WO2020164728A1

WO2020164728A1 - Particles for selective laser sintering, process of producing the particles and their use

Info

Publication number: WO2020164728A1
Application number: PCT/EP2019/053765
Authority: WO
Inventors: Yong Heng SO; Lisa TAN
Original assignee: Robert Bosch Gmbh
Priority date: 2019-02-15
Filing date: 2019-02-15
Publication date: 2020-08-20
Also published as: CN113396178A; DE112019006869T5

Abstract

A process including (a) providing a suspension including: (a1) at least one polymeric material selected from the group consisting of polypropylene, polypropylene copolymers and combinations thereof provided in form of solid particles that are suspended in the suspension; (a2) at least one filler; (a3) a first solvent capable of dissolving the at least one polymeric material; and (a4) a second solvent miscible with the first solvent. The process may further include (b) solubilizing the suspension by heating, to dissolve the polymeric material, (c) precipitating and (d) recovering the polymer composite particles from the solution. The recovered particles may be used in a selective laser sintering process for producing a sintered object, for example by i) providing a layer of the particles in a particle bed; ii) applying laser to the layer so as to fuse the particles together; and repeating steps i) and ii) thereby providing the sintered object.

Description

PARTICLES FOR SELECTIVE LASER SINTERING, PROCESS OF PRODUCING

THE PARTICLES AND THEIR USE

TECHNICAL FIELD

[0001] The present disclosure relates to a process of producing particles for selective laser sintering, to the particles produced by such process and to the use of such particles in selective laser sintering.

BACKGROUND

[0002] One of the major limitations of selective laser sintering is the small selection of available materials that can be processed. Currently, the most widely used materials are polymers based on polyamide (PA), with PA12 making up approximately 95% of the selective laser sintering market.

[0003] Therefore, there is a need to provide for new materials which can be used for selective laser sintering.

SUMMARY

[0004] It is therefore, object of the invention to provide an improved process of producing composite polymer particles for selective laser sintering.

[0005] Various embodiments may provide a process for producing polymer composite particles comprising at least one polymeric material and at least one filler for selective laser sintering. The process may include (a) providing a suspension. The suspension may include (a1 ) at least one polymeric material selected from the group consisting of polypropylene, polypropylene copolymers and combinations thereof. The at least one polymeric material may be provided in form of solid particles that are suspended in the suspension. The suspension may further include (a2) at least one filler. The suspension may further include (a3) a first solvent capable of dissolving the at least one polymeric material at a first elevated temperature. The suspension may further include (a4) a second solvent miscible with the first solvent, wherein the at least one polymeric material is insoluble in said second solvent. The process may further include (b) solubilizing the suspension by heating the suspension to a temperature equal to or higher than the first elevated temperature to dissolve the polymeric material. The process may further include (c) precipitating the polymer composite particles from the solution. Precipitating may comprise cooling. The process may further include (d) recovering the formed polymer composite particles.

[0006] Various embodiments may provide for polymer composite particles suitable for selective laser sintering. The polymer composite particles may be obtainable by the process according to various embodiments.

[0007] Various embodiments may provide for the use of the particles in a selective laser sintering process for producing a sintered object. The use may include i) providing a layer of the particles in a particle bed. The use may further include ii) applying laser to the layer so as to fuse the particles together. The use may further include repeating steps i) and ii), for example for a plurality of times, thereby providing the sintered object.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

- FIG.1 shows a schematic flowchart of the process 100 according to various embodiments;

- FIG. 2A shows a schematic of a temperature profile including a cooling process according to various embodiments;

- FIG. 2B shows a schematic of a temperature profile including an improved cooling process according to various embodiments;

- FIG. 2C shows a schematic of a temperature profile including a further improved cooling process according to various embodiments;

- FIG. 3 shows two scanning electron microscopy (SEM) images of particles which are not according to the invention;

- FIG. 4 shows two SEM images of particles according to a first example, in two different scales;

- FIG. 5 shows two SEM images of particles according to a second example, in two different scales; - FIG. 6 shows a graphic 610 with the distribution of the particles sizes and the cumulative distribution in graph 624;

- FIG. 7 shows a SEM image of a resulting material according to a first comparative example.

DETAILED DESCRIPTION

[0009] FIG.1 shows a schematic flowchart of a process 100 according to various embodiments. The process 100 may include providing a suspension in a step 1 10. The suspension may include a first solvent, and may include the at least one polymeric material. The suspension may further include a filler. For example, the filler and the solvent may be mixed in a step 1 10, e.g. sonicated, and the polymeric material may be added afterwards. The filler may include or consist of a clay. The first solvent may be capable of dissolving the at least one polymeric material at a first elevated temperature. The process may further include adding a second solvent to the suspension for example in a step 120, a second solvent miscible with the first solvent, wherein the at least one polymeric material is insoluble in said second solvent. The polymeric material may be added to the suspension together with the second solvent, for example before the second solvent, together with the second solvent or after the second solvent. The process may further include a step 130 of solubilizing the suspension by heating the suspension to a temperature equal to or higher than the first elevated temperature to dissolve the polymeric material. Solubilizing may be carried out under reflux. The process may further include a step 140 of precipitating the polymer composite particles from the solution, wherein precipitating comprises cooling. The process may further include a step 150 of recovering the formed polymer composite particles.

[0010] According to various embodiments, the concentration of polymeric material in the first solvent may be selected from 1 wt% to 20 wt%, for example from 8 wt% to 12 wt%.

[0011] According to various embodiments, the process for producing polymer composite particles, may mean a process of producing polymer composite particles.

[0012] According to the disclosed process, it is possible to obtain polymer composite particles with a homogenous size distribution and essentially free of sharp edges. The polymer composite particles are mostly single particles and in some cases single particles accompanied by binary particles (potato shaped), but otherwise essentially free of agglomerates. These particles present good properties for use in selective laser sintering.

[0013] According to various embodiments, the average roundness (Rnd) of the polymer composite particles may be 0.8 or higher. The roundness of a particle may be calculated with the formula:

wherein the A is the area the measured area of the particle, and r_max is the maximum length of the particle as diameter.

[0014] According to various embodiments, the at least one polymeric material may be selected from the group consisting of polypropylene (PP), polypropylene copolymers and combinations thereof. For example, the polypropylene copolymers may be selected from grafted polypropylene, preferably grafted-PP (g-PP), such as maleic anhydride grafted polypropylene. For example the at least one polymeric material may include PP and grafted-PP, wherein the weight ratio of PP to grafted-PP (g-PP) may be selected from 100:1 to 100:20.

[0015] According to various embodiments, the polymeric material may be provided in form of solid particles, for example in powder form.

[0016] According to various embodiments, the first solvent may be capable of dissolving the polymeric material. For example, the first solvent and the polymeric material may be selected so that the solvent is capable of dissolving the polymeric material, at a first elevated temperature. The first elevated temperature may be, for example at or higher than 130 ^°C for PP in p-xylene/1 -hexanol mixture, further for example, higher than 136 ^°C for PP in p-xylene/1 -hexanol mixture, at a pressure of 1 atm.

[0017] According to various embodiments, the solvent mixture of first and second solvents may retain the property of being able to dissolve the at least one polymeric material at a temperature equal to or higher than the first elevated temperature. The elevated temperature may be higher than the boiling point of the first solvent, for example higher than 125°C, preferably in the range from 135°C to 155°C.

[0018] In the context of the present invention and according to various embodiments, temperature values may include a variation of +/- 2 ^°C. [0019] According to various embodiments, the first solvent may be selected from the group consisting of p-xylene, m-xylene, o-xylene, toluene, decaline, ethylbenzene, chlorobenzene, or a combination thereof.

[0020] According to various embodiments, the second solvent may be miscible with the first solvent. The second solvent may be different from the first solvent.

[0021] According to various embodiments, the at least one polymeric material may be insoluble in the second solvent.

[0022] According to various embodiments, the second solvent may be an alcohol, preferably having a boiling point higher than a boiling point of the first solvent.

[0023] According to various embodiments, the alcohol may be selected from: aliphatic straight chain primary alcohols with 6 or more carbon atoms, straight chain secondary alcohols with 7 or more carbon atoms, a combination thereof.

[0024] According to various embodiments, the volume ratio of the first solvent to the second solvent may be selected from 1 :1 to 1 :4, for example it may be selected from 1 :2 to 1 :3.

[0025] According to various embodiments, the clay may be an unmodified smectite, modified smectite, or a mixture thereof. For example the clay may include or consist of montmorillonite, modified montmorillonite, or a mixture thereof.

[0026] According to various embodiments, the clay may include particles of a size D50 smaller than the polymer composite particles’ size, for example smaller than 15 micrometer.

[0027] According to various embodiments, the concentration of clay (for example OMMT) relative to the polymeric material may be selected from 0.1 wt % to 10 wt%, preferably from 0.5 wt% to 2 wt%.

[0028] According to various embodiments, the first solvent may be capable of dissolving and/or exfoliating and/or dispersing the at least one filler, for example the first solvent may be capable of exfoliating the at least one filler. This may be done, for example, with the aid of ultrasonication.

[0029] In the context of the present invention and according to various embodiments, the term“suspension” may refer to a mixture of a solvent with at least one of: the at least one polymeric material, the at least one filler, the second solvent. When the polymeric material is dissolved (also referred to as solubilized) in the first solvent, the suspension may be referred to as solution.

[0030] According to various embodiments, for example in the case wherein the filler cannot be dissolved, exfoliated or dispersed in any of the first or second solvents, a third solvent may be added to the suspension. The third solvent may be capable of exfoliating and/or dispersing the at least one filler prior to providing the suspension. The third solvent may be miscible between the first solvent and the second solvent.

[0031] According to various embodiments, the process may include the step of exfoliating, optionally involving ultrasonication, the at least one filler in the first solvent or in the mixture of the first and second solvents, and optionally further including the third solvent. For example, the filler may be OMMT and the first solvent may be p-xylene.

[0032] According to various embodiments, the process may include adding the at least one polymeric material to the first solvent. At this stage, the suspension may already include the filler as described above. The process may further include adding a second solvent to the suspension, wherein the second solvent is different from the first solvent. For example, the at least one polymeric material may be added to the suspension and the second solvent may be added afterwards to the suspension. Alternatively, the second solvent and the at least one polymeric material may be added in reverse order or simultaneously to the suspension. In one example, the first solvent may be p-xylene and the second solvent may be 1 -hexanol.

[0033] According to various embodiments, the process may include solubilizing the suspension thereby providing a solution, by heating the suspension to a temperature equal to or higher than the first elevated temperature to dissolve the polymeric material . For example, the suspension may be heated to dissolve the polymeric material, for example until a clear solution is obtained bleating may be performed under reflux, for example to a temperature selected between 15 ^°C below (for example 13 ^°C below) the boiling point of the second solvent, the temperature may be further selected to be above the boiling point of the first solvent. For example, for a p-xylene/1 -hexanol mixture, the temperature may be higher than 125 °C, preferably selected from 135 °C to 155 °C, for example at ambient pressure. Solubilizing may be carried out under reflux. Solubilizing may be performed under stirring. For example, neat PP may be completely dissolved in a p-xylene/1-hexanol solution at a temperature equal or above 136 ^°C (with or without filler), heated, when stirring for 30 min or more.

[0034] According to various embodiments, the process may further include precipitating the formed polymer composite particles from the suspension thereby forming a precipitate. For forming the precipitate, the solubilized solution may be cooled down, in a cooling process.

[0035] According to various embodiments, the cooling process may include a first cooling step, wherein the solution is cooled from a temperature equal to or higher than the first elevated temperature Ti to a second temperature T2 said second temperature T2 being below the first elevated temperature Ti, and kept at the second temperature for a first time period An . In accordance with various embodiments, the cooling process may further include a second cooling step, wherein the solution is cooled to a third temperature T3 below the second temperature T₂, and kept at the third temperature T3 for a second time period At2, and thereafter the solution may be further cooled, for example to room temperature. In accordance with various embodiments, in the second cooling step the solution may be cooled from the second temperature T2 to a first intermediate temperature Tn lower than the second temperature T₂, and kept at the first intermediate temperature Tn for a first intermediate time period An. In accordance with various embodiments, the solution may be cooled from the first intermediate temperature Tn to a second intermediate temperature T₁₂ lower than the first intermediate temperature Tn, and kept at the second intermediate temperature T₁₂ for a second intermediate time period D 12 before the solution is cooled to the third temperature T3.

[0036] According to various embodiments, the third temperature may be the precipitation temperature, meaning a temperature under which a precipitation starts to form.

[0037] According to various embodiments, the second temperature T2 may be chosen between the first elevated temperature Ti and the third temperature T3. In some embodiments, the second temperature T₂ may be selected from T3 + 10 ^°C and T3 + 14 ^°C.

[0038] According to various embodiments, the first intermediate Tn may be chosen between the second temperature T₂ and the third temperature T3. In some embodiments, the first intermediate Tn may be selected from T₂ - 6 ^°C and T₂ - 4 ^°C. [0039] According to various embodiments, the second intermediate T 12 may be chosen between the first intermediate temperature Tn and the third temperature T3. In some embodiments, the second intermediate T 12 may be selected from Ti - 3 ^°C and T2 - 1 ^°C.

[0040] According to various embodiments, each time period of the first time period A_M , the second time period DE, the first intermediary time period An, the second intermediary time period D12, as far as provided, may be selected from 1 minutes to 60 minutes, for example from 5 minutes to 60 minutes.

[0041] The solution/precipitate mixture may be further cooled to a lower temperature, e.g. a temperature lower than 40 ^°C, such as 20 ^°C or room temperature, and then the precipitate may be recovered. The cooling rate from the third temperature to a lower temperature (for example to room temperature) may be adjusted, for example linearly, for example so that the cooling from the third elevated temperature to the lower temperature takes a time of e.g. 20 minutes to 2 h.

[0042] According to various embodiments, the cooling may be quiescently. In other cases, for example, using stirring during cooling resulted in the formation of aggregates, which is undesired for selective laser sintering particles.

[0043] According to various embodiments, a cooling rate may be selected between 1 ^°C/minute and 10 ^°C/minute, for example between 1 ^°C/minute and 5 ^°C/minute. For example, at least one of a cooling rate from (i) the first elevated temperature to the second temperature, (ii) the second temperature to the third temperature for the case in which no intermediate temperatures are used, (iii) a cooling rate from the second temperature to the first intermediate temperature, (iv) a cooling rate from the first intermediate temperature to the second intermediate temperature, (v) a cooling rate from the second intermediate temperature to the third temperature, may be selected between 1 ^°C/minute and 10 ^°C/minute, for example between 1 ^°C/minute and 5 ^°C/minute. It was found that, with the first and second cooling steps, it was possible to obtain polymer composite particles with relative good morphology, e.g. without sharp edges and with narrow size distribution, which are suitable for selective laser sintering. Without wanting to be bound by theory, it is believed that the nucleation mainly takes place during the first cooling step, while particle growth mainly takes place during the second cooling step. It is believed that particle growth takes place at the third temperature, while the steps between the temperature equal to or higher than the first elevated temperature (Ti) and the third temperature promote the controlled nucleation of the of the polymer composite particles.

[0044] According to various embodiments, the first cooling step may include a temporary heating step, wherein, the temperature of the solution may be risen from a temperature equal or above the second temperature to a temporary heating temperature during a time period, wherein the temporary heating temperature is lower than the highest temperature used during the solubilization of the suspension, for example the temporary heating temperature may be lower than the first elevated temperature. It was found that incorporating the temporary heating step increases the homogeneity and size of the polymer composite particles. For example, it was possible to reduce formation of potato shapes particles (binary particles) with the heating step included in the cooling process. Further, the obtained particles size distribution is more homogeneous. For example, for PP in p-xylene including OMMT, the heating temperature of the heating step may be 122 and the heating duration may be 10 minutes +/- 20%. The heating duration may be counted as included in the cooling duration of the first cooling step, thus the total cooling duration may for the first cooling step be 30 minutes +/- 20%.

[0045] FIG. 2A shows a schematic of a temperature profile including a cooling process according to various embodiments. The graph 210 has a vertical axis representing temperature and a horizontal axis representing time. In a first cooling step, the solution is cooled from a temperature equal to or higher than the first elevated temperature Ti to a second temperature T2 lower than the first elevated temperature Ti, and kept at the second temperature T2for a first time period An . The cooling process may further include a second cooling step, wherein the solution is cooled to a third temperature T3 below the second temperature T2, and kept at the third temperature T3 for a second time period At2, and thereafter the solution may be further cooled, for example to room temperature. The first cooling step, when the temperature is kept at the second temperature for a first time period An may include a temporary heating step 212, for a temporary time period Att, as previously.

[0046] FIG. 2B shows a schematic of a temperature profile including an improved cooling process according to various embodiments. The graph 210 has a vertical axis representing temperature and a horizontal axis representing time. The cooling process may include a first cooling step, wherein the solution is cooled from a temperature equal to or higher than the first elevated temperature Ti to a second temperature T2 said second temperature T2 being below the first elevated temperature Ti, and kept at the second temperature T2for a first time period DH . The cooling process may further include a second cooling step, wherein the solution may be cooled from the second temperature T₂ to a first intermediate temperature Tu lower than the second temperature T2, and kept at the first intermediate temperature Tu for a first intermediate time period D11. In accordance with various embodiments, the solution may be cooled from the first intermediate temperature Tu to a second intermediate temperature T12 lower than the first intermediate temperature Tu, and kept at the second intermediate temperature T12 for a second intermediate time period D 12 before the solution is cooled to the third temperature T3, and thereafter the solution may be further cooled, for example to room temperature.

[0047] FIG. 2C shows a schematic of a temperature profile including a further improved cooling process according to various embodiments. The cooling process of FIG. 2C is identical to the cooling process described in connection with FIG. 2B, with the exception that the first cooling step includes a temporary heating step 212, for a temporary time period An, as previously described.

[0048] For example, for PP in p-xylene with OMMT, the first elevated temperature T 1 may be 140 ^°C, the second temperature T2 may be 122 ^°C and the first time period DH may be 30 minutes, the first intermediate temperature Tu may be 1 16 ^°C and the first intermediary time period D11 may be 10 minutes, the second intermediate temperature T12 may be 1 14 ^°C and the second intermediary time period D12 may be 30 minutes, and the third temperature T3 may be 1 10 ^°C and the second time period A may be 60 minutes.

[0049] According to various embodiments, the recovery step (d) may include filtering the suspension obtained in the precipitating step (c), for example the filter may be used to remove the solvents and residual dissolved polymeric material from the polymer composite particles. The recovery step (d) may include washing the particles obtained by filtering with a fourth solvent, said fourth solvent being miscible with at least the second solvent and the at least one polymeric materials being insoluble in the fourth solvent. Optionally, recovering the precipitate may include sonicating the precipitate before filtering the precipitate. The precipitate may be dried, for example under vacuum. [0050] According to various embodiments, the precipitate includes polymer composite particles suitable for selective laser sintering. After drying, about 100 wt%, e.g. at least 99 wt%, of the precipitate, may be utilized for selective laser sintering without requiring further treatment. For example more than 96 vol% of the precipitate may be of particles having a size within the desired range. The polymer composite particles are single spherical particles, essentially free of sharp edges, and of a composite including the filler and the polymeric material. The desired range may be from 20 micrometer to 100 micrometer, for example from 45 micrometer to 100 micrometer. The polymer composite particles may be essentially spherical. The polymer composite particles’ composition may comprise PP/OMMT composite.

[0051] According to various embodiments, the polymer composite particles may be used in a selective laser sintering process of producing a sintered object. The use may include i) providing a layer of the polymer composite particles in a particle bed (also referred to as powder bed). The use may further include ii) applying laser to the layer so as to fuse the polymer composite particles together. Thereby, one layer of the sintered object may be provided. The use may further include repeating steps i) and ii) thereby providing at least a part of the sintered object or the final sintered object, which is produced layer-by- layer. The high achieved roundness, e.g. greater than 0.8, of the polymer composite particles allow for a good flow of the polymer composite particles in the particle bed, thereby providing a good homogeneity in the layers. Further, since the polymer composite particles have a narrow size distribution, fusion and control of the fusion of the polymer composite particles is facilitated, allowing for better control on the final properties of the sintered object, which is further improved by the homogeneity of the composite.

Examples

[0052] FIG. 3 shows two SEM images of particles which are not according to the invention. In both images, the scale bar represents 100 micrometers. The particles are cryogenically-milled with the typical irregular shape and rough particle surface. Such particles are not ideal to be used for selective laser sintering, but may be used, for example, as polymeric material in the process of the present disclosure.

[0053] In a first example, 4.31 mg of OMMT as filler was sonicated in 5 ml_ p-xylene (as first solvent) to aid the exfoliation of the clay layers. 431 mg of neat PP (as one polymeric material) and 10 mL of 1 -hexanol (as second solvent) were then added to the suspension and the resulting suspension was subsequently heated to 140 °C under reflux with stirring until a clear solution was obtained. The solution was maintained at 140 °C, the first elevated temperature, for 45 min to ensure complete dissolution. The solution was then cooled quiescently to 1 10 °C according to the cooling process as explained in the following, and maintained at this temperature until precipitation of the polymer composite particles of PP/OMMT was complete. The solution was cooled from the temperature of 140 °C to 122 °C and maintained at 122 °C during 30 minutes, this was followed by cooling to the second temperature of 1 16 °C and maintained at 1 16 °C for 10 minutes. According to the second cooling step, the solution was cooled to 1 14 °C and maintained at 1 14 °C during 30 minutes, and further cooled to the third temperature of 1 10 °C and maintained at 1 10 °C during 60 minutes. The suspension was then further cooled to room temperature, sonicated for 15 min, and then filtered to yield powder of the PP/OMMT composite particles. The powder was subsequently washed with isopropyl alcohol and then dried under vacuum at room temperature to yield polymer composite particles with mean size D50 of 26 micrometers. The polymer composite particles were spherical shaped or potato shaped, with smooth surfaces and essentially free of sharp corners, thereby suitable for selective laser sintering. SEM images of the obtained polymer composite particles are shown in FIG. 4. In the top image of FIG. 4, the scale bar represents 100 micrometers. In the bottom image of FIG. 4, the scale bar represents 50 micrometers.

[0054] In a second example, polymer composite particles were prepared as in Example 1 , with the exception that a modified cooling process was used. The solution was cooled from the temperature of 140 °C to 122 °C, and maintained at 122 °C during 15 minutes, this was followed by a temporary heating step, in which the solution was kept at 126 °C during 10 minutes, and then cooled back to 122 °C at which temperature it was kept for 5 minutes and, further cooled to the second temperature of 1 16 °C and maintained at 1 16 °C during 10 minutes. According to the second cooling step, the solution was cooled to 1 14 °C and maintained at 1 14 °C during 30 minutes, and cooled to the third temperature of 1 10 °C and maintained at 1 10 °C during 60 minutes. The suspension was then further cooled to room temperature, sonicated for 15 minutes, and then filtered to yield powder of the PP/OMMT composite particles. The composite powder was subsequently washed with isopropyl alcohol and then dried under vacuum at room temperature to yield polymer composite particles with mean size D50 of 63 micrometers. The polymer composite particles were spherical shaped and substantially free of potato shaped particles. The polymer composite particles showed smooth surfaces and were essentially free of sharp corners, thereby suitable for selective laser sintering. SEM images of the obtained polymer composite particles are shown in FIG. 5. In the top image of FIG. 5, the scale bar represents 200 micrometers. In the bottom image of FIG. 5, the scale bar represents 50 micrometers.

[0055] FIG. 6 shows a graphic 610 showing the distribution of the polymer composite particles sizes as percentage volume (vertical axis 614) as function of the particle size in micrometers (horizontal axis 612), for particles of the second example. The cumulative distribution is shown in the graph 624, as percentage cumulative volume (vertical axis 624) as function of the particle size in micrometers (horizontal axis 622), for particles of the second example. The D50 particle size of 63 micrometers may be obtained from the graph, at the horizontal axis 622 (particle size) where the cumulative volume curve crosses the 50% (D50 point 262) on the vertical axis.

[0056] In a first comparative example, particles were prepared as in Example 2, however stirring was used during the cooling phase. The resulting material had agglomerates with irregular shape and high porosity, therefore not suitable to be used with selective laser sintering. An example of the resulting material of the first comparative example is shown in FIG. 7. The scale bar represents 20 micrometers.

Claims

1. Process for producing polymer composite particles comprising at least one polymeric material and at least one filler for selective laser sintering, comprising:

(a) providing a suspension comprising:

(a1 ) at least one polymeric material selected from the group consisting of

polypropylene, polypropylene copolymers and combinations thereof, wherein said at least one polymeric material is provided in form of solid particles that are suspended in the suspension;

(a2) at least one filler;

(a3) a first solvent capable of dissolving the at least one polymeric material at a first elevated temperature; and

(a4) a second solvent miscible with the first solvent, wherein the at least one polymeric material is insoluble in said second solvent;

(b) solubilizing the suspension by heating the suspension to a temperature equal to or higher than the first elevated temperature to dissolve the polymeric material;

(c) precipitating the polymer composite particles from the solution, wherein precipitating comprises cooling; and

(d) recovering the formed polymer composite particles.

2. The process according to claim 1 , wherein the polypropylene copolymers are selected from grafted polypropylene, preferably maleic anhydride grafted

polypropylene.

3. The process according to claim 1 or 2, wherein the suspension further comprises a third solvent miscible with the first and second solvent and capable of dissolving, exfoliating, dispersing, or a combination thereof, the at least one filler. 4. The process according to any one of claims 1 to 3, wherein the filler is an inorganic filler that is optionally organically modified, preferably a clay filler, more preferably organically modified montmorillonite (OMMT).

5. The process according to any one of claims 1 to 4, wherein the first solvent is selected from the group consisting of p-xylene, m-xylene, o-xylene, toluene, decaline, ethylbenzene, chlorobenzene, or a combination thereof.

6. The process according to any one of claims 1 to 5, wherein the second solvent is an alcohol, preferably having a boiling point higher than the first solvent.

7. The process according to claim 6, wherein the alcohol is selected from the group of aliphatic straight chain primary alcohols with 6 or more carbon atoms or aliphatic straight chain secondary alcohols with 7 or more carbon atoms.

8. The process according to any one of claims 3 to 7, the process comprises the step of exfoliating, optionally involving ultrasonication, the at least one filler in the first solvent or in the third solvent miscible with the first and second solvent and capable of exfoliating and/or dispersing the at least one filler prior to providing the suspension.

9. The process according to any one of claims 1 to 8, wherein the first elevated temperature is equal or higher than the boiling point of the first solvent, preferably higher than 125°C more preferably in the range of between 135 and 155°C.

10. The process according to any one of claims 1 to 9, wherein the solubilizing step (b) is carried out under reflux.

11. The process according to any one of claims 1 to 10, wherein the precipitating step (c) comprises a cooling process, said cooling process comprising:

- a first cooling step, wherein the solution is cooled from a temperature equal to or higher than the first elevated temperature (Ti) to a second temperature (T2) said second temperature (T2) being below the first elevated temperature (Ti), and kept at the second temperature for a first time period (D ), and

- optionally a second cooling step, wherein the solution is cooled to a third temperature (T3) below the second temperature (T2), and kept at the third temperature (T3) for a second time period (Dΐ2) .

12. The process according to any one of claims 1 to 11 , wherein the recovery step (d) comprises filtering the suspension obtained in the precipitating step (c) and optionally washing the particles obtained by filtering with a fourth solvent, said fourth solvent being miscible with at least the second solvent and the at least one polymeric materials being insoluble in the fourth solvent.

13. Polymer composite particles for selective laser sintering obtainable by the process of any of the previous claims.

14. Use of the particles of claim 13 in a selective laser sintering process for producing a sintered object.

15. Use according to claim 14, comprising:

i) providing a layer of the particles in a particle bed;

ii) applying laser to the layer so as to fuse the particles together;

repeating steps i) and ii) thereby providing the sintered object.