Process for recycling particulate semi-crystalline polyamide for 3D printing

Abstract

The invention relates to a process for recycling a particulate semi-crystalline polyamide, the initial particulate semi-crystalline polyamide having already been used in a layer-by-layer printing process, the process comprising: - a step of contacting the initial particulate semi-crystalline polyamide with water and at least one acid; - a step of heating the initial particulate semi-crystalline polyamide to a temperature above Tc – 20°C and below Tf, Tc denoting the crystallization temperature of the initial particulate semi-crystalline polyamide and Tf being the melting temperature of the initial particulate semi-crystalline polyamide, and - a step of collecting the treated particulate semi-crystalline polyamide, in which the treated particulate semi-crystalline polyamide has an inherent viscosity lower than the inherent viscosity of the initial particulate semi-crystalline polyamide.

Description

Title of the invention: Process for recycling a particulate semi-crystalline polyamide for 3D printing. Field of the invention

[0001] The present invention relates to a method for recycling a particulate semi-crystalline polyamide. More particularly, the invention relates to a method for manufacturing a particulate semi-crystalline polyamide that can be reused several times in a layer-by-layer manufacturing process of objects by powder agglomeration through melting or sintering induced by radiation. Technical background

[0002] The construction of three-dimensional articles by additive manufacturing (also commonly called 3D printing) can be used to produce prototypes or various parts, for example in the automotive, nautical, aeronautical, aerospace, medical (in particular for the manufacture of prostheses, hearing systems, cellular tissues...), textile, clothing, fashion, decoration, housings for electronics, telephony, home automation, computer science, lighting, sports and industrial tooling fields.

[0003] There are several 3D printing techniques, including: - manufacturing by fused filament deposition (FFF) or "fused deposition modeling" in English (FDM), using spools of plastic filament as consumables; - stereolithography (SLA), where a light projector photopolymerizes (solidifies) resin; - powder fusion or sintering (SLS, SMS...), methods where particles of plastic or metal material are fused using focused or non-focused electromagnetic radiation; - material jetting (HSS, MJF, "material jetting" or "binder jetting" in English), in which microscopic droplets of material or binding agent are deposited on a bed of powder allowing the absorption of unfocused infrared radiation.

[0004] Among these techniques, the powder sintering manufacturing process (also called powder fusion agglomeration, including the SLS and MJF technologies mentioned above) is particularly interesting. This technology makes it possible, for example, to obtain fine and complex geometries that are impossible to achieve using conventional molding techniques. According to this process, a layer of polymer powder, for example polyamide, is conventionally, Selectively and briefly irradiated in a chamber by radiation, generally electromagnetic (e.g., laser beam, infrared radiation, UV radiation), the result is that the powder particles impacted by the radiation melt. The molten particles coalesce and solidify, leading 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.

[0005] In the case of selective laser sintering (SLS), the procedure is typically carried out as follows. A thin layer of powder, for example, polyamide, is deposited onto a horizontal plate held in a chamber heated to a temperature between the crystallization temperature Te and the melting temperature Tf of the powder.The laser agglomerates powder particles at various points within the powder layer according to a geometry corresponding to the object, for example, using a computer that stores the object's shape and reproduces it as slices. Next, the horizontal plate is lowered by a value corresponding to the thickness of a powder layer (for example, between 0.05 and 2 mm, and generally around 0.1 mm), and then a new layer of powder is deposited. 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 created. The entire assembly is then slowly cooled, and the object solidifies as soon as its temperature drops below the crystallization temperature (Te). The parts that have not been agglomerated remain in a powder state. Inside the chamber, the resulting object is surrounded by powder.After cooling, the object is separated from the powder, which can then be reused for another operation.

[0006] It is preferable that the polymer (particularly polyamide) used for 3D printing according to this process have the largest possible difference between its melting temperature Tf and its crystallization temperature Te in order to avoid deformation (or "curling") during manufacturing. Indeed, immediately after the laser beam is applied, the sample temperature is higher than the powder's crystallization temperature, but the addition of a new, cooler layer of powder rapidly lowers the part's temperature below Te, leading to deformation.

[0007] Furthermore, the highest possible enthalpy of fusion (AHf) is required to obtain good geometric definition of the manufactured parts. Indeed, if this enthalpy is too low, the energy supplied by the laser is sufficient to sinter the powder particles near the areas being built by thermal conduction, and thus the geometric accuracy of the part is no longer satisfactory.

[0008] For sintering processes using polyamide powder, the use of polyamide with the following properties is also preferred: - the molar mass of the polyamide particles is preferably low enough, both so that the fusion of the particles does not require too much energy and so that the inter-particle coalescence is sufficient during the passage of the radiation in order to obtain an object that is as non-porous as possible. - The molar mass of the polyamide part (3D object) is preferably sufficiently high to ensure acceptable mechanical properties. Ideally, the polyamide should continue to polymerize after melting. Acceptable mechanical properties typically refer to an elongation at break greater than 15% for objects built in both horizontal dimensions or "flat" in the sintering device.

[0009] During each build, also called a "run," a large portion of the powder is not used: typically, about 85% of the powder is not targeted by the laser. It is therefore advantageous to be able to reuse this powder during the next build (or subsequent "run"). The polyamide powder should, as far as possible, retain its initial properties: color, average molar mass, and thermal properties.

[0010] Furthermore, during sintering, the surrounding powder, i.e., the powder not exposed to radiation, generally remains well above its glass transition temperature (Tg) for several hours. This can lead to an increase in the molar mass and therefore the viscosity of the polyamide (a phenomenon known as "viscosity rise"). Consequently, coalescence between powder particles becomes increasingly difficult during successive runs. It then becomes necessary to increase the radiation power each time the powder is reused in subsequent runs.

[0011] This reuse of powders is not infinite, even when adjusting the radiation parameters. Indeed, the molar mass continues to increase to such an extent that a very high radiation power no longer allows for proper coalescence of the powder, or even leads to degradation.

[0012] More specifically, the macromolecular architecture of the recycled polyamide powders is no longer linear: it becomes branched, with polymer chains of very high molar masses preventing proper coalescence. The dispersity D and the average molar mass Mz of these recycled powders have increased significantly compared to the virgin powder. These two phenomena are visible when performing size-exclusion chromatography analyses. These explanations concerning the agglomeration of Polyamide powders under laser beam are valid regardless of the radiation or agent causing the melting.

[0013] It is therefore necessary to propose a process for recycling polyamide powders whose properties have been altered by exposure to heat during a three-dimensional printing process. Summary of the invention

[0014] The invention relates to a method for recycling a particulate semi-crystalline polyamide, the initial particulate semi-crystalline polyamide having already been used in a layer-by-layer printing process, the method comprising: - a step of bringing the initial particulate semi-crystalline polyamide into contact with water and at least one acid; - at least one heating step of the initial particulate semi-crystalline polyamide to a temperature above Te - 20°C and below Tf, where Te is the crystallization temperature of the particulate semi-crystalline polyamide and Tf is the melting temperature of the initial particulate semi-crystalline polyamide, and - a step of collecting the treated particulate semi-crystalline polyamide,

[0015] wherein the treated particulate semi-crystalline polyamide has an inherent viscosity lower than the inherent viscosity of the initial particulate semi-crystalline polyamide.

[0016] In embodiments, the treated particulate semi-crystalline polyamide has a dispersity Df lower than the dispersity Di of the initial particulate semi-crystalline polyamide.

[0017] In embodiments, the dispersity Df of the treated particulate semi-crystalline polyamide is between 1 and 4, preferably between 2 and 3.5.

[0018] In embodiments, the treated particulate semi-crystalline polyamide has an average molar mass Mzf lower than the average molar mass Mz of the initial particulate semi-crystalline polyamide.

[0019] In embodiments, the step of bringing the initial particulate semi-crystalline polyamide into contact can be done at a temperature of 15 to 100°C.

[0020] In embodiments, the process according to the invention comprises a single heating step.

[0021] In embodiments, when the process according to the invention comprises a single heating step to a temperature Tl, the duration of the heating step is between 1 hour and 40 hours, preferably between 10 and 30 hours, preferably between 20 and 25 hours.

[0022] In embodiments, the process according to the invention comprises more than one heating step, preferably two heating steps.

[0023] In embodiments, where the process according to the invention comprises two heating steps, the duration of the first heating step at a temperature Tl is between 2 and 10 hours, preferably between 2 and 8 hours, even more preferably between 3 and 6 hours, and preferably of all about 5 hours. According to the invention, the first heating step is carried out at a constant temperature Tl.

[0024] Preferably, the heating temperature during the first stage is between Tc-20°C and Tc+20°C, preferably from Te -10°C to Te +10°C.

[0025] In embodiments, where the process according to the invention comprises two heating steps, the duration of the second heating step is between 4 and 20 hours, preferably between 8 and 17 hours, more preferably between 10 and 15 hours, and even more preferably about 12 hours. In a preferred embodiment, the second heating step is carried out at a constant temperature T2.

[0026] In embodiments, the temperature T2 of the second heating step is between Tf-60°C and Tf-5°C, preferably between Tf-55°C and Tf-10°C, Tf representing the melting temperature of the particulate semi-crystalline polyamide at the end of the first heating step.

[0027] In embodiments, the difference between the temperature T2 of the second heating stage and the temperature Tl of the first heating stage is at least 1°C, preferably at least 2°C, even more preferably at least 3°C; and the difference between the temperature T2 of the second heating stage and the temperature Tl of the first heating stage is preferably from 1°C to 10°C, preferably still from 2°C to 7°C, preferably still from 3°C to 5°C. Preferably, the temperatures Te and Tf are measured on the initial particulate semi-crystalline polyamide by ISO 11357-3:2018 Plastics — Differential Scanning Calorimetry (DSC) — Part 3.

[0028] In embodiments, the process according to the invention further includes a step of cooling the mixture before the collection step.

[0029] In embodiments, the step of collecting the treated particulate semi-crystalline polyamide includes a step of separating the mixture by a solid-liquid separation means in order to obtain a liquid fraction and a solid fraction comprising the treated particulate semi-crystalline polyamide, followed by a step of drying the solid fraction comprising the treated particulate semi-crystalline polyamide.

[0030] In embodiments, the collection of the treated particulate semi-crystalline polyamide includes a drying step after the separation step in order to obtain dry particulate treated semi-crystalline polyamide.

[0031] Preferably, the solid-liquid separation step can be implemented by a mechanical filtration process chosen from flotation, sedimentation, decantation, centrifugation, membrane separation and combinations thereof.

[0032] Preferably, the solid-liquid separation means can be chosen from a centrifuge, a filter, a centrifuge, a decanter, a hydrocyclone, a sedimentation tank and combinations thereof.

[0033] In embodiments, the solid fraction obtained during the separation step has a moisture content of 50% by weight or less, preferably 40% by weight or less, preferably 30% by weight or less, more preferably 15% by weight or less.

[0034] In embodiments, the drying step makes it possible to obtain the treated and dried semi-crystalline polyamide, that is to say having a moisture content of less than 5% by weight, preferably less than 2% by weight, preferably less than 1% by weight.

[0035] In embodiments, the process according to the invention may further include an addition of at least one acid to the particulate semi-crystalline polyamide treated during collection, preferably between solid-liquid separation and drying, preferably in an amount of 500 to 3000 ppm, preferably even more in an amount of 1000 to 2000 ppm, by weight, of acid relative to the weight of polyamide.

[0036] Preferably, the treated particulate semi-crystalline polyamide has an inherent viscosity lower than the inherent viscosity of the initial semi-crystalline polyamide.

[0037] In embodiments, the inherent viscosity of the treated particulate semi-crystalline polyamide, as measured according to ISO 307:2019 in an m-cresol solvent at a temperature of 20°C, is greater than or equal to 0.7 (g / 100 g)1, preferably between 0.7 (g / 100 g)1 and 1.5 (g / 100 g)', more preferably between 0.7 (g / 100 g)1 and 1.3 (g / 100 g)', 1 and preferably of all is 1.15 ±0.1 (g / 100 g)'1.

[0038] In a particularly preferred embodiment, the initial particulate semi-crystalline polyamide is in powder form.

[0039] In some embodiments, the process according to the invention includes a preliminary sieving step before the collection step and / or before the contacting step. In some embodiments, the preliminary sieving step is carried out by filtration through a sieve having a mesh size of between 3 x D90 and 6 x D90 of the powder, preferably 3 x D90 to 5 x D90 of the powder, and more preferably a porosity of 4 x D90 of the powder.

[0040] When the preliminary sieving step is implemented before the contacting step if the powder to be recycled is highly agglomerated, i.e. if the sieving residue is at least 15%, preferably at least 20%.

[0041] In embodiments, the treated particulate semi-crystalline polyamide is in powder form having a sieve retention of less than 15% by weight, preferably less than 12% by weight, preferably even less than 10% by weight, the sieve having a mesh size of between 1.5 x D90 to 3 x D90 of the powder, preferably 1.75 x D90 to 2 x D90 of the powder, more preferably a porosity of 1.75 x D90 of the powder.

[0042] In embodiments, the treated particulate semi-crystalline polyamide is in powder form having a sieve residue at 160 pm of less than 10% by weight, preferably less than 5% by weight, preferably even less than 4% by weight.

[0043] In embodiments, the initial particulate semi-crystalline polyamide is a homopolyamide, a copolyamide or a mixture thereof, the homopolyamide resulting from the condensation of amino acid or aminocarboxylic acid monomers, preferably alpha, omega-aminocarboxylic acids, and / or lactam monomers, or from the polycondensation 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, the copolyamide resulting from the condensation of at least two different monomers.

[0044] In alternative embodiments, the initial particulate semi-crystalline polyamide is a homopolyamide, a copolyamide or mixtures thereof, the polyamide having a C / N ratio greater than or equal to 8, preferably greater than or equal to 9, advantageously greater than or equal to 10.

[0045] Preferably, the initial particulate semi-crystalline polyamide is chosen from PA 11 and PA 12.

[0046] Preferably, PA 11 comprises at least 90%, preferably at least 95%, more preferably 100% of repeating motifs 11, in molar percentage.

[0047] Preferably, the PA 12 comprises at least 90%, preferably at least 95%, more preferably 100% of repeating 12 motifs, in molar percentage.

[0048] In one embodiment, the initial particulate semi-crystalline polyamide is PA 11 having an inherent viscosity before treatment of at least 1.5 + / - 0.10 (g / 100 g)1.

[0049] In one embodiment, the initial particulate semi-crystalline polyamide to be recycled is PA 12 having an inherent viscosity before treatment of at least 1.5 + / -0.10 (g / 100 g)1

[0050] In embodiments, at least one acid is chosen from among the acids of formula HxPyOz in which x, y and z are integers chosen from the range from 1 to 7, boric acid, the salts of these acids, their esters, their anhydrides and their mixtures.

[0051] In embodiments, during the contacting step, at least one acid is present in an amount of between 0.1 and 1.5%, preferably between 0.4 and 1.2%, more preferably between 0.6 and 1.1%, by weight, relative to the weight of the initial particulate semi-crystalline polyamide.

[0052] In embodiments, the amount of acid present in the treated particulate semi-crystalline polyamide is between 3000 and 10000 ppm, preferably from 4000 to 8000 ppm, by weight, relative to the weight of the initial particulate semi-crystalline polyamide.

[0053] In embodiments, the process according to the invention includes the addition of antioxidants after the collection step.

[0054] In embodiments, the method according to the invention comprises: - a step of collecting the initial particulate semi-crystalline polyamide, in an object manufacturing apparatus by powder agglomeration by fusion using electromagnetic radiation, - a step of contacting the initial particulate semi-crystalline polyamide with water and at least one acid, - at least one heating step of the initial particulate semi-crystalline polyamide to a temperature above Te - 20°C and below Tf, where Te is the crystallization temperature of the initial particulate semi-crystalline polyamide and Tf is the melting temperature of the initial particulate semi-crystalline polyamide, - a step of collecting the treated particulate semi-crystalline polyamide. Detailed description

[0055] The invention is now described in more detail and in a non-limiting manner in the following description.

[0056] Unless otherwise indicated, all percentages relating to quantities are mass percentages.

[0057] In this text, the quantities indicated for a given species may apply to that species according to all its definitions (as mentioned in this text), including more restricted definitions.

[0058] The invention relates to a method for recycling a particulate semi-crystalline polyamide, the initial particulate semi-crystalline polyamide having already been used in a layer-by-layer printing process, the method comprising: - a step of contacting an initial particulate semi-crystalline polyamide with water and at least one acid, - at least one heating step of the initial particulate semi-crystalline polyamide to a temperature above Te - 20°C and below Tf, where Te is the crystallization temperature of the particulate semi-crystalline polyamide and Tf is the melting temperature of the initial particulate semi-crystalline polyamide, - a step of collecting the treated particulate semicrystalline polyamide,

[0059] wherein the treated particulate semicrystalline polyamide has an inherent viscosity lower than the inherent viscosity of the initial particulate semicrystalline polyamide.

[0060] By "initial" particulate semicrystalline polyamide, we mean the particulate semicrystalline polyamide before it is subjected to the treatment process according to the invention.

[0061] By "polyamide" for the purposes of the invention, we mean the polycondensation products of lactams, amino acids, or diamine / diacid pairs. It may be a homopolymer, that is, a polymer resulting from the polycondensation of the same repeating unit, that is, the same monomer (for example, "Z" polyamides).It can also be a copolymer resulting from the polycondensation 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 (a monomer different from the first monomer) (i.e., "X / Y" polyamides). Finally, it can be a mixture of homopolymer and copolymer.

[0062] These monomers can be linear or branched or substituted as appropriate.

[0063] “Z” type polyamides are derived from the condensation of amino acid or aminocarboxylic acid monomers, preferably alpha, omega-aminocarboxylic acids, and / or lactam monomers.

[0064] Examples of amino acids or aminocarboxylic acids include aminocaproic acid, amino-7-heptanoic acid, amino-11-undecanoic acid, N-heptylamino-11-undecanoic acid (HAU) and amino-12-dodecanoic acid; preferably amino-11-undecanoic acid.

[0065] By way of example of lactams, one can cite those having from 3 to 12 carbon atoms on the main ring and which can be substituted. One can cite for example [3,[3-dimethylpropriolactam, α,α-dimethylpropriolactam, amylolactam, capryllactam, oenantholactam, butyrolactam, caprolactam, oenantholactam and lauryllactam; preferably lauryllactam.

[0066] As an example of this type of preferred polyamide, we can cite PA 11 and PA 12.

[0067] Polyamides of the "XY" type are obtained from the polycondensation 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 a diamine aliphatic, cycloaliphatic or aromatic, in particular containing from 2 to 20 carbon atoms, preferably from 6 to 14 carbon atoms.

[0068] Examples of dicarboxylic acids include acids with between 4 and 18 carbon atoms, preferably with 9 to 12 carbon atoms, such as adipic acid, sebacic acid, azelaic acid, suberic acid, isophthalic acid, butanedioic acid, dodecanedicarboxylic acid, octadecanedicarboxylic acid, 1,4-cyclohexyldicarboxylic acid, terephthalic acid, brassylic acid, and also dimerized fatty acids.

[0069] Examples of diamines include aliphatic diamines having 6 to 12 atoms, and diamines can also be arylic and / or saturated cyclic. Examples include hexamethylenediamine, piperazine (Pip), tetramethylenediamine, roctamethylenediamine, 1,9-nonanediamine, 1,10-decamethylenediamine, dodecamethylenediamine, 1,5-diaminohexane, 2,2,4-trimethyl-1,6-diaminohexane, diamine polyols, isophoronediamine (IPD), methyl pentamethylenediamine (MPDM), bis(3-methyl-4-aminocyclohexyl)methane (BMACM), methaxylyenediamine, trimethylhexamethylenediamine, trimethylhexamethylenediamine, isomers of bis(4-aminocyclohexyl)methane (BACM), 2,2-bis(3-methyl-4-aminocyclohexyl)propane (BMACP), paraaminodicyclohexylmethane (PACM), 2,6-bis(aminomethyl)norbornane (BAMN).

[0070] As an example of this type of polyamide, we can cite PA 612 resulting from the condensation of hexamethylene diamine and 1,12-dodecanedioic acid; PA 6.13 resulting from the condensation of hexamethylene diamine and brassylic acid; PA 9.12 resulting from the condensation of 1,9-nonanediamine and 1,12-dodecanedioic acid; PA 10.10 resulting from the condensation of 1,10-decanediamine and sebacic acid; PA 10.12 resulting from the condensation of 1,10-decanediamine and 1,12-dodecanedioic acid.

[0071] Polyamide can also be a copolymer resulting from the polycondensation of several monomers such as those mentioned above. In particular, polyamide can also be a copolyamide resulting from the condensation of:

[0072] - of at least two different monomers, for example of at least two α,βα acids different amino carboxylic acids or

[0073] - of two different lactams or

[0074] - of a lactam and an α,co-aminocarboxylic acid of different carbon numbers Or

[0075] -of at least one α,co-aminocarboxylic acid (or a lactam), at least one diamine and at least one dicarboxylic acid, or

[0076] - of an aliphatic diamine with an aliphatic dicarboxylic acid and at least one another monomer chosen from among the aliphatic diamines different from the previous one and the aliphatic diacids different from the previous one.

[0077] One would not depart from the invention by using a mixture of polyamides, which may be mixtures of aliphatic polyamides, semi-aromatic polyamides, and / or cycloaliphatic polyamides.

[0078] For example, powder compositions comprise at least one polyamide selected from the following PEBA polyamides, copolyamides and / or copolymers comprising at least one of the following XY or Z monomers: 5.12, 513, 514, 516, 518, 536, 610, 612, 613, 614, 616, 618, 636, 6T, 9, 109, 1010, 1012, 1013, 1014, 1016, 1018, 1036, 10T, 11, 12, 124, 129, 1210, 1212, 1213, 1214, 1216, 1218, 1236, 12T, MXD10, MXD12, MXD14, and mixtures thereof; in particular selected from PA 11, PA 12, PA 6.12, PA 6.13, PA 9.12, PA 10.10, PA10.12, PA 6 / 12, PA11 / HAU, PA 11 / 10.10, and mixtures thereof.

[0079] Preferably, the polyamide has a C / N ratio greater than or equal to 8, preferably greater than or equal to 9, advantageously greater than or equal to 10. For the purposes of this invention, "C / N ratio" means the average number of carbon atoms per nitrogen atom per repeating unit. In the case of a PA Z homopolyamide, where Z designates a motif obtained from an amino acid or a lactam, the number of carbon atoms per nitrogen atom is the number of carbon atoms in the motif. For example, PA 11 obtained by polycondensation of amino-11-undecanoic acid has a C / N ratio of 11. In the case of a homopolyamide of the PA XY type, with X denoting a motif obtained from a diamine and Y denoting a motif obtained from a diacid, the number of carbon atoms per nitrogen atom is the average of the number of carbon atoms present in the XY unit.For example, PA 1012, obtained by polycondensation of decanediamine, a diamine with the C10 group, and dodecanedioic acid, a diprotic acid with the C12 group, has a C / N ratio of 11, calculated as follows: (10+12) / 2 = 11. For copolyamides, for example with the structure XaYa / XbYb, the number of carbon atoms per nitrogen atom is calculated according to the same principle. The calculation is performed in the molar proportion of the different amide units, that is, the XaYa and XbYb units. Thus, the 11 / 1010 copolyamide, containing 90% molar of 11 and 10% molar of 1010, has a C / N ratio of 10.9: 90% x11 + 10% x [(10+10) / 2] = 10.9.

[0080] Advantageously, the initial particulate semi-crystalline polyamide is selected from polyamide 11, polyamide 12, polyamide 10.10, polyamide 10.12, or polyamide 6.10, polyamide 12.12.

[0081] Preferably, the initial particulate semi-crystalline polyamide is chosen from PA 11 and PA 12.

[0082] When the initial polyamide to be recycled is PA 11, its inherent viscosity is greater than 1.5 + / - 0.10 (g / 100 g). When the initial polyamide is PA 12, its inherent viscosity is greater than 15 + / - 0.10 (g / 100 g).

[0083] The choice of process temperatures T1 and T2 depends on the lowest crystallization temperature of the polyamides in the mixture and the highest melting temperature of the mixture.

[0084] The particulate semi-crystalline polyamide can be in powder or granule form. The granules thus treated can then be ground to make powders. Preferably, the particulate semi-crystalline polyamide is in powder form.

[0085] By "polyamide powder" is meant a powder comprising at least particles comprising at least one polyamide; for the purposes of the invention, the "polyamide powder" may thus comprise components other than polyamide (in particles comprising polyamide or in particles lacking polyamide). The polyamide powder preferably has a median volume diameter D50 which may be between 10 and 150 pm, preferably between 20 and 100 pm, more preferably between 30 and 60 pm. In embodiments the median volume diameter of the powder is 10 to 20 pm, or 20 to 30 pm, or 30 to 35 pm, or 35 to 40 pm, or 40 to 45 pm, or 45 to 50 pm, or 50 to 55 pm, or 55 to 60 pm, or 60 to 70 pm, or 70 to 80 pm, or 80 to 90 pm, or 90 to 100 pm, or 100 to 110 pm, or 110 to 120 pm, or 120 to 130 pm, or 130 to 140 pm, or 140 to 150 pm. Such median diameter ranges are particularly well suited for use in a 3D article construction process.The median volume diameter (D50) of particles refers to the particle size distribution in a sample. D50 indicates that 50% of the particles in the sample have a diameter less than or equal to the D50 value. It can be determined according to ISO 13319:2007, for example using a Beckman Coulter Multisizer 3 Counter particle size analyzer.

[0086] By "polyamide granules" means particles comprising at least one polyamide of which at least one dimension is between 500 pm and 1 cm.

[0087] The initial particulate semi-crystalline polyamide is preferably a semi-crystalline polyamide for recycling, i.e., one that has already been subjected to a 3D printing process at least once. A particulate semi-crystalline polyamide for recycling may have an inherent viscosity greater than 1.5, preferably 2, a yellowing index (YI) greater than 10, and / or include antioxidant residues and end chains with oxidized groups.

[0088] The process according to the invention makes it possible to produce a particulate semi-crystalline polyamide exhibiting good reusability from one construction to the next. It This process also allows for the recycling of particulate semi-crystalline polyamide whose physical properties have been altered, such as inherent viscosity, oxidation of chain ends, and antioxidant degradation, restoring its structural properties to those of virgin particulate semi-crystalline polyamide. To clarify, reuse applies to powder that can be subjected to a new 3D printing cycle. Conversely, recycling applies to powder whose physicochemical characteristics render it unsuitable for a new 3D printing cycle.

[0089] Yellowing is quantified by the yellowness index (YI) measured according to ASTM E313-96 (D65), in particular using a Konica Minolta spectrocolorimeter illuminating D65 under 10° in included specular reflection (SCI) mode.

[0090] The process according to the invention makes it possible to reduce at least one of the parameters selected from among the dispersity, the average molar mass, and the inherent viscosity of the initial particulate semicrystalline polyamide and their combination. The treated particulate semicrystalline polyamide, preferably in powder form, can exhibit a dispersity Df lower than the dispersity Di of the initial particulate semicrystalline polyamide. The dispersity index Df of the treated semicrystalline polyamide powder can be between 1 and 4, preferably between 2 and 3.5.

[0091] The dispersity D is calculated according to the formula D = Mw / Mn, where Mw represents the mass-average molar mass and Mn represents the number-average molar mass. More precisely, the mass-average molar mass Mw is the average of the molar masses weighted by the mass of chains of each length. The molar mass Mn is the number-weighted average of the molar masses of each length. This corresponds to the molar mass of a chain of average length.

[0092] Preferably, the treated particulate semi-crystalline polyamide has an average molar mass Mzf lower than the average molar mass Mz of the initial particulate semi-crystalline polyamide.

[0093] The different average molar masses Mn, Mw and Mz can be determined by size exclusion chromatography according to the operating conditions described in the article "Molar mass analysis of polyamides 11 and 12 by size exclusion chromatography in HFiP" (Laun et al. Polymer 49 (2008) 4502-4509), according to the formula:

[0094] _ Dyx2

[0095] Ni is the number of macromolecules of molar mass Mi. Thus, when n = 0, Mx corresponds to Mn; when n = 1, Mx corresponds to Mw; and when n = 2, Mx corresponds to Mz.

[0096] The process according to the invention may also include a step of collecting an initial semi-crystalline polyamide powder, in an apparatus for manufacturing an object by agglomerating powder by fusion using electromagnetic radiation, before the step of contacting the initial semi-crystalline polyamide powder with water and at least one acid.

[0097] During the contacting step, the water may be in liquid or gaseous form. According to the invention, the water is in free form, that is, not bound to a matrix or gel. The contacting step can be carried out at a temperature between 15°C and 100°C, the increase in temperature accelerating the dispersion of the particulate semi-crystalline polyamide in the water.

[0098] The contacting step can also be carried out at a temperature above 100°C to introduce water in gaseous form.

[0099] During the contacting step, at least one acid may be present partly or entirely in the water or in the particulate semicrystalline polyamide to be treated, preferably partly in the water and partly in the particulate semicrystalline polyamide. During the contacting step, the acid may be present in an amount of between 0.1 and 1.5%, preferably between 0.4 and 1.2%, and more preferably between 0.6 and 1.1%, by weight, relative to the weight of the initial particulate semicrystalline polyamide. The at least one acid is selected from among the acids of the formula HxPyOz, where x, y, and z are integers selected from the range of 1 to 7, boric acid, salts of these acids, their esters, their anhydrides, and mixtures thereof. In the formula for acid HxPyOz, H is the hydrogen atom, P is the phosphorus atom, O is the oxygen atom, x, y and z are integers chosen from the range of 1 to 7.The acid usable in the process of the invention is preferably selected from hypophosphorous acid (H3PO2), phosphorous acid (H3PO3), orthophosphoric acid (H3PO4), pyrophosphoric acid (also called diphosphoric acid) (H4P2O7), metal phosphates, metal phosphites, metal hypophosphites, phosphoric and phosphorous esters and anhydrides, and mixtures thereof. These acids are also commonly used as catalysts for the polycondensation reaction of certain polyamides. Where appropriate, all or part of the acid present at the time of contact may originate from the synthesis of the polyamide, and / or all or part of this acid may be added at the time of contact. Preferably, said at least one acid comprises a mixture of hypophosphorous acid (H3PO2). 3PO2 and phosphoric acid H3PO4, in a weight ratio in the range of 10 / 90 to 90 / 10.

[0100] After this contact, the particulate semi-crystalline polyamide is subjected to at least one heating step. However, the process according to the invention may also comprise two or more heating steps.

[0101] Preferably, the process according to the invention comprises two heating steps (and only two heating steps) as defined in this application.

[0102] When the process according to the invention includes a single heating step to a temperature Tl, the duration of the heating step is between 1 hour and 40 hours, preferably between 10 and 30 hours, preferably between 20 and 25 hours.

[0103] When the process according to the invention comprises two heating steps, the first heating step may last between 2 and 10 hours, preferably between 2 and 8 hours, even more preferably between 3 and 6 hours, and preferably of all approximately 5 hours. According to the invention, the duration of the first heating step does not include the warm-up time of the heating equipment. In other words, the first heating step is carried out at a constant temperature Tl. For the purposes of the invention, a temperature is considered constant if it remains within a range of ±0.5°C relative to a target temperature (or an average temperature). In other words, the first heating step corresponds to a temperature plateau; that is, the initial semi-crystalline polyamide is subjected to a heating step at the constant temperature Tl.The temperature Tl of the first heating stage can be between Tc-20°C and Te + 20°C, preferably between Tc-10°C and Te + 10°C.

[0104] After this first step, the particulate semi-crystalline polyamide is subjected to a second heating step. The duration of the second heating step can be between 4 and 20 hours, preferably between 8 and 17 hours, more preferably between 10 and 15 hours, and even more preferably about 12 hours.

[0105] The second heating stage can be carried out either according to a temperature plateau, i.e. at a constant temperature T2, or according to a temperature ramp, i.e. the temperature is progressively increased from T1 to T2 during the entire duration of the heating.

[0106] Preferably, the second heating step is carried out at a constant temperature T2, i.e. according to a plateau during which the particulate semi-crystalline polyamide is heated at the constant temperature T2 for the entire duration of the heating.

[0107] The temperature T2 of the second heating step is between Tf-60°C and Tf-5°C, preferably between Tf-55°C and Tf-10°C, Tf representing the melting temperature of the polyamide at the end of the first heating step.

[0108] The difference between the temperature T2 of the second heating stage and the temperature Tl of the first heating stage can be at least 1°C, preferably at least 2°C, even more preferably at least 3°C; and the difference between the temperature T2 of the second heating stage and the temperature Tl of the first heating stage is preferably 1 to 10°C, preferably still 2 to 7°C, preferably still 3 to 5°C.

[0109] The crystallization and melting temperatures can be measured according to ISO 11357-3:2018 Plastics — Differential scanning calorimetry (DSC) — Part 3. More specifically, the temperature Tf of the particulate semi-crystalline polyamide to be treated is measured during the first DSC heating. This first DSC heating is not the first heating step of the process according to the invention.

[0110] The treatment process according to the invention makes it possible to increase the melting temperature of the particular semi-crystalline polyamide and / or its enthalpy of fusion AHf. According to the invention, the melting temperature of the treated polyamide is higher than the melting temperature of the initial polyamide and / or the enthalpy of fusion T of the treated polyamide is greater than the enthalpy of fusion T of the initial polyamide.

[0111] The process according to the invention may further include a step of cooling the mixture before the collection step. The cooling may be carried out between 50°C and 100°C, until reaching a temperature between 10°C and 50°C, for example until reaching ambient temperature (20 to 25°C).

[0112] The collection of the treated polyamide may include a step of separating the mixture. The mixture may be separated by a solid-liquid separation means to obtain a liquid fraction and a solid fraction comprising the treated particulate semi-crystalline polyamide, followed by a step of drying the solid fraction comprising the treated particulate semi-crystalline polyamide.

[0113] The solid-liquid separation step can be implemented by mechanical filtration employing wettability such as flotation, density such as sedimentation, decantation and centrifugation), as well as particle size such as membrane separation.

[0114] The solid-liquid separation means can be selected from a centrifuge, a filter, a centrifuge, a decanter, a hydrocyclone, a sedimentation tank, and combinations thereof. Preferably, the solid-liquid separation means can be a filter or a centrifuge.

[0115] The solid-liquid separation step makes it possible to obtain a solid fraction having a moisture content of 50% by weight or less, preferably 25% by weight or less, preferably 20% by weight or less, and more preferably 15% by weight or less. For example, this moisture content can be from 5 to 15% by weight, from 15 to 20% by weight, 20 to 25% by weight, 25 to 30% by weight, 30 to 40% by weight, 40 to 50% by weight.

[0116] The process according to the invention may further include adding at least one acid to the solid fraction containing the particulate semi-crystalline polyamide during collection, preferably between solid-liquid separation and drying, preferably in an amount of 500 to 3000 ppm, and even more preferably in an amount of 1000 to 2000 ppm, by weight, of acid relative to the weight of polyamide. The at least one acid is preferably the same as that present during the contacting step. Adding acid during drying reduces the drying time and the overall process duration, improves process control, and increases energy efficiency.

[0117] The amount of acid present in the treated particulate polyamide is between 3000 and 10000 ppm, preferably 4000 to 8000 ppm, by weight, relative to the weight of the initial polyamide.

[0118] The drying step yields the treated and dried semi-crystalline polyamide. In particular, the drying step removes solvent residues, for example, in an oven or agitated dryer. The dried particulate semi-crystalline polyamide has a moisture content of less than 5% by weight, preferably less than 2% by weight, and preferably less than 1% by weight (for example, from 1 to 5%, or from 0.5 to 2%). Preferably, the drying step is a vacuum drying step.

[0119] In embodiments, the process according to the invention includes the addition of antioxidants after the collection step.

[0120] In some embodiments, the process according to the invention includes a preliminary sieving step before the filtration step or before the contacting step, preferably before the contacting step. In some embodiments, the preliminary sieving step is carried out by filtration through a sieve having a mesh size of between 3 x D90 and 6 x D90 of the powder, preferably 3 x D90 to 5 x D90 of the powder, and more preferably a porosity of 4 x D90 of the powder.

[0121] The treated semi-crystalline polyamide can be in powder form and have a sieve retention of less than 15% by weight, preferably less than 12% by weight, preferably even less than 10% by weight, the sieve having a mesh size of between 1.5 x D90 to 3 x D90 of the powder, preferably 1.75 x D90 to 2 x D90 of the powder, more preferably a porosity of 1.75 x D90 of the powder.

[0122] D90 refers to the particle size distribution in a sample. The D90 value indicates that 90% of the particles in the sample have a diameter less than or equal to the D90 value.

[0123] For example, this sieve rejection can be from 0.1 to 10% by weight, or from 0.1 to 5% by weight, or from 0.1 to 4% by weight.

[0124] When the polyamide is PAU, the processed polyamide may be in powder form and have a sieve residue at 160 µm of less than 15% by weight, preferably less than 12% by weight, and even more preferably less than 10% by weight. For example, this sieve residue may be from 0.1 to 15% by weight, or from 0.1 to 12% by weight, or from 0.1 to 10% by weight.

[0125] The treated particulate semicrystalline polyamide has an inherent viscosity that is strictly different from the inherent viscosity of the initial semicrystalline polyamide; that is, the treatment allows the equilibrium molar mass of the particulate semicrystalline polyamide to be achieved in the presence of water. The treated particulate semicrystalline polyamide has an inherent viscosity greater than or equal to 0.7 (g / 100 g)-l, preferably between 0.7 and 1.5, more preferably between 0.7 and 1.3, and even more preferably 1.15 ± 0.1 (g / 100 g)-l. These values ​​are particularly relevant for PA11 and PA12. Such inherent viscosity values ​​allow articles produced by 3D printing from the treated semicrystalline polyamide to have good mechanical properties.However, when the initial particulate semi-crystalline polyamide is a semi-crystalline polyamide to be recycled, the treated particulate semi-crystalline polyamide has a lower inherent viscosity than the inherent viscosity of the initial semi-crystalline polyamide. Examples

[0126] The following examples illustrate the invention without limiting it.

[0127] The crystallization and melting temperatures and the enthalpy of fusion of the powders were measured according to ISO 11357-3:2018 (DSC) on a DSC TA Q2000 apparatus by carrying out a first heating at 20°C / min from -20°C to 240°C (for PA12 powder) and from -20 to 250°C (for PA11 powder), during which the temperature and enthalpy of fusion of the polyamide powders are determined, followed by a cooling at -20°C / min from 240°C to -20°C (for PA12 powder) and from 250°C to -20°C (for PA11 powder), during which the crystallization temperature is determined. The median volume 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, except when using m-cresol as the solvent and a temperature of 20°C.The viscosity inherent to 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 expressed in (g / 100 g)1. The average molar masses Mn, Mw and Mz were determined by size exclusion chromatography under the operating conditions described in the article “Molar mass analysis of polyamides 11 and 12 by size exclusion chromatography in HFiP” (Laun et al. Polymer 49 (2008) 4502-4509) ), according to the formula: .

[0128] L^+ï

[0129] Ni is the number of macromolecules of molar mass Mi. Thus, when n = 0, Mx corresponds to Mn; when n = 1, Mx corresponds to Mw; and when n = 2, Mx corresponds to Mz.

[0130] The physico-chemical characteristics of virgin polyamide powders are shown in Table 1; those of polyamide powders to be recycled at the end of their life in Table 2; and those of polyamide powders after treatment in Table 3. Example 1 - Recycling of polyamide 11 powder

[0131] Fifty successive 3D prints are performed on a P396 machine (marketed by EOS GmbH) using PA1101 powder (PA11 powder marketed by EOS GmbH), following the manufacturer's recommendations regarding both machine settings and powder reuse rate. After completing the 50 print runs, the geometry and mechanical performance of the 3D objects are no longer compliant. This powder is therefore at the end of its life and exhibits the physicochemical properties described in the table below.

[0132] 210 g of recycled PA11 polyamide powder having an inherent viscosity of 1.64 (g / 100g) ', having a filtration residue of 10.6% on a 160 µm sieve, containing 5600 ppm of orthophosphoric acid, and 390 g of deionized water are introduced into an autoclave reactor (usable volume IL). The powder is dispersed in water heated to 60°C using a stirrer (inclined-blade propeller). This dispersion is then heated to 160°C and maintained at an isothermal temperature (+ / - 0.5°C) for 20 hours; this treatment temperature corresponds to Te + 1°C. After 20 hours of isothermal heating, the dispersion is cooled to room temperature, drained, and then filtered using a sintered assembly (porosity 3) and a vacuum flask, and finally dried at 70°C under vacuum. The contact time between the PA11 powder and the water, including the heating and cooling stages, is approximately 26 hours (excluding filtration and drying). The PA11 powder is placed on a 160 µm square mesh vibrating sieve to determine the amount of residue.The inherent viscosity is only 1.00 (g / lOOg). The recycled PAU powder has a macromolecular architecture compatible with use in 3D printing. Example 2 - Recycling of polyamide 12 powder

[0133] Fifty successive 3D prints are carried out on a P396 machine (marketed by EOS GmbH) using PA2200 powder (PA 12 powder marketed by EOS GmbH) following the manufacturer's recommendations regarding both machine settings and powder reuse rate. After completing the 50 print runs, the geometry and mechanical performance of the The 3D objects are no longer compliant. This powder is therefore at the end of its life and exhibits the physicochemical properties described in the table below.

[0134] 210 g of polyamide PA12 in the form of a recyclable powder having a viscosity An inherent concentration of 1.63 g / 100 g, with a filtration residue of 9.2% on a 160 µm sieve, containing 5143 ppm of hypophosphorous acid, along with 390 g of deionized water and 1 g of an aqueous solution of hypophosphorous acid (50% concentration), are introduced into an autoclave reactor (usable volume IL). The powder is dispersed in water heated to 60°C using a propeller-type stirrer. This dispersion is then heated to 150°C and maintained at an isothermal temperature (+ / -0.5°C) for 20 hours; this treatment temperature corresponds to Te + 5°C. After 20 hours of isothermal heating, the dispersion is cooled to room temperature before being drained and filtered using a sintered filter (porosity 3) and a vacuum flask. The contact time between the PA12 powder and the water is approximately 15 hours (excluding filtration and drying). The PA12 powder is placed on a vibrating sieve with a 160 µm square mesh to determine the amount of residue.The measured melting temperature is only 189°C, hence a difference between the melting temperature and the crystallization temperature of only 47°C, the inherent viscosity is only 1.24 (g / lOOg)'. .

[0135] [Tables 1] Characteristics of virgin polyamide powders Examples Tf DHf Te Inherent viscosity D Mz Refusal Ex. 1 201°C 115 J / g 159°C 1.15 (g / 100 g)-l 2.9 100,000 g / mol < 1% Ex. 2 185°C 117 J / g 140°C 1.00 2.3 79,000 g / mol < 1%

[0136] [Tables2] Characteristics of polyamide powders to be recycled at the end of their life Examples Tf DHf Te Inherent viscosity D Mz Refuse Ex. 1 205°C 110 J / g 157°C 1.64 (g / 100 g)-l 4.3 250,000 g / mol 10.6% Ex. 2 189°C 103 J / g 138°C 1.63 3.3 221,000 g / mol 9.2%

[0137] [Tables3] Characteristics of treated polyamide powders Example Tf (°C) AHf (J / g) Te (°C) D Inherent viscosity (g / 100g)■') Mz Refusal t amas 160pm Ex. 1 203°C 119 J / g 159°C 3 1.00 97,000 g / ml 6.2% Ex. 2 188°C 112 J / g 142°C 3.2 1.24 150,000 g / mol 7.6%

[0138] The recycling process according to the invention makes it possible to reduce the inherent viscosity of the polyamide powder, its dispersity, the average molar mass Mz as well as to reduce the amount of agglomerated powders. Conclusion

[0139] The process according to the invention makes it possible to restore to the powders to be recycled the physico-chemical characteristics compatible with the requirements of 3D printing technologies

Claims

Demands

1. A process for recycling a particulate semi-crystalline polyamide, the initial particulate semi-crystalline polyamide having already been used in a layer-by-layer printing process, the process comprising: - a step of contacting the initial particulate semi-crystalline polyamide with water and at least one acid; - at least one step of heating the initial particulate semi-crystalline polyamide to a temperature above Te - 20°C and below Tf, where Te is the crystallization temperature of the initial particulate semi-crystalline polyamide and Tf is the melting temperature of the initial particulate semi-crystalline polyamide; and - a step of collecting the treated particulate semi-crystalline polyamide, wherein the treated particulate semi-crystalline polyamide has an inherent viscosity lower than the inherent viscosity of the initial particulate semi-crystalline polyamide.

2. A treatment method according to claim 1 wherein the treated particulate semi-crystalline polyamide has a dispersity index Df lower than the dispersity index Di of the initial particulate semi-crystalline polyamide.

3. A treatment method according to any one of the preceding claims wherein the dispersity index Df of the treated particulate semi-crystalline polyamide is between 1 and 4, preferably between 2 and 3.

5.

4. A treatment method according to any one of the preceding claims wherein the treated particulate semi-crystalline polyamide has an average molar mass Mzfin lower than the average molar mass Mz; of the initial particulate semi-crystalline polyamide.

5. A method according to any one of the preceding claims, wherein the collection of the treated particulate semicrystalline polyamide comprises a step of separating the mixture by a solid-liquid separation means in order to obtain a liquid fraction and a solid fraction comprising the treated particulate semicrystalline polyamide, followed of a drying step of the solid fraction comprising the treated particulate semi-crystalline polyamide.

6. A process according to the preceding claim, comprising adding acid to particulate semi-crystalline polyamide during collection, preferably between separation and drying, preferably in an amount of 500 to 3000 ppm, preferably again in an amount of 1000 to 2000 ppm, by weight, of acid relative to the weight of polyamide.

7. A process according to any one of the preceding claims, wherein the inherent viscosity of the treated particulate semi-crystalline polyamide, as measured according to ISO 307:2019 in an m-cresol solvent at a temperature of 20°C, is greater than or equal to 0.7 (g / 100 g)-l, preferably is between 0.7 and 1.5, more preferably between 0.7 and 1.3, even more preferably is 1.15 + 0.1 (g / 100 g)-1.

8. A method according to any one of the preceding claims, wherein the initial particulate semi-crystalline polyamide is in powder form.

9. A process according to any one of the preceding claims, wherein the particulate semi-crystalline polyamide being treated is in powder form having a sieve retention of less than 15% by weight, preferably less than 12% by weight, preferably even less than 10% by weight, the sieve having a mesh size of between 1.5 x D90 and 3 x D90 of the powder, preferably 1.75 x D90 and 2 x D90 of the powder, more preferably a porosity of 1.75 x D90 of the powder.

10. A process according to any one of the preceding claims, wherein the initial particulate semi-crystalline polyamide is a homopolyamide, a copolyamide or a mixture thereof, the polyamide having a C / N ratio greater than or equal to 8, preferably greater than or equal to 9, advantageously greater than or equal to 10.

11. A method according to any one of the preceding claims, wherein the acid is selected from the acids of formula HxPyOz in which x, y and z are integers selected from the range of 1 to 7, boric acid, salts of these acids, their esters, their anhydrides and their mixtures.

12. A method according to any one of the preceding claims, wherein the amount of acid present in the semi-crystalline polyamide particulate matter content is between 3000 and 10000 ppm, preferably 4000 to 8000 ppm, by weight, relative to the initial particulate semi-crystalline polyamide.

13. A process according to any one of the preceding claims wherein the collection of the treated particulate semicrystalline polyamide includes a drying step after the separation step in order to obtain a dry particulate treated semicrystalline polyamide.

14. Processing method according to any one of the preceding claims comprising the addition of antioxidants after the collection step.