diameter of the particles corresponding to the arithmetic mean of the particle diameters weighted by their volume. This value is measured according to ISO 13319-1:2021, for example on a Coulter counter multisizer 3 granulometer. The term "viscosity" is understood to mean the inherent viscosity as measured in an Ubbelohde-type viscometer according to ISO 307:2019, except when using m-cresol 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, all divided by the concentration of polymer dissolved in the solvent. The term "3D printing" refers to a technique for producing parts through additive manufacturing, by selectively melting a powder using electromagnetic radiation such as a laser or infrared light. A "VOC" means a volatile organic compound, i.e. an organic compound having a vapor pressure of 0.01 KPa or more at a temperature of 293.15 K, or having a corresponding volatility under the particular conditions of use. The best known are butane, toluene, ethanol (90° alcohol), acetone and benzene. Process for recycling a used polyamide composition According to a first aspect, the invention thus aims to provide a method for recycling a used polyamide composition into a recycled polyamide powder having a monomodal melting endotherm and a single melting temperature. fusion (T,"#*), said method comprising the steps of: bringing a used polyamide composition into contact with a solvent in order to to obtain a mixture; ii. heating the mixture to solubilize the polyamide in the solvent; iii. cooling the mixture to the precipitation temperature (T,) of the polyamide in said solvent, whereby a polyamide powder is obtained precipitate characterized by a non-monomodal melting endotherm and more of a melting temperature, (T,"**) being the highest melting temperature high; and iv. maintaining the temperature of the mixture at a temperature at most equal to T,, in particular included in the range from T,- 0.1°C to T,-15°C, until that the precipitated polyamide powder is characterized by an endotherm of monomodal melting and a melting temperature (T;"**); and v. recovery of the recycled polyamide powder obtained. The term "monomer" in the following description should be taken to mean "repeating unit". The case where a repeating unit is made up of the association of a diamine with a diacid is special. It is considered that It is the association of a diamine and a diacid, that is to say the diamine.diacid couple, which corresponds to the monomer. This is explained by the fact that individually, the diamine or the diacid does not allow amide-type functions to be obtained. For the purposes of the invention, the term "polyamide" means the condensation products of lactams, amino acids or diamine-diacid pairs. It may be a homopolymer, i.e. a polymer resulting from the condensation of the same repeating unit, i.e. the same monomer, or a copolymer resulting from the condensation of at least two repeating units, i.e. two different monomers, called "co-monomers", i.e. at least one monomer and at least one co-monomer (monomer different from the first monomer) to form a copolymer such as a copolyamide (abbreviated CoPA), as defined below. Copolyamide (abbreviated CoPA) means the polymerization products of at least two different monomers chosen from: amino acid or aminocarboxylic acid monomers, and preferably alpha, omega-aminocarboxylic acids; lactam-type monomers; the pairs of monomers of the “diamine-diacid” type resulting from the reaction between a diamine and a dicarboxylic acid; and their mixtures, with monomers with different carbon numbers in the case of mixtures between an amino acid type monomer and a monomer of lactam type. These monomers may be linear or branched or substituted where appropriate. According to embodiments, the polyamide is a homopolymer. According to a first type, the polyamide comes from the condensation of an aliphatic, cycloaliphatic or aromatic dicarboxylic acid, in particular containing from 4 to 36 carbon atoms, preferably from 6 to 18 carbon atoms, and an aliphatic, cycloaliphatic or aromatic diamine, in particular containing from 2 to 20 carbon atoms, preferably from 6 to 14 carbon atoms. Examples of dicarboxylic acids include 1,4-cyclohexyldicarboxylic acid, butanedioic acid, adipic acid, azelaic acid, suberic acid, sebacic acid, dodecanedicarboxylic acid, octadecanedicarboxylic acid, terephthalic acid and isophthalic acid, but also dimerized fatty acids. Examples of diamines include tetramethylenediamine, hexamethylenediamine, 1,10-decamethylenediamine, dodecamethylenediamine, trimethylhexamethylenediamine, isomers of bis(4-aminocyclohexyl)methane (BACM), bis(3-methyl-4-aminocyclohexyl)methane (BMACM), 2,2-bis(3-methyl-4-aminocyclohexyl)propane (BMACP), para- aminodicyclohexylmethane (PACM), isophoronediamine (IPDA), 2,6-bis(aminomethyl)norbornane (BAMN) and piperazine (Pip). Advantageously, the polyamide is selected from PA 4.6, PA 4.10, PA 4.12, PA 4.14, PA 4.18, PA 6.10, PA 6.12, PA 6.14, PA 6.18, PA 9.12, PA 10.10, PA 10.12, PA 10.14 and PA 10.18. In the PA XY notation, X represents the number of carbon atoms derived from the diamine residues and Y represents the number of carbon atoms derived from the diacid residues, as is conventional. In some embodiments, the polyamide is selected from polyamide 11, polyamide 6, polyamide 10.10, polyamide 10.12, or polyamide 6.10. Preferably, the polyamide is PA 11. Steps i) and ii) The indefinite article “a” or definite article “the” before the term “polyamide” used in the process according to the invention means, within the framework of this disclosure, “at least one polyamide”, and respectively “said at least one polyamide”. Thus, in a first step 1), a used polyamide composition, i.e. a used composition comprising “at least one” polyamide, is brought into contact with a solvent in order to obtain a mixture. Preferably, a single polyamide is used in the process. However, it is possible to use a mixture of several, in particular two, polyamides. Preferably, such a mixture comprises a majority polyamide, re- in particular having more than 80% by weight of the total weight of polyamides used in step i), and this in such a way as to obtain a coprecipitation of the mixture of polyamides. In certain embodiments, the solvent which is brought into contact with the polyamide may be chosen from: ethanol, propanol, butanol, isopropanol, heptanol, formic acid, acetic acid, N-methylpyrrolidone, N-butylpyrrolidone, butyrolactam, caprolactam. Preferably, the solvent which is brought into contact with the polyamide is a C1-C3 aliphatic alcohol, more preferably ethanol, and even more preferably technical grade ethanol of 96% purity (containing water and denatured with 2-butanone and propan-2-ol). The polyamide may have a weight fraction in the solvent of 0.01 to 0.30; and preferably of 0.1 to 0.3. It may in particular have a weight fraction of 0.01 to 0.05; 0.05 to 0.1; or of 0.1 to 0.15 or of 0.15 to 0.2; or of 0.2 to 0.25; or of 0.25 to 0.3. The mixture obtained is then heated in step ii) to dissolve the polyamide, i.e. until a homogeneous mixture is obtained. The heating of the mixture can in particular be carried out at a temperature between 100°C and 180°C, and preferably between 120°C and 160°C. In some embodiments, heating the mixture may for example be carried out at a temperature of 100°C to 105°C; or 105°C to 110°C; or 110°C to 115°C; or 115°C to 120°C; or 120°C to 125°C; or 125°C to 130°C; or 130°C to 135°C; or 135°C to 140°C; or 140°C to 145°C; or 145°C to 150°C; or 150°C to 155°C; or 155°C to 160°C; or 160°C to 165°C; or 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. In some embodiments, heating the mixture, including maintaining the mixture at the dissolution temperature, may last from 1 to 6 hours, and preferably from 1 to 3 hours. Thus, heating the mixture may last from 1 hour to 1 hour and 30 minutes; or from 1 hour and 30 minutes to 2 hours; or from 2 hours to 2 hours and 30 minutes; or from 2 hours and 30 minutes to 3 hours; or from 3 hours to 3 hours and 30 minutes; or from 3 hours and 30 minutes to 4 hours; or from 4 hours to 4 hours and 30 minutes; or from 4 hours and 30 minutes to 5 hours; or from 5 hours to 5 hours and 30 minutes; or from 5 hours and 30 minutes to 6 hours. In some embodiments, the heating comprises at least one step during which the temperature increases in order to reach a maximum temperature between 100°C and 200°C, in particular between 120°C and 160°C. In some embodiments, the heating comprises at least one step in which the temperature remains substantially constant at a value between 100°C and 200°C, especially between 120°C and 160°C, Step iii) Then, in step iii), the mixture is cooled in order to cause the polyamide to precipitate in powder form. The precipitation temperature (T,) can vary for the same polyamide depending on the solvent. Similarly, for the same solvent, it can vary depending on the polyamide. Indeed, the precipitation of the polyamide is accompanied by a release of heat leading to a slight rise in the internal temperature. At the end of precipitation there is no more release of heat and the internal temperature drops back to its set temperature. This precipitation temperature can be between 80°C and 130°C, in particular between 100 and 120°C, in particular when the solvent is a C1-C2 aliphatic alcohol. This cooling can be carried out to a temperature greater than or equal to 50°C. Thus, the cooling can, for example, be carried out to a temperature of 50°C. Thus, the cooling can, for example, be carried out to a temperature ranging from 50°C to 60°C; or from 60°C to 70°C; or from 70°C to 80°C; or from 80°C to 90°C; or from 90°C to 100°C; or from 100°C to 110°C; or from 110°C to 120°C; or from 120°C to 130°C. Furthermore, this cooling may be carried out at a rate of between | and 100°C per hour, preferably between 10 and 60°C per hour, and more preferably between 20 and 50°C per hour. For example, the cooling may be carried out at a rate of 1 to 5°C per hour; 5 to 10°C per hour; 10 to 15°C per hour; or 15 to 20°C per hour; or 20 to 25°C per hour; or 25 to 30°C per hour; or 30 to 35°C per hour; or 35 to 40°C per hour; or 40 to 45°C per hour; or 45 to 50°C per hour; or 50 to 55°C per hour; or 55 to 60°C per hour; or 60 to 65°C per hour; or from 65 to 70°C per hour; or from 70 to 75°C per hour; or from 75 to 80°C per hour; or from 80 to 85°C per hour; or from 85 to 90°C per hour; or from 90 to 95°C per hour; or from 95 to 100°C per hour. In certain embodiments, and in order to promote precipitation, a quantity of polyamide may be introduced in step i) of loading the raw materials. Preferably, this quantity of polyamide is less than or equal to 20% by mass, and preferably less than or equal to 10% by mass relative to the total mass of polyamide used in step i). The polyamide may be identical or different from that dissolved in the solvent, preferably identical. The polyamide may in particular be chosen from polyamide 11, polyamide 6, polyamide 10.10, polyamide 10.12 and polyamide 6.10. Thus, the added quantity of polyamide can represent from 0.1% to 1% by mass; or from 1% to 2% by mass; or from 2% to 3% by mass; or from 3% to 4% by mass; or from 4% to 5% by mass; or from 5% to 8% by mass; or from 8% to 12% by mass; or from 12% to 16% by mass; or from 16% to 20% by mass of the polyamide relative to the total mass of polyamide used in step 1). Step iii) is advantageously carried out with stirring. For a given stirring system, the stirring speed makes it possible to control the volume average diameter of the particles. As a general rule, the higher the stirring speed, the lower the average diameter of the polyamide particles. Conversely, the lower the stirring speed, the higher the average diameter of the polyamide particles. Step iv) During the cooling step, when the precipitation temperature of the polyamide in said solvent is reached, a precipitation phase begins. The start of this precipitation phase corresponds to the start of step iv) of the process according to the invention. In step iv), the mixture is then maintained at a temperature, close to this precipitation temperature (T,) of the polyamide in the solvent, at most equal to and in particular included in the range from -0.1°C to -15°C of this precipitation temperature, and this for a sufficient time to allow the production of a precipitated polyamide powder having a monomodal melting endotherm and an increased melting temperature. In other words, the process includes in step iv) a temperature stage during which the temperature is kept constant for a period t. More particularly, the temperature is kept constant throughout the duration of the polyamide precipitation phase, namely a period t,, then for an additional period t, making it possible to perfect the crystalline mesh of the precipitated polyamide and thus to obtain a polyamide powder having a monomodal melting endotherm and an increased melting temperature. In general, the duration t, is generally much less than the duration t: so that the total duration t of the temperature plateau is generally very close to t,. The additional duration required to obtain a monomodal melting endotherm can be determined by analyzing samples taken at different differential scanning calorimetry (DSC) intervals according to the ISO11357-3 standard. For example, at the end of the precipitation phase of polyamide 11, i.e. at the end of the period t,, the inventors were able to observe by DSC, in 1** heating, the obtaining of a bimodal melting endotherm, characterized by two distinct melting temperatures. By maintaining the temperature constant at a temperature close to the precipitation temperature of the polyamide in the solvent, for a sufficient additional duration t,, the inventors were able to observe the transformation of the endotherm bimodal melting of polyamide particles into a monomodal melting endotherm, reflected on the DSC thermogram by the disappearance of the peak associated with the lowest melting temperature, in favor of the peak associated with the highest melting temperature. Advantageously, this temperature plateau of a total duration t, +t, therefore makes it possible both to increase the difference T, .T, but also to obtain a monomodal melting endotherm. According to embodiments, in step iv), the mixture is maintained at a constant temperature for a duration t, of at least 2 hours, in particular between 3 and 12 hours, from the end of the precipitation of the polyamide. This additional duration after the end of the precipitation of the polyamide 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 10 to 11 hours; or 11 to 12 hours. In certain embodiments, in step iv), the mixture is maintained at a constant temperature for a duration t of at least 2 hours, in particular between 3 and 12 hours, from the start of precipitation of the polyamide. This duration from the start of precipitation of the polyamide 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 10 to 11 hours; or 11 to 12 hours Step v) and vi) At the end of the temperature plateau carried out in step iv), the precipitated polyamide particles are recovered from the mixture in the form of a powder in step v) by conventional solid-liquid separation means. This step generally includes cooling the mixture obtained so that the reactor can be emptied and thus the precipitated polyamide particles obtained from the solvent can be separated, in particular by filtration. The process for manufacturing the polyamide powder may also comprise a step vi) of drying the polyamide powder obtained in step iv) or recovered in step v). The drying step may for example be carried out in a stirred or rotary dryer. In some embodiments, the drying may be carried out at a temperature of 10°C to 150°C, in particular 50°C to 100°C, preferably 25°C to 85°C, and more preferably 70°C to 80°C. The drying may for example be carried out at a temperature of 10°C to 20°C; or 20°C to 30°C; or 30°C to 40°C; or 40°C to 50°C; or 50°C to 60°C; or 60°C to 70°C; or 70°C to 80°C; or 80°C to 90°C; or 90°C to 100°C; or 100°C to 110°C; or 110°C to 120°C; or from 120°C to 130°C; or from 130°C to 140°C; or from 140°C to 150°C; or from 150°C to 160°C. In some embodiments, the drying may be carried out under vacuum at a pressure of less than 100 mbar, preferably less than 50 mbar. Thus, the drying may be carried out at a pressure of 1 to 10 mbar; or 10 to 20 mbar; 20 to 30 mbar; 30 to 40 mbar; 40 to 50 mbar; 50 to 60 mbar; 60 to 70 mbar; 70 to 80 mbar; 80 to 90 mbar; 90 to 100 mbar; 100 to 150 mbar; 150 to 200 mbar; 200 to 250 mbar; or 250 to 300 mbar; or 300 to 500 mbar; or 500 to 700 mbar; or from 700 mbar to less than 1 bar (absolute pressure). Alternatively, drying can be carried out at atmospheric pressure. Advantageously, drying the organic solvent promotes the elimination of VOCs possibly present in the initial used composition. Step vii) The particles recovered in step v), optionally dried in step vi), may optionally be subjected to a step vii) aimed at separating them from the inorganic materials, in particular in the form of fillers, possibly present in the used polyamide composition used in step i). Examples of mineral fillers that may be present in used polyamide compositions include hollow beads, fibers, for example glass or carbon fibers, talc, carbon black, nanotubes, carbon or not. Mineral fillers can be separated from polyamide by taking advantage of density differences. For example, this separation can be achieved by decantation, using cyclones, etc. The precipitated polyamide particles, separated from the mineral fillers by conventional means such as by decanting the mixture into a suitable liquid, for example a mixture of water and glycerol, can be recovered at the end of step vii). They can optionally be dried under conditions similar to those of step v). Polyamide powder and mineral fillers can be recovered and reused separately. Advantageously, the mineral fillers, such as fibers, recovered at the end of step vii) are covered with crystallized polyamide which makes them particularly compatible with a polymer matrix for use as a filler in a subsequent use. Thus, according to embodiments, the invention relates to the mineral fillers capable of being obtained according to the recycling process described above, in particular according to steps i) to vii). According to other embodiments, the mineral fillers possibly present in the used polyamide composition can be separated and recovered before precipitation of the polyamide, in particular before step iii). Indeed, during step i), the Polyamide is generally dissolved in the solvent while the mineral fillers remain in suspension. These can then be separated and recovered by conventional solid-liquid separation techniques, such as filtration. Polyamide powder capable of being obtained according to the recycling process of the invention According to a second aspect, the invention relates to a polyamide powder having a monomodal melting endotherm and a single melting temperature (T;, "**) capable of being obtained by the recycling process as described above. In some embodiments, the polyamide powder has an inherent viscosity of 0.8 to 1.7, and preferably 1.0 to 1.5. Thus, the powder may, for example, have an inherent viscosity of 0.8 to 0.9; or 0.9 to 1.0; or 1.0 to 1.1; or 1.1 to 1.2; or 1.2 to 1.3; or 1.3 to 1.4; or 1.4 to 1.5; or 1.5 to 1.6; or 1.6 to 1.7. In the above, the inherent viscosity is expressed in (g / 100 g)!. The inherent viscosity is measured using a micro-Ubbelohde tube. The measurement is carried out at 20°C on a 75 mg sample of powder at a concentration of 0.5% (m / m) in m-cresol. The inherent viscosity is expressed in (g / 100 g) and is calculated according to the following formula: Inherent viscosity = In(t, / t9) x 1 / C, with C = m / px 100, in which t, ​​is the flow time of the solution, tg is the flow time of the solvent, m is the mass of the sample whose viscosity is determined and p is the mass of the solvent. In some embodiments, the precipitated polyamide powder may have a crystallization temperature (T.) of 100°C to 200°C, and preferably of 130°C to 180°C. The polyamide powder may in particular have a crystallization temperature of 100°C to 110°C; or of 110°C to 120°C; or of 120°C to 130°C; or of 130°C to 140°C; or of 140°C to 150°C; or of 150°C to 160°C; or of 160°C to 170°C; or of 170°C to 180°C; or of 180°C to 190°C; or of 190°C to 200°C. In certain embodiments, the polyamide powder has a fusion enthalpy greater than or equal to 60 J / g, preferably greater than or equal to 100 J / g. This fusion enthalpy may for example be 60 to 80 J / g; or 80 to 100 J / g; or 100 to 110 J / g; or 110 to 120 J / g; or 120 to 130 J / g; or 130 to 140 J / g; or 140 to 150 J / g; or 150 to 160 J / g. In certain embodiments, the polyamide powder may have a melting temperature T[, between 130°C and 260°C, and preferably between 160°C and 210°C. The polyamide powder may in particular have a melting temperature of 130°C to 140°C; or of 140°C to 150°C; or of 150°C to 160°C; or of 160°C to 170°C; or of 170°C to 180°C; or of 180°C to 190°C; or of 190°C to 200°C; or of 200°C to 210°C; or of 210°C to 220°C; or of 220°C to 230°C; or from 230°C to 240°C; or from 240°C to 250°C; or from 250°C to 260°C. The melting temperature (T;) of the precipitated polyamide powder is determined during the first heating as explained above. According to the method of the invention, a single melting temperature of the polyamide is observed at the end of the temperature plateau at the end of step iv). In some embodiments, the polyamide powder may have an apparent specific surface area of ​​0.1 to 50 m° / g, and preferably 1 to 10 m° / g. The precipitated polyamide powder may therefore have a specific surface area of ​​0.1 to 1 m' / g; or 1 to 5 m' / g; or 5 to 10 mg; or from 10 to 20 m” / g; or from 20 to 30 m? / g; or from 30 to 50 m? / g. The apparent specific surface area (ASS) is measured according to the BET (BRUNAUER-EMMET-TELLER) method, known to those skilled in the art. It is notably described in The Journal of the American Chemical Society, volume 60, page 309, February 1938, and corresponds to the international standard ISO 9277: 2010. The specific surface area measured according to the BET method corresponds to the surface porosity of the powder, i.e. it includes the surface formed by the pores on the surface of the particles. According to certain embodiments, the polyamide powder obtained according to the process of the invention is characterized in that it has: a volume average diameter of between 10 and 200 um, in particular between 20 and 100 um, and preferably between 40 and 80 um; a diameter Dv10 greater than 5 um, in particular between 10 and 70 um, and preferably between 20 and 60 um; a median diameter in volume Dv50 between 10 and 200 um, in particular between 20 and 100 um, and preferably between 30 and 90 um; a diameter Dv90 less than 350 um, in particular between 30 and 200 um, and preferably between 50 and 150 um; a span factor between 0.1 and 1.5; and preferably between 0.5 and 1.0. an enthalpy of fusion greater than 60 J / g; and preferably included between 100 and 160 J / g an inherent viscosity of between 0.5 and 2.0, and preferably between 1.0 and 1.5. In a preferred embodiment, the polyamide powder having a monomodal melting endotherm and a single melting temperature (T;"*) capable of being obtained by the recycling process, is characterized in that it has a span factor of between 0.1 and 1.5, preferably between 0.1 and 1.0 and more preferably between 0.5 and 1.0. Polyamide 11 powder According to another aspect, the invention relates to a polyamide 11 powder characterized in that it presents a monomodal melting endotherm and a single melting temperature in heating T, equal to Tf,"* <comprise entre 195°C et 205°C, notamment d’environ 200°C et / ou une température de cristallisation T, comprise entre 150 et 165°C, notamment d’environ 158°C. Polyamide 11 powder is in particular a powder characterized by one or more of the following characteristics: a volume average diameter of between 10 and 200 um, in particular between 20 and 100 um, and preferably between 40 and 80 um; a diameter Dv10 greater than 5 um, in particular between 10 and 70 um, and preferably between 20 and 60 um; a median diameter in volume Dv50 between 10 and 200 um, in particular between 20 and 100 um, and preferably between 30 and 90 um; a diameter Dv90 less than 350 um, in particular between 30 and 200 yum, and preferably between 50 and 150 um; a span factor between 0.1 and 1.5; and preferably between 0.5 and 1.0. an enthalpy of fusion greater than 100 J / g; and preferably included between 110 and 160 J / g an inherent viscosity between 0.8 and 1.8, and preferably between 1.0 and 1.5. Preferably, the polyamide 11 powder is characterized in that it has a monomodal melting endotherm and a single melting temperature in 1* heating T, equal to Tf,"**between 195°C and 205°C, and a span factor between 0.1 and 1.5, preferably between 0.1 and 1 and more preferably between 0.5 and 1.0. Composition in powder form for 3D printing, in particular by selective laser sintering According to yet another aspect, the invention relates to a composition in powder form for 3D printing, in particular by selective laser sintering, comprising a polyamide powder as defined above, in association with one or more usual fillers or additives, i.e. suitable for 3D printing technologies. This composition is advantageously ready to use. This composition may include additives that help improve the processing properties of the powder for its use in 3D printing technologies. The additives generally represent less than 5% by weight relative to the total weight of the composition. Preferably, the additives represent less than 1% by weight of the total weight of the composition. Among the additives, mention may be made of flow agents, stabilizing agents (light, in particular UV, and heat), optical brighteners, dyes, pigments, energy-absorbing additives (including UV absorbers). Among the flow agents, mention may be made, for example, of a hydrophilic or hydrophobic silica. Advantageously, the flow agent represents from 0.01 to 0.5% by weight relative to the total weight of the composition. Preferably, the composition comprises 0.1 to 0.4% by weight of flow agent. The composition may also include one or more fillers, making it possible in particular to improve the mechanical properties (breaking stress and elongation at break) of the parts obtained by 3D printing. The fillers generally represent less than 50% by weight, and preferably less than 40% by weight relative to the total weight of the final powder. Among the fillers, we can cite reinforcing fillers, in particular mineral fillers such as carbon black, talc, nanotubes, carbon or not, fibers (glass, carbon, etc.), ground or not. The additives or fillers may be mixed with the polyamide before the polyamide powder manufacturing process, during the polyamide powder manufacturing process (e.g., in step i) before dissolving the polyamide or in step iv) after precipitation), or after the polyamide powder manufacturing process. Preferably, the additives are introduced after the polyamide powder manufacturing process, by mixing the polyamide powder and said additives. The composition may comprise the polyamide in a weight proportion preferably greater than or equal to 80%, or 81%, or 82%, or 83%, or 84%, or 85%, or 86%, or 87%, or 88%, or 89%, or 90%, or 91%, or 92%, or 93%, or 94%, or 95%, or 96%, or 97%, or 98%, or 99%, or 99. 1%, or 99.2%, or 99.3%, or 99.4%, or 99.5%, or 99.6%, or 99.7%, or 99.8%, or 99.9%, or 99.91%, or 99.92%, or 99.93%, or 99.94%, or 99.95%, or 99.96%, or 99.97%, or 99.98%, or 99.99%. In embodiments, the polyamide contained in the composition is polyamide 11. In embodiments, polyamide 11 has a melting temperature (T1) of between 185°C and 205°C. In embodiments, the difference between the melting temperature (T;) and the crystallization temperature (T.) of polyamide 11 is between 35 and 45°C. Use of a polyamide powder obtained according to the recycling process of the invention or of a composition in powder form comprising it, in a process for agglomerating powder by fusion The invention also relates to a method for manufacturing a polyamide object by agglomeration of powder by fusion using electromagnetic radiation, the powder being a polyamide powder or a composition in the form of powder as defined above. The electromagnetic radiation can be infrared, ultraviolet or visible radiation. Preferably, it is laser radiation (the manufacturing process is then called “selective laser sintering”). According to this method, a thin layer of powder is deposited on a horizontal plate maintained in an enclosure heated to a so-called construction temperature. The term 'construction temperature' designates 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 chosen within the range T, - T, of the polyamide powder resulting from the manufacturing process, preferably between T, - 5°C and T, + 5°C, and more preferably between T4; - 10°C and T, + 10°C. The electromagnetic radiation then 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 reproducing this shape in the form of slices). Next, the horizontal plate is lowered by a distance corresponding to the thickness of a layer of powder, and a new layer is deposited. The thickness of a layer is typically between 0.05 and 2 mm, and generally around 0.1 mm. The electromagnetic radiation provides the energy necessary to sinter the powder particles into a geometry corresponding to this new slice of the object and so on. The procedure is repeated until the object is manufactured. Powders are used in the agglomeration process by fusion or sintering. These powders can have a volume average diameter of 10 µm up to 200 µm and are advantageously of volume average diameter between 20 and 100 µm. Preferably the volume average diameter is between 40 and 80 um. The invention also relates to a manufactured article, in particular by 3D printing, obtained by sintering using electromagnetic radiation of a powder as previously described. This article can be chosen from prototypes and models, particularly in the automotive, nautical, aeronautical, aerospace, medical (prosthetics, hearing systems, cellular tissues, etc.), textile, clothing, fashion, decoration, housings for electronics, telephony, home automation, IT, lighting fields. More generally, the invention also relates to the use of a manufacturing method as previously described to increase the difference (T; -Te) between the melting temperature (T;) and the crystallization temperature (T.) of a polyamide. Examples The following examples illustrate embodiments of the present invention without limiting it. In all the following examples: - The particle size distribution of the powders was characterized by measuring the particle size distribution on a Coulter Counter-Multisizer 3 device (Beckmann Coulter) in accordance with ISO 13319-1:2021. From this, the volume average diameter as well as the diameters Dv10, Dv50 and Dv90 were determined. The span value is calculated from these volume average diameters. - the analysis of the thermal characteristics is carried out by DSC according to the ISO 11357-3 standard "Plastics — Differential Scanning Calorimetry (DSC) Part 3: Determination of temperature and enthalpy of melting and crystallization”. The temperatures which are of particular interest here are the melting temperature during the first heating (T,) and the crystallization temperature (T.). Indeed, in a manner known to those skilled in the art (in the field of manufacturing 3D objects by agglomeration of powder by fusion), the difference "T; - T." corresponds to T4 - T… - The inherent viscosity of polyamides is measured in an Ubbelohde type viscometer according to ISO 307:2019, except when using m-cresol as solvent and a temperature of 20°C. - the acidity (similar to the concentration at the COOH chain end of the polyamide) and the basicity (similar to the concentration at the NH chain end of the polyamide) are measured by potentiometry. The acidity is measured according to the following method: a sample of polyamide is dissolved in benzyl alcohol at a concentration of 0.6% by mass; then, this sample is assayed by potentiometry with a 0.02N tetrabutylammonium hydroxide solution. The basicity is measured according to the following method: a sample of polyamide is dissolved in meta-cresol at a concentration of 0.6% by mass; then, this sample is assayed by potentiometry with a 0.02N perchloric acid solution. Example 1 according to the invention: Recycling of a used polyamide composition containing glass fibers Scraps and cores recovered following the injection of Rilsan® BZM30 O TLDA grade were first coarsely crushed to make them easier to handle. Their composition is as follows: 70 wt% of partially oxidized PA11 and 30 wt% of glass fibers (and antioxidant residues). In a reactor (1L useful), 85 g of this crushed raw material and 425 g of technical ethanol (purity 96%) are loaded, mechanical stirring is carried out using propeller turbine type blades. The stirrer is started at a speed of 500 rpm throughout the test, then the medium is heated to 160°C, followed by a one-hour isotherm to solubilize only the used polyamide. Controlled cooling at a rate of -60°C / h to 110°C is carried out to precipitate the polyamide, followed by a 4-hour isotherm at this same temperature to achieve crystalline perfection. The crystallization exotherm is detected at 115°C and only disrupts the thermal regulation for a few minutes. Then, controlled cooling is restarted at this same rate of -60°C / h to 20°C, the reactor is then drained and the dispersion is dried in an oven at 75°C at atmospheric pressure. It was possible to separate the used PA11 powder from the glass fibers by decantation, using a water / glycerol mixture (45 / 55% by volume). It then appears that 61% by mass of the used PA11 precipitated directly in powder form and the remaining 39% by mass allowed the glass fibers to be coated (see [Fig. 1]). Pre-coated with PA11, the glass fibers are more easily incorporated and have better compatibility with polyamide matrices compared to natural glass fibers. In addition to being recyclable, these glass fibers are now more easily usable in polyamide-based compositions. The obtained PAI1 powder has the following characteristics: an inherent viscosity of 1.30, a volume average diameter of 61 μm as well as diameters Dv10 = 29 μm, Dv50 = 68 μm and Dv90 = 91 μm therefore a span = 0.91. The DSC analysis of this PA11 powder shows a monomodal melting endotherm in 1% heating with a single melting temperature at 201°C associated with a fusion enthalpy of 136 J / g, as well as a single crystallization temperature T, = 158°C. The difference Tf, - T, is now equal to 43°C, Example 2 according to the invention: Recycling of a used polyamide composition containing carbon fibers Scraps and cores recovered following the injection of Rilsan® BSR30 grade were first coarsely crushed to make them easier to handle. Their composition is as follows: 70 wt% of partially oxidized PA11 and 30 wt% of carbon fibers (and antioxidant and carbon black residues). In a reactor (1L useful), 85 g of this crushed raw material and 425 g of technical ethanol (purity 96%) are loaded, mechanical stirring is carried out using propeller turbine blades. The stirrer is started at a speed of 500 rpm throughout the test, then the medium is heated to 160°C, followed by a one-hour isotherm to solubilize the partially oxidized polyamide only. Controlled cooling at a rate of -60°C / h to 110°C is carried out to precipitate the polyamide, followed by a 4-hour isotherm at this same temperature to achieve crystalline perfection. The crystallization exotherm is detected at 115°C and only disrupts thermal regulation for a few minutes. Then, controlled cooling is restarted at the same speed of -60°C / h down to 20°C, the reactor is then drained and the dispersion is dried in an oven at 75°C at atmospheric pressure. It was possible to separate the used PA11 powder from the carbon fibers by decantation using a water / glycerol mixture (45 / 55% by volume). It then appears that 52% by mass of the partially oxidized PA11 precipitated directly in powder form and the remaining 48% by mass allowed the carbon fibers to be coated (see [Fig.2]). Pre-coated with PA11, the carbon fibers are more easily incorporated and have better compatibility with polyamide matrices compared to natural carbon fibers. In addition to being recyclable, these glass fibers are now more easily usable in polyamide-based compositions. The obtained PAI1 powder has the following characteristics: it is black in color (the carbon black has not been separated), an inherent viscosity of 1.42, a volume average diameter of 55 um as well as diameters Dv10 = 29 um, Dv50 = 58 um and Dv90 = 77 um therefore a span = 0.83. The DSC analysis of this PA11 powder shows a monomodal melting endotherm in 1 heating with a single melting temperature at 200°C associated with a melting enthalpy of 132 J / g, as well as a single crystallization temperature T, = 159°C. The difference Tf; - T. is now equal to 42°C. Example 3 according to the invention: Recycling of a used polyamide composition polluted by VOCs Used tubes were collected from gasoline lines during the dismantling of various vehicles. These tubes were originally obtained by extrusion of Rilsan® BESN Black P20 TL grade. They were first roughly crushed to make them easier to handle. These used PA11 tubes contain 4% by mass of VOCs (mainly gasoline, toluene, xylene, and trimethylbenzene). The content is determined by thermogravimetric analysis and the composition by gas chromatographic analysis. In a reactor (1L useful), 85 g of this crushed raw material and 425 g of technical ethanol (purity 96%) are loaded, mechanical stirring is carried out using propeller turbine blades. The stirrer is started at a speed of 500 rpm throughout the test, then the medium is heated to 160°C, followed by a one-hour isotherm to solubilize the partially oxidized polyamide only. Controlled cooling at a rate of -60°C / h to 115°C is carried out to precipitate the polyamide, followed by a 4-hour isotherm at this same temperature to achieve crystalline perfection. The crystallization exotherm is detected at 120°C and only disrupts thermal regulation for a few minutes. Then, controlled cooling is restarted at the same rate of -60°C / h down to 20°C, the reactor is then drained and the dispersion is dried at 90°C under vacuum (50 mbar) for 6 hours. The VOC content of PA11 powder is now 0.35% by mass (mainly ethanol with traces of pollutants < 0.1% by mass). The dissolution / precipitation process in ethanol therefore appears capable of extracting the pollutants from PA11 and then removing them by entrainment during vacuum drying. The obtained PAI1 powder has the following characteristics: it is black in color (the carbon black has not been separated), an inherent viscosity of 1.45, a volume average diameter of 51 μm as well as diameters Dv10 = 25 μm, Dv50 = 49 μm and Dv90 = 62 μm therefore a span = 0.76. The DSC analysis of this PA11 powder shows a monomodal melting endotherm in 1 heating with a single melting temperature at 199°C associated with a melting enthalpy of 136 J / g, as well as a single crystallization temperature T, = 159°C. The difference Tf; - T. is now equal to 40°C. Example 3a (comparative): elimination of VOCs from a used composition by drying The same crushed raw material as in Example 3 was placed directly into a dryer to extract VOCs from PA11. After 12 hours of drying at 90°C under 50 mbar vacuum, it was only possible to remove 0.5% by mass of these VOCs. It was necessary to heat the dryer to 150°C for a further 12 hours under 50 mbar vacuum to remove 3.5% by mass of VOCs. The resulting crushed PA11 powder, however, still contains a residual content of 0.5% by mass of VOCs. Furthermore, its particle size does not make it suitable for 3D printing. In addition to recovering a powder that can be used directly in 3D printing, the drying of example 3 according to the invention is advantageously less energy-intensive for eliminating VOCs. Example 4 (comparative): preparation of a polyamide 11 Polyamide 11 is obtained by polycondensation of 11-aminoundecanoic acid in the presence of 3,000 ppm of orthophosphoric acid used as a catalyst. This PAI1 has an inherent viscosity of 1.40 associated with a concentration of COOH groups at the end of the chain equal to 55 mmol / kg and NH groups at the end of the chain equal to 51 mmol / kg as well as a melting temperature of 189°C (2: DSC heating according to ISO 11357-3:2018). In a reactor (1L useful), 85 g of the PA11 produced in example 1 and 425 g of technical ethanol (purity 96%) are loaded into a reactor, mechanical stirring is carried out using propeller turbine type blades. The stirrer is started at a speed of 500 rpm throughout the test, then the medium is heated to 160°C, followed by a one-hour isotherm to solubilize the polyamide 11. Controlled cooling at a rate of -60°C / h to 115°C is carried out to precipitate the polyamide, followed by a 4-hour isotherm at this same temperature to achieve crystalline perfection. The crystallization exotherm is detected at 120°C and only disrupts the thermal regulation for a few minutes. Then, controlled cooling is restarted at this same rate of -60°C / h to 20°C, the reactor is then drained and the dispersion is dried in an oven at 75°C at atmospheric pressure. The obtained PAI1 powder has the following characteristics: an inherent viscosity of 1.28, a volume average diameter of 40 μm as well as diameters Dv10 = 27 μm, Dv50 = 42 μm and Dv90 = 53 μm therefore a span = 0.62. The DSC analysis of this PA11 powder shows a monomodal melting endotherm in 1 heating with a single melting temperature at 200°C associated with a melting enthalpy of 140 J / g, as well as a single crystallization temperature T. = 157°C, The gap Ty - T, is now equal to 43°C. Example 5 (comparative) following US 2008 / 0166496 A diamine-terminated PA 11 is prepared by polymerization of 250 g of 11-aminoundecanoic acid in the presence of 1.25 g of 4, 4'-diaminocyclohexylmethane (PACM, mixture of isomers). The obtained polyamide 11 has an inherent viscosity of 1.42 associated with a concentration of COOH groups at the end equal to 19 mmol / kg and of NH groups at the end of the chain equal to 67 mmol / kg. In a reactor (1L useful), 85 g of this diamine-terminated PAI1 and 425 g of technical ethanol (purity 96%) are loaded, mechanical stirring is carried out using propeller turbine blades. The stirrer is started at a speed of 500 rpm throughout the test. The medium is heated to 152°C, followed by a one-hour isotherm at this temperature. The medium is then cooled to 112°C at a speed of 25°C / h and then maintained at this temperature for one hour, during this cooling phase when the internal temperature reaches 125°C, the double jacket temperature must be 2 to 3°C lower than the internal temperature. The crystallization exotherm is detected and only disturbs the controlled cooling for a few minutes. After one hour at this temperature, the medium is cooled to room temperature. Then the reactor is drained and the ethanol is distilled in a stirred dryer at 70°C / 400 mbar and then the powder is dried at 84°C / 20 mbar.. The obtained PAI1 powder has the following granulometric characteristics: a volume average diameter of 89 μm as well as diameters Dv10 = 65 μm, Dv50 = 93 μm and Dv90 = 123 μm therefore a span = 0.62. The DSC analysis of this PA11 powder shows a bimodal fusion endotherm in 1 heating with a shoulder at 193°C and a peak at 202°C associated with a fusion enthalpy of 140 J / g, as well as a single crystallization temperature T, = 162°C. It is the lower of the two melting temperatures which is used to calculate the difference T, - T, which is therefore equal to 29°C.

Claims

Claims

1. Method for recycling a used polyamide composition, cn unc recycled polyamide powder with a melting endotherm monomodal and a single melting temperature (T,"*), said process including the steps of: L bringing a used polyamide composition into contact with a solvent in order to obtain a mixture; ii. heating the mixture in order to solubilize the polyamide in the solvent; lil. cooling the mixture to the preci- temperature pitation (T,) of the polyamide in said solvent, whereby obtains a precipitated polyamide powder characterized by a non-monomodal melting endotherm and more than one tem- melting temperature, (T,"**) being the melting temperature higher; and iv. maintaining the temperature of the mixture at a temperature at more equal to T, notably included in the range from T,- 0.1°C to T,-15°C, until the polyamide powder precipitate is characterized by a melting endotherm monomodal and a melting temperature (T,,"#*); and v. recovery of the recycled polyamide powder obtained.

2. The method of claim |, wherein the solvent which is put into contact with polyamide is an alcohol, in particular an alkaline alcohol C,-C4 phatic, preferably ethanol.

3. A method according to any one of claims 1 or 2, wherein the polyamide is polyamide 11, polyamide 6, or polyamide 10.10, or polyamide 10.12, or polyamide 6.

10.

4. A method according to any preceding claim, in which in step iv), the mixture is maintained at a temperature for a duration of at least 2 hours, in particular between 3 and 12 hours, from the end of the precipitation of the polyamide.

5. A method according to any one of the preceding claims, in which composition further comprises organic compounds volatile organic compounds (VOCs).

6. A method according to any preceding claim, in which composition includes mineral fillers, in particular fibers, in particular glass and / or carbon fibers.

7. A method according to claim 6, further comprising a step vii) of separation and recovery of mineral charges if necessary present in the precipitated polyamide powder, especially after step iv), v) or vi).

8. Polyamide powder having a monomodal melting endotherm and a single melting temperature (T,"*) which can be obtained by the recycling process according to one of claims 1 to 7.

9. Polyamide powder according to claim 8, characterized in that it has a span factor of between 0.1 and 1.5; preferably between 0.1 and 1.0 and more and preferably between 0.5 and 1.

0.

10. | Powder according to any one of claims 8 or 9, in which the polyamide is polyamide 11.

11. | Powder according to any one of claims 9 and 10, wherein the difference between the melting temperature (T,"**) and the crystallization temperature- lisation (T,) is between 35 and 45°C.

12. Polyamide 11 powder according to any one of the claims 8 to 11 showing a monomodal melting endotherm and a single melting temperature (T,,"**) between 195°C and 205°C furthermore having at least one of the following characteristics: a volume average diameter between 10 and 200 um, in particular between 20 and 100 um, and preferably between 40 and 80 um; a diameter Dv10 greater than 5 um, including between 10 and 70 um, and preferably between 20 and 60 um ; a median diameter in volume Dv50 between 10 and 200 um, in particular between 20 and 100 um, and preferably- typically between 30 and 90 um; a diameter Dv90 less than 350 um, including in particular between 30 and 200 um, and preferably between 50 and 150 um ; a span factor between 0.1 and 1.5; preferably between 0.1 and 1 and more preferably between 0.5 and 1.0; an enthalpy of fusion greater than 100 J / g; and preferably- partially between 110 and 160 J / g; and / or an inherent viscosity of between 0.8 and 1.8, and preferably- typically between 1.0 and 1.

5.

13. Composition in powder form for 3D printing, in particular by laser sintering, including: - a polyamide powder according to any one of claims 8 at 12; and - at least one filler or additive.

14. Method for manufacturing polyamide objects by agglomeration of powder by fusion using electromagnetic radiation, the powder being as defined in any one of the claims 8a to 13.

15. Manufactured article obtained by fusion using electric radiation- electromagnetic of a powder according to any one of claims 8 to 12 or a composition according to claim 13.