WO2023083809A1

WO2023083809A1 - Polymer powder for manufacturing high-definition components of low roughness

Info

Publication number: WO2023083809A1
Application number: PCT/EP2022/081129
Authority: WO
Inventors: Geoffroy CAMMAGE; Hervé Ster; Arnaud Lemaitre
Original assignee: Arkema France
Priority date: 2021-11-09
Filing date: 2022-11-08
Publication date: 2023-05-19
Also published as: FR3128959A1

Abstract

The invention is directed primarily to a thermoplastic polymer powder composition suitable for additive manufacture by selective melting, wherein the polymer powder has - a particle size characterized by: o a mean volume diameter Dv < 55 µm, and o a span of less than 1.20, and - such that a ratio d is between 0.40 and 0.55, the formula of ratio d being as follows: [Math 10] (formula (I)) in which d_p is the apparent density of the powder as measured according to standard ISO 787-11:1981; and d_m is the density of the material as measured on the powder after melting according to standard ISO 1183-1.

Description

Description Title: Polymer powder for the manufacture of high definition and low roughness parts

[Technical area]

This patent application relates to a thermoplastic polymer powder for the additive manufacturing of high definition and low roughness parts.

[prior technique]

3D printing by selective melting of thermoplastic polymer powders (SLS, MJF, HSS, etc.) makes it possible to build parts with complex geometries. Nevertheless, the definition of the parts thus printed as well as their surface roughness are not yet completely satisfactory.

It is known to improve the surface condition of parts built by selective melting by a physical and/or chemical treatment. However, these treatments substantially increase the production time and therefore the manufacturing cost of the parts.

Some processes have also been described in which the polymer powder to be used in 3D printing is processed. Thus, European patent EP1742986 B1 teaches that a polyamide powder having a melting enthalpy and a melting temperature far from the crystallization temperature makes it possible to improve the definition of parts built by selective melting.

Patent EP 1 413 595 B1 describes a process for increasing at least one of the following two parameters of a polyamide: (i) its melting temperature and (ii) its enthalpy of fusion AHf in which this polyamide is brought into contact in the solid state with water or water vapor at a temperature close to its crystallization temperature Te for a time sufficient to effect this increase, then the water (or water vapor) is separated polyamide and the polyamide is dried.

Furthermore, patents EP 2 115 043 B1 and EP 2 627 687 B1 teach that the heat treatment of PAEK powders makes it possible to improve the definition of constructed parts.

However, these methods are specific to certain polymers and do not always provide complete satisfaction.

[Summary of Invention]

The object of the invention is therefore to propose a polymer powder making it possible to improve the definition and the surface appearance of parts produced by selective melting.

Indeed, the present invention is based on the observation that a powder of suitable morphology and having an appropriate particle size makes it possible to improve the definition of the parts and reduces the surface roughness of items produced by selective fusion 3D printing methods.

More specifically, it has been observed that a powder with a narrow particle size distribution with few fine particles makes it possible to optimize the fusion of the targeted grains, in that it reduces the risk of fusion of neighboring grains, and therefore improves the definition built parts. Moreover, the low concentration of large particles limits the texturing of the surface and therefore the loss of definition and the roughness of the surface which could result therefrom.

Finally, it has been observed that the use of a powder whose apparent density is such that the ratio with that of the material is between 0.40 and 0.55 makes it possible to optimize the definition of the parts. The apparent density of the powder varies in particular according to the morphology of the powder but also according to its crystallinity. Such a powder also makes it possible to avoid construction defects on parts, in particular round or bent, while maintaining good productivity.

Also, according to a first aspect, the subject of the invention is a thermoplastic polymer powder composition suitable for additive manufacturing by selective melting, in which the polymer powder has a particle size characterized by: o a volume-average diameter Dv <55 pm, and o a span less than 1.2, and such that a ratio d is between 0.40 and 0.55, the ratio d being of the following formula:

[Math 1]

, dp d = ~ ~ am in which dp is the apparent density of the powder measured according to standard ISO 787-11: 1981; and d _m is the density of the material measured on the powder after fusion according to standard ISO 1183-1.

According to one embodiment, the thermoplastic polymer comprises or consists of a thermoplastic polymer chosen from the group consisting of polyesters, polyvinyl chloride, polyacetal, polyolefins such as polypropylene and polyethylene, polystyrene, polycarbonate, poly-(N-methylmethactlumide, PMMI), polymethylmethacrylate (PMMA), ionomers, polyamides, thermoplastic elastomers such as polyetherblock amides, PAEKs, and mixtures thereof, and in particular it comprises or consists of PA 11 , PA 12, semi-aromatic polyamide such as PA 11/10T, a PEBA or a PAEK such as PEKK, PEEK, PEEK-PEDEK and PEEK-PEmEK.

According to one embodiment, the composition further comprises a flow agent. Advantageously, the polymer powder has a span of less than 1.00, and preferably less than 0.90.

Preferably, the polymer powder has a ratio d of between 0.45 and 0.55, and in particular between 0.47 and 0.51.

Advantageously, the polymer powder also has an inherent viscosity of 0.65 to 1.50, preferably 0.85 to 1.40, and more preferably 1.00 to 1.30.

According to a second aspect, the invention relates to a process for manufacturing the composition of the powder described, comprising the steps of:

(i) Prepolymerization of the thermoplastic polymer monomer(s) and subsequent granulation;

(ii) Grinding into a powder;

(iii) Possible subsequent sieving of the prepolymer powder obtained; And

(iv) Subjecting the obtained prepolymer powder to solid phase polycondensation to obtain a polymer powder.

According to a third aspect, the invention relates to the use of the composition described or obtained according to the above process for the manufacture of articles by additive manufacturing by means of selective melting, in particular chosen from SLS, MJF and HSS.

Finally, according to a fourth aspect, the invention relates to an article capable of being obtained by additive manufacturing by means of selective melting of the composition described or obtained according to the process described.

[Brief description of figures]

The invention will be better understood with regard to the following description and the figures, which show:

Fig. 1 a 3D printing device by sintering of the SLS type (English acronym for "selective laser sintering", selective laser sintering);

Fig. 2 the evolution of the thickness e of the powder deposited after melting by irradiation as a function of the number of layers n of 100 μm each in a construction by laser sintering, calculated for powders having a ratio d of 0.3 (•) , 0.5 (■) and 0.7 (♦). [Description of Embodiments]

Definition of terms

The term “thermoplastic polymer” is understood to denote a polymer having the property of softening when it is sufficiently heated, and which, on cooling, becomes hard again. The polymer has a molar mass as measured by CES (steric exclusion chromatography) greater than 5000 g/mol.

The term “average diameter by volume” or “Dv” is also understood to mean the average diameter by volume of a pulverulent material, as measured according to standard ISO 13319-1:2021, for example on a Coulter Counter particle counter. -Multisizer 3 (Beckmann Coulter). There are different diameters. More specifically, the Dv50 denotes the median diameter by volume, and the Dv10 and Dv90 respectively denote the diameters below which 10 or 90% by volume of the particles are located.

The term "span" means a ratio describing the width of the particle size distribution of a powder, with the following formula:

[Math 2]

in which :

Dv10 designates the diameter below which there are 10% by volume of the particles of the polymer powder;

Dv50 designates the diameter below which 50% by volume of the particles of the polymer powder are found (by definition Dv50 is also the median diameter by volume), and

Dv90 designates the diameter below which 90% by volume of the particles of the polymer powder are found, these diameters being measured as indicated above.

Finally, the term “3D printing by sintering” means processes in which a layer of polymer powder is irradiated by electromagnetic radiation (for example laser beam, infrared radiation, UV radiation), so as to selectively melt the powder particles. affected by radiation. The molten particles coalesce and solidify to lead to the formation of a solid mass. This process can produce 3D articles by repeatedly irradiating a succession of freshly applied powder layers.

The term “surface roughness” is understood to denote the arithmetic mean deviation R _a of the surface profile of a sample according to the ISO4287:1997 standard, for example using a PERTHOMETER S8P device. In the case of 3D printing, the build orientation can impact roughness, so it can be useful to distinguish between the bottom and top of an item, where the bottom is the first build layer and the top is the last build layer.

The term “viscosity” is understood to denote the inherent viscosity as measured in a viscometer of the Ubbelohde type according to standard ISO 307:2019, except when using m-cresol as solvent and at a temperature of 20° C. The dimension of the inherent viscosity is the reciprocal 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 nomenclature used to designate polyamides follows the ISO 1874-1 standard. In particular, in the PA X notation, X represents the number of carbon atoms of the polyamide units resulting from the condensation of an amino acid or lactam. In the notation PA XY designating a polyamide resulting from the condensation of a diamine with a dicarboxylic acid or an acid derivative possessing di-functionality, X represents the number of carbon atoms of the diamine and Y represents the number of carbon atoms of the dicarboxylic acid or acid derivative. The notation PA X/Y, PA X/Y/Z, etc. refers to copolyamides in which X, Y, Z, etc. represent homopolyamide units as described above. In a broad sense, the present invention provides a composition of thermoplastic polymer powder suitable for additive manufacturing by selective melting, in which the polymer powder has a particle size characterized by an average diameter Dv <55 μm, a specific particle size distribution, characterized by a span of less than 1.2, and finally a ratio d between the density of the powder and that of the material between 0.40 and 0.55. The use of powders with a low d ratio most often compromises the properties of the manufactured articles, particularly in terms of mechanical strength. Moreover, such a powder is difficult to convert due to the powdery behavior of the particles. Finally, a bed of such powder is not able to support the weight of the item being built. The article then collapses during printing, and cannot be completed. The use of powders with a high d ratio makes it possible to achieve better mechanical properties. However, such powders include large particles, and the constructed articles then often exhibit high roughness. Also, it was found that a ratio d as defined above constituted a good compromise between the two requirements of roughness and mechanical properties.

It has also been observed that a powder with such a ratio d makes it possible to absorb more quickly the differences in thickness of the layer of powder at the start of construction, linked to the contraction of the powder at the sintered places. These variations in thickness can create stresses in the part and also affect its quality insofar as the laser power may not be sufficient to melt the polymer so as to ensure its perfect coalescence. Such defects can cause porosity which risks deteriorating the mechanical properties.

Finally, it was found that the surface roughness Ra of an article built by 3D construction, if it is directly linked to the particle size characteristics of the powder used, in particular to the average diameter Dv, also depends on the span, the thickness ep the layer of constructions, and the ratio d between the density of the powder and that of the material. For several polyamides, a correlation was found in the following form:

[

The effect of the ratio d on the roughness is particularly unexpected.

Overall, the thermoplastic polymer powder compositions according to the invention make it possible to obtain parts having low roughness, and moreover make it possible to avoid construction defects on parts, in particular round or bent parts, while maintaining good productivity.

A. Composition of thermoplastic polymer powder

The thermoplastic polymer powder composition proposed according to the invention comprises a polymer powder which has: a particle size characterized by: o a volume-average diameter Dv <55 μm, and o a span of less than 1.20, and such that a ratio d is between 0.40 and 0.55, the ratio d being of the following formula:

[Math 4]

in which dp is the apparent density of the powder measured according to standard ISO 787-11: 1981; and d _m is the density of the material measured on the powder after melting by vertical thrust in water (21° C.) according to standard ISO 1183-1.

According to the invention, the thermoplastic polymer powder has a specific particle size. Indeed, the thermoplastic polymer powder of the invention has a volume-average diameter Dv of less than 55 μm. According to certain embodiments, the thermoplastic polymer powder has a volume-average diameter Dv of between 30 and 55 μm, in particular between 35 and 50 μm and very particularly between 40 and 45 μm.

In addition to the average diameter alone, the particle size distribution of the thermoplastic polymer powder can also have a significant impact on the performance in 3D printing by sintering.

Thus, according to the invention, the thermoplastic polymer powder has a span, as defined above, of less than 1.2, and in particular less than 1, and very particularly less than 0.9. According to one embodiment, the span of the thermoplastic polymer powder is between 0.20 and 1.20, or between 0.30 and 1.10, or between 0.35 and 1.00, or between 0.40 and 0.90.

According to one embodiment, the volume diameter Dv10 of the powder is preferably greater than 15 μm. According to certain embodiments, the thermoplastic polymer powder has a volume diameter Dv10 of between 15 and 50 μm, or between 25 and 45 μm or between 30 and 40 μm.

According to one embodiment, the volume diameter Dv50 of the powder is preferably between 30 and 60 μm. According to certain embodiments, the thermoplastic polymer powder has a volume diameter Dv50 of between 35 and 55 μm, or between 40 and 50 μm.

According to one embodiment, the thermoplastic polymer powder has a volume diameter Dv90 of less than 120 μm, in particular less than 100 μm, and very particularly less than 90 μm. According to certain embodiments, the thermoplastic polymer powder has a volume diameter Dv90 of between 40 and 120 μm, or between 45 and 100 μm, or between 50 and 80 μm.

The thermoplastic polymer powder in the composition according to the invention is moreover characterized by a specific ratio d, which is between 0.40 and 0.55. The ratio d is of the following formula:

[M

As explained above, the ratio d as specified expresses the compromise proposed according to the invention between the search for a maximum density of the powder and therefore a high ratio d on the one hand, and the fact that such powders generate articles with high roughness and require a greater number of layers to absorb the difference in thickness resulting from the difference in density at the start of construction (8 to 10 layers or almost 1 mm on the article being constructed, see Fig. 2). Thus, it was found that a ratio d between 0.40 and 0.55 allowed to best reconcile the advantage of having a dense powder with a view to good mechanical properties of the printed articles and that of having a powder that is not too dense with a view to limiting constraints and defects at the start of construction.

The apparent density d _p of the thermoplastic polymer powder depends, in addition to its particle size, in particular on the shape of the particles, their porosity and their crystalline structure. The thermoplastic polymer powder preferably has a bulk density d _p of 0.200 to 0.600, preferably of 0.300 to 0.550, and extremely preferably of 0.400 to 0.500.

The density d _m of the thermoplastic polymer depends on the polymer considered and generally ranges from 0.850 to 1.850, preferably from 0.900 to 1.450, and extremely preferably from 0.950 to 1.150.

The thermoplastic polymers capable of being used in the context of the present invention can in particular be chosen from polyesters, polyvinyl chloride, polyacetal, polyolefins such as polypropylene and polyethylene, polystyrene, polycarbonate, poly( N-methylmethacrylimide, PMMI), polymethylmethacrylate (PMMA), ionomers, polyamides, thermoplastic elastomers such as polyetherblock amides (PEBA), PAEKs, and their copolymers and mixtures.

Among the polyamides, mention may in particular be made of aliphatic polyamides such as PA 6, PA 6.6, PA 11 and PA 12 and their copolymers, and semi-aromatic polyamides such as PA 11/10T for example.

Among the PAEK polymers, mention may be made in particular of polyetherketoneketone (PEKK), polyetheretherketone (PEEK) and their copolymers such as polyetheretherketone-polyetherdiphenyletherketone (PEEK-PEDEK) and polyetheretherketone-polyethermetaetherketone (PEEK-PEmEK).

According to one embodiment, the polymer comprises or consists of a polyamide, in particular PA 11, PA 12, a PEBA, or a semi-aromatic polyamide such as PA 11/10T or a PAEK such as PEKK , as well as mixtures or copolymers thereof.

The thermoplastic polymer powder generally comprises at least 50% by weight of thermoplastic polymer relative to the total weight of powder.

According to certain embodiments, the powder comprises at least 50%, or at least 75%, or at least 80%, or at least 85%, or at least 90%, or at least 92.5%, or at least 95 %, or at least 97.5%, or at least 98%, or at least 98.5%, or at least 99%, or at least 99.5% by weight of thermoplastic polymer relative to the total weight of the powder of thermoplastic polymer of the invention.

According to certain embodiments, the thermoplastic polymer powder can comprise a single thermoplastic polymer, for example only a polyester, a polyolefin, a polyamide, a polyester, a PEBA or a PAEK. According to some embodiments, the powder according to the invention comprises several different polymers by at least one of their properties. These properties can be in particular the molar mass, the crystallinity, but also the thermal properties or even the particle size.

According to some embodiments, the thermoplastic polymer powder has a viscosity of 0.65 to 1.50, preferably 0.85 to 1.40, and more preferably 1.00 to 1.30. These viscosity ranges are particularly advantageous and make it possible to obtain a good compromise to have both good coalescence properties during sintering (sufficiently low viscosity) and good mechanical properties of the sintered object (sufficiently high viscosity).

The thermoplastic polymer powder may comprise, in addition to the thermoplastic polymer(s), one or more usual additives and fillers.

The additives generally represent less than 5% by weight relative to the total composition weight. Preferably, the additives represent less than 1% by weight of the total powder weight. 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 0.4% by weight relative to the total weight of composition. In other embodiments, the powder composition does not include a flow agent.

The thermoplastic polymer powder may also include one or more fillers. The fillers generally represent less than 50% by weight, and preferably less than 40% by weight relative to the total weight of final powder. Among the fillers, let us mention reinforcing fillers, in particular mineral fillers such as carbon black, talc, nanotubes, carbon or not, and fibers, in particular glass or carbon fibers, ground or not, or even glass in another form, for example in the form of flakes or balls, hollow or not.

B. Manufacturing process of thermoplastic polymer powder

The thermoplastic polymer powder can in particular be obtained by grinding thermoplastic polymer in the form of extruded granules or scales, according to conventional techniques.

The grinding can be carried out on equipment known for this purpose, for example by means of a counter-rotating pin mill (pin mill), a hammer mill (hammer mill) or in a whirl mill.

When the powder comprises several polymers and/or certain additives and/or certain reinforcing fillers, some or all of them may be incorporated by mixing in the molten state, for example by extrusion (compounding) and granulation followed by grinding of the granules. Alternatively, it is also possible to add other polymers and/or certain additives and/or certain reinforcing fillers by dry blending. Preferably, the flow agent is added by dry mixing.

According to one embodiment, and in particular for polyamides, the process for manufacturing the composition of the powder comprises the steps of:

(ii) Grinding into a powder;

(iii) Possible subsequent sieving of the prepolymer powder obtained;

The additives and/or reinforcing fillers can then be added to the prepolymer, by melt mixing (compounding) or dry mixing, between step (i) and (ii), or subsequently, by dry mixing.

C. Use of powder

The polymer powder composition as described above is useful in particular for being implemented in a 3D printing process by sintering. Preferably, the composition of the invention is used in a selective laser sintering process (SLS, Selective Laser Sintering, in English), a sintering process of the MJF (Multi Jet Fusion) type or a sintering process of the HSS type. (High Speed Sintering).

The SLS process is widely known. In this context, particular reference may be made to documents US 6,136,948 and WO 96/06881.

In this type of process, a thin layer of powder is deposited on a horizontal plate held in an enclosure heated to a temperature called the construction temperature. Most often, heating to the building temperature is carried out by means of IR radiation lamps, for example halogen lamps, which generally have an emission maximum at a wavelength between 750 nm and 1250 nm. The build temperature refers to the temperature to which the powder bed, of a constituent layer of a three-dimensional article under construction, is heated during the layer-by-layer sintering process of the powder. Electromagnetic radiation, for example in the form of a laser, then provides the energy needed to sinter the powder particles at different points of the powder layer according to a geometry corresponding to an object, for example using a computer having in memory the shape of an object and restoring the latter in the form of slices. Then the horizontal plate is lowered by a height corresponding to the thickness of a layer of powder, and a new layer of powder is spread, heated and then sintered in the same way. The procedure is repeated until the object has been made.

The layer of powder deposited on a horizontal plate can have, before sintering, for example a thickness of 20 to 200 μm, and preferably of 50 to 150 μm. After sintering, the thickness of the layer of agglomerated material is a little lower, and may for example have a thickness of 10 to 150 μm, and preferably from 30 to 100 μm.

For the MJ F and HSS process, the entire layer of building material is exposed to radiation, but only a part covered with a melting agent is melted to become a layer of a 3D part. The melting agent is a compound capable of absorbing radiation and converting it into thermal energy, for example black ink. It is applied selectively to the selected region of the building material. The melting agent is able to penetrate the layer of the building material and transmits the absorbed energy to the neighboring building material, thereby causing it to melt or be sintered. By melting, bonding and subsequent hardening of each layer of the building material, the object is formed.

In the particular case of the MJF, a detailing agent is additionally added to the edges of the zone to be melted to allow the parts to have a better definition.

Advantageously, the use of the polymer powder composition of the invention in these processes does not require any particular modification. As mentioned above, on the other hand, it makes it possible to obtain parts with lower roughness and better definition.

The polymer powder composition according to the invention can be recycled and reused in several successive constructions. In this case, it can be reused alone or mixed with other powders, whether recycled or not.

By way of example is described below the use of the composition of thermoplastic polymer powder described in a method for constructing a three-dimensional object layer-by-layer by sintering caused by electromagnetic radiation in a device 1, such as the one schematized in Figure 1.

The electromagnetic radiation may for example be infrared radiation, ultraviolet radiation, or preferably laser radiation. In particular, in a device 1 such as that shown schematically in Figure 1, the electromagnetic radiation may comprise a combination of infrared radiation 100 and laser radiation 200.

The sintering process is a layer-to-layer manufacturing process to construct an 80 three-dimensional object.

The device 1 comprises a sintering chamber 10 in which are arranged a supply tray 40 containing the thermoplastic polymer powder and a movable horizontal plate 30. The horizontal plate 30 can also act as a support for the object three-dimensional 80 under construction. However, objects made from thermoplastic polymer powder generally do not need additional support and can generally be self-supported by unsintered powder from previous layers.

According to the method, thermoplastic polymer powder is taken from the supply tray 40 and deposited on the horizontal plate 30, forming a thin layer 50 of powder constituting the three-dimensional object 80 under construction. The layer of powder 50 is heated using infrared radiation 100 to reach a substantially uniform temperature equal to the predetermined minimum construction temperature Te.

The energy required to sinter the thermoplastic polymer powder particles at different points of the powder layer 50 is then supplied by laser radiation 200 from the laser 20 moving in the plane (xy), according to a geometry corresponding to that of the object. The molten powder re-solidifies forming a sintered part 55 while the rest of the layer 50 remains in the form of unsintered powder 56. A single passage of a single laser radiation 200 is generally sufficient to ensure the sintering of the powder. Nevertheless, in certain embodiments, it is also possible to envisage several passages at the same place and/or several electromagnetic radiations reaching the same place to ensure the sintering of the powder.

Then, the horizontal plate 30 is lowered along the axis (z) by a distance corresponding to the thickness of a layer of powder, and a new layer is deposited. The laser 20 provides the energy needed to sinter the powder particles according to a geometry corresponding to this new slice of the object and so on. The procedure is repeated until object 80 has been produced.

The temperature in the sintering chamber 10 of the layers lower than the layer being built can be lower than the construction temperature. However, this temperature generally remains above, or even well above, the glass transition temperature of the powder. It is particularly advantageous for the temperature of the bottom of the enclosure to be maintained at a temperature Tb, called "tank bottom temperature", such that Tb is less than Te by less than 40° C., preferably less than 25° C. C and more preferably less than 10°C.

Once the object 80 is completed, it is removed from the horizontal plate 30 and the unsintered powder 56 can be sieved before being returned, at least in part, to the supply bin 40 to serve as recycled powder.

D. Part likely to be manufactured

The use of a composition according to the invention thus makes it possible to manufacture three-dimensional articles of good quality, having a very satisfactory surface appearance, in particular low roughness and precise and well-defined dimensions and contours. The composition of thermoplastic polymer powder allows the manufacture by 3D printing by sintering of parts which have properties, in particular mechanical properties, at least analogous to the parts obtained if not superior compared to conventional thermoplastic polymer powders.

The invention will be explained in more detail in the following examples.

[Examples]

Example 1

A polyamide 11 powder composition was prepared according to the following method.

First of all, a polyamide 11 prepolymer was synthesized from 1.2 kg of amino-11-undecanoic acid in the presence of 0.5 kg of water, 5 g of hypophosphorous acid (titer 50%, expressed in% by weight in the aqueous solution) and 9.8 g of phosphoric acid (title 75%, expressed in% by weight in the aqueous solution). The mixture was heated to a temperature of 190°C in 2 hours with stirring as soon as the temperature reached 160°C or the pressure exceeded 8.5 bars. During the synthesis, the water initially charged with amino-11-undecanoic acid was eliminated by evaporation at constant pressure (P=10 bars). After having drawn off a quantity of water of 430 g, the prepolymer is extracted from the reactor under pressure through a die. It was then cooled using two steel rollers with circulation of cold water to be solidified, cooled and crushed into scales.

The prepolymer thus obtained was mixed in a suitable container with 3.3 g of carbon black. This mixture was introduced into a twin-screw extruder to be melted and intimately mixed and then extruded. The mixture was then cooled using two steel rollers with circulation of cold water to be solidified and cooled and then crushed into scales.

The additive prepolymer recovered in the form of flakes is then ground in a hammer mill equipped with an internal selector until a powder with a median diameter by volume Dv50 of 64 μm is obtained. The powder thus obtained is then subjected to solid phase polycondensation in a dryer at 140-155°C under vacuum in order to increase the inherent viscosity of the polyamide up to 1.15.

The pigmented polyamide 11 powder obtained was then sieved using a RUSSEL type FINEX 22 ultrasonic swirling sieve, using an 80 μm square mesh.

The powder composition obtained was then characterized in terms of particle size and density as indicated below. The results are collated in Tables 1 and 2 below. Characterization of powders

Granulometry

The particle size of the powders was characterized by measuring the particle size distribution on a Coulter Counter-Multisizer 3 device (Beckmann Coulter) in application of the ISO 13319-1:2021 standard. From this, the mean diameter and the diameter corresponding to the ^1st , ^5th and ^9th deciles of the distribution were determined, then the span was calculated according to the following formula:

[

Report d

The apparent density of the powder d _p was measured in application of the ISO 787-11:1981 standard using a 250 mL precision glass test tube graduated from 2 to 2 mL and comprising a non-graduated upper part of at least 50 mL. The powder is introduced slowly into the test tube inclined at 45°, a volume of powder comprised between 220 and 250 ml. This volume of powder is then weighed in order to calculate the apparent density and then divided by the density of the water to deduce its density.

The density of the material d _m was measured after melting the powder under a heating press (T > T _f + 40° C. and pressure of 2 tonnes). The density of the film obtained, that is to say that of the material, is measured by vertical thrust in water (21°C) according to the ISO 1183-1 standard using a Sartorius AC 210P hydrostatic balance with kit of density YDK 01.

The ratio d between the apparent density d _p of the powder and the density of the material d _m measured on the powder after melting was calculated according to the following formula:

[Math 7]

[Table 1]

Properties of thermoplastic polymer powders

[Table 2]

Properties of printed articles

The polymer powder obtained was then used to manufacture by 3D printing by sintering, more specifically by SLS, a 1 BA XY specimen (1 BA specimen according to the ISO 527-1 BA standard, called "XY" because printed in the horizontal plane of the printer) by laser sintering on a P100 machine (marketed by the company EOS) by adjusting the thickness of the powder layer to 100 μm and using the following set of parameters: [T able 3]

Specimens are visually assessed for print definition. The results are collated in Table 2 above.

The specimens are also characterized by their upper and lower surface roughness by means of the arithmetic mean deviation Ra of the surface profile of a sample according to the ISO4287:1997 standard, using a PERTHOMETER S8P device (the result corresponds to the average of three values taken on three different base lengths).

The specimens are then mechanically polished using a sander equipped with a 320 grit emery cloth, then the surface roughness measurement of these specimens is repeated under the same conditions as those described above. The results are collated in Table 2 below.

Example 2

The PA11 powder prepared in example C2 is sieved using a RUSSEL type FINEX 22 ultrasonic swirl sieve, using an 80 μm square mesh.

The particle size characteristics of the polyamide 11 powder obtained were determined as indicated in Example 1. The results are collated in Table 1 above. Furthermore, the density of the material was measured as explained in Example 1 (see Table 2 above).

The powder obtained is used to manufacture test pieces as indicated in example 1. These test pieces are then characterized by their surface roughness on the top and on the bottom. The results are collated in Table 2 above.

Example 3

First of all, a polyamide 12 prepolymer was synthesized from 1 kg of lauryllactam in the presence of 0.1 kg of water. The mixture was heated to a temperature of 260° C. in 4 hours with stirring as soon as the temperature reached 160° C. or the pressure exceeded 8.5 bars. During the synthesis, the water initially charged with the lauryllactam was eliminated by evaporation at constant pressure (P=20 bars). After having drawn off a quantity of water of 30 g, the prepolymer is extracted from the reactor under pressure through a die. It was then cooled using two steel rollers with circulating cold water to be solidified, cooled and crushed into scales.

The prepolymer recovered in the form of flakes is then ground in a hammer mill equipped with an internal selector while using a second dynamic selector at the outlet in order to eliminate the finest particles until a powder with a median diameter in Dv50 volume of 82pm. The powder thus obtained is then subjected to solid phase polycondensation in a dryer at 140-155°C under vacuum in order to increase the inherent viscosity of the polyamide up to 1.10.

The powder thus obtained is sieved using a RUSSEL ultrasonic unclogging sifter with ultrasonic declogging type FINEX 22, using a square mesh of 80 μm.

The particle size characteristics of the polyamide 12 powder obtained were determined as indicated in Example 1. The results are collated in Table 1 above. Furthermore, the density of the material was measured as explained in Example 1 (see Table 2 above).

The powder obtained is used to manufacture test specimens as indicated in example 1, using the following set of parameters:

[Table 4]

These specimens are then characterized by their surface roughness on the top and on the bottom. The results are collated in Table 2 above.

Example C1

First of all, a low viscosity polyamide 11, called in the following “prepolymer”, was synthesized from 1.2 kg of amino-11-undecanoic acid in the presence of 0.5 kg of water, 5 g of hypophosphorous acid (50% titer) and 9.8 g of phosphoric acid (75% titer). The mixture is heated to a temperature of 190° C. in 2 hours with stirring as soon as the temperature reaches 160° C. or the pressure exceeds 8.5 bars. During the synthesis, the water initially loaded with 11-amino-undecanoic acid is eliminated by evaporation at constant pressure (P=10 bars). After having drawn off a quantity of water of 430 g, the prepolymer is extracted from the reactor under pressure through a die. It is then cooled using two steel rollers with circulation of cold water to be solidified, cooled and crushed into scales.

The prepolymer recovered in the form of flakes is then ground in a hammer mill fitted with an internal selector until a powder with a median diameter by volume Dv50 of 74 μm is obtained. The powder thus obtained is then subjected to polycondensation in the solid phase in a dryer at 140-155°C under vacuum in order to increase the viscosity of the polyamide up to 1.18.

Example C2

The PA11 prepolymer recovered in the form of scales from example C1 is ground with the same grinding parameters as C1 while using a second dynamic selector at the outlet in order to eliminate the finest particles until a powder having a volume median diameter Dv50 of 91 µm. The powder thus obtained is then subjected to polycondensation in the solid phase in a dryer at 140-155°C under vacuum in order to increase the viscosity of the polyamide up to 1.16. A PA11 powder having the particle size characteristics indicated in Table 1 above is obtained.

Example C3

The PA11 powder prepared according to Example C1 is sieved using a RUSSEL type FINEX 22 ultrasonic unclogging nutating sieve, using an 80 μm square mesh. A PA11 powder having the particle size characteristics indicated in Table 1 above is obtained.

The powder obtained is used to manufacture test pieces as indicated in example 1. These test pieces are then characterized by their surface roughness on the top and on the bottom. The results are collated in Table 2 above. Example C4

Polyamide 12 granules (Rilsamid® AECNO TL marketed by Arkema) are extruded using a die to obtain fibers with a diameter of 60 μm which are cooled so that they can be micro-granulated over a length of 70 μm. The polyamide powder thus obtained has an inherent viscosity of 1.09.

The powder obtained is used to manufacture test pieces as indicated in example 3. These test pieces are then characterized by their surface roughness on the top and on the bottom. The results are collated in Table 2 above.

Example C5

A polyamide 12.12 powder was obtained according to example 1 of patent CN104356643B. First, dodecanedioic acid is dissolved in ethanol at 60°C, a solution of dodecanediamine in ethanol is gradually introduced with pH monitoring. When pH=7, the solution then contains a stoichiometric nylon 12.12 salt which precipitates. This salt is recovered after filtration and drying to be placed in an autoclave in order to polymerize at a temperature of 250°C to obtain PA12.12 with an inherent viscosity of 1.15, which is then extracted from the reactor through a die and cooled to be granulated. These granules are then dissolved in ethanol at a temperature of 140°C and a pressure of 8 bar and then precipitated directly in powder form by cooling. This powder is recovered after draining and drying. The polyamide powder thus obtained has an inherent viscosity of 1.12.

The particle size characteristics of the polyamide 12.12 powder obtained were determined as indicated in Example 1. The results are collated in Table 1 above. Furthermore, the density of the material was measured as explained in Example 1 (see Table 2 above).

Example 4

The PA 12.12 powder prepared according to Example C5 (Example 1 of patent CN104356643B) is sieved using a RUSSEL type FINEX 22 ultrasonic swirling sieve, using an 80 μm square mesh and then subjected to defining. The particle size characteristics of the polyamide 12.12 powder obtained were determined as indicated in Example 1. The results are collated in Table 1 above. Furthermore, the density of the material was measured as explained in Example 1 (see Table 2 above).

All the results show that a powder with the particle size as claimed makes it possible to very significantly improve the roughness of the parts obtained by 3D printing from it. In this context not only the average diameter counts but also the span which illustrates the width of the particle size distribution. Thus, the roughness obtained with the powder of examples 1 to 3 is notably lower than that obtained with the powder of example C3, having an average diameter Dv10, Dv50 and Dv90 close, but a higher span.

The study also reveals a favorable effect of the value of the average diameter, since a high roughness is observed for the parts obtained from the powders of examples C2 and C5, whereas these powders have a very low span value.

Furthermore, the examples show that the roughness is correlated with the parameters studied according to the following equation:

[Math 8]

the best understanding being obtained for powder compositions according to the invention.

[List of cited documents]

EP1742986 B1

EP 1 413 595 B1

EP 2 115 043 B1 EP 2 627 687 B1

US 6,136,948

WO 96/06881

CN104356643B

Claims

22

CLAIMS Composition of thermoplastic polymer powder suitable for additive manufacturing by selective melting, in which the polymer powder has a particle size characterized by: o a volume-average diameter Dv <55 μm, as measured according to standard ISO 13319-1: 2021, for example on a Coulter Counter-Multisizer 3 particle counter (Beckmann Coulter), and o a span of less than 1.20, the average diameters in volumes used to calculate the span also being measured according to the ISO 13319-1 standard :2021, for example on a Coulter Counter-Multisizer 3 particle counter (Beckmann Coulter), and

- such that a ratio d is between 0.40 and 0.55, the ratio d being of the following formula:

[Math 9] dp d = —

^Ü dm in which dp is the apparent density of the powder measured according to standard ISO 787-11: 1981; and d _m is the density of the material measured on the powder after fusion according to standard ISO 1183-1. Composition according to Claim 1, in which the thermoplastic polymer comprises or consists of a thermoplastic polymer chosen from the group consisting of polyesters, polyvinyl chloride, polyacetal, polyolefins such as polypropylene and polyethylene, polystyrene, polycarbonate, poly(N-methylmethactlumide, PMMI), polymethylmethacrylate (PMMA), ionomers, polyamides, thermoplastic elastomers such as polyetherblock amides, PAEKs, and mixtures thereof. Composition according to Claim 2, in which the thermoplastic polymer comprises or consists of PA 11 , PA 12, semi-aromatic polyamide such as PA 11/10T, a PEBA or a PAEK such as PEKK, PEEK, PEEK- PEDEK and PEEK-PEmEK.

4. Composition according to one of claims 1 to 3, further comprising a flow agent.

5. Composition according to one of claims 1 to 4, in which the polymer powder has a span of less than 1.00.

6. Composition according to claim 5, in which the polymer powder has a span of less than 0.90.

7. Composition according to one of claims 1 to 6, in which the polymer powder has a ratio d of between 0.45 and 0.55.

8. Composition according to claim 7, in which the polymer powder has a ratio d of between 0.47 and 0.51.

9. Composition according to one of claims 1 to 8, in which the polymer powder has an inherent viscosity of 0.65 to 1.50, preferentially from 0.85 to 1.40, and more preferably from 1, 00 to 1.30, as measured in an Ubbelohde type viscometer according to standard ISO 307:2019, except when using m-cresol as solvent and at a temperature of 20°C.

10. Process for manufacturing the composition of the powder according to one of claims 1 to 10, comprising the steps of:

(ii) Grinding into a powder;

(iii) Possible subsequent sieving of the prepolymer powder obtained; And

11. Use of the composition according to one of claims 1 to 9 or obtained with the method according to claim 10 for the manufacture of articles by additive manufacturing by means of selective melting.

12. Use according to claim 11, in which the additive manufacturing is chosen from selective laser sintering (Selective Laser Sintering or SLS), sintering of the Multi Jet Fusion (MJF) type and sintering of the High Speed Sintering (HSS) type. Article capable of being obtained by additive manufacturing by means of selective melting of the composition according to one of Claims 1 to 9 or capable of being manufactured by the process according to Claim 10.