Patent Publication Number: US-9428610-B2

Title: Powder comprising polymer-coated inorganic particles

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Application No. DE 102011078722.4, filed Jul. 6, 2011, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a powder based on polymer-coated fillers which has advantages in terms of the stability of the production process, and density, to the use of the said powder in shaping processes, and also to mouldings produced by a layer-by-layer process by which regions of a powder layer are selectively melted, with use of the said powder. After cooling and solidification of the regions previously melted layer-by-layer, the moulding can be removed from the powder bed. The mouldings according to the invention moreover exhibit less susceptibility to warpage than conventional mouldings. 
     2. Description of the Related Art 
     A task frequently encountered in very recent times is the rapid provision of prototypes. Particularly suitable processes are those which are based on pulverulent materials and in which the desired structures are produced layer-by-layer through selective melting and solidification. Supportive structures for overhangs and undercuts can be omitted here, because the powder bed surrounding the molten regions provides sufficient support. Nor is there any need for the subsequent operation of removing supports. The processes are also suitable for producing short runs. 
     The selectivity of the layer-by-layer process here can be provided by way of example by applying susceptors, absorbers, or inhibitors, or by masks, or by way of focussed introduction of energy, for example through a laser beam, or by way of glass fibres. The energy is introduced by way of electromagnetic radiation. 
     A process which has particularly good suitability for the purpose of rapid prototyping is selective laser sintering. In this process, plastics powders are briefly irradiated selectively in a chamber by a laser beam, and the powder particles which encounter the laser beam therefore melt. The molten particles coalesce and rapidly resolidify to give a solid mass. This process can provide simple and rapid production of three-dimensional products by repeated irradiation of a succession of freshly applied layers. 
     The laser sintering (rapid prototyping) process for producing mouldings from pulverulent polymers is described in detail in the U.S. Pat. No. 6,136,948 and WO 96/06881. A wide variety of polymers and copolymers is claimed for the said application, examples being polyacetate, polypropylene, polyethylene, ionomers and polyamide. 
     Other processes with good suitability are the selective inhibition bonding (SIB) processes described in WO 01/38061, and a process described in EP 1 015 214. Both processes operate with large-surface-area infrared heating for melting of the powder. The selectivity of the melting process is achieved in the first case by applying an inhibitor, and in the second process it is achieved by a mask. DE 103 11 438 describes another process. In this, the energy required for the fusion process is introduced through a microwave generator, and the selectivity is achieved by applying a susceptor. 
     Other suitable processes are those operating with an absorber which is either present in the powder or is applied by ink-jet processes, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6 and DE 10 2004 020 452.7. 
     The rapid prototyping or rapid manufacturing processes mentioned (RP or RM processes) can use pulverulent substrates, in particular polymers, preferably selected from polyesters, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, or a mixture thereof. 
     WO 95/11006 describes a polymer powder which is suitable for the laser sintering process and which, when melting behaviour is determined by differential scanning calorimetry with a scanning rate of from 10 to 20° C./min, exhibits no overlap of the melting and recrystallization peak, has a degree of crystallinity of from 10 to 90%, likewise determined by DSC, has a number-average molecular weight Mn of from 30 000 to 500 000, and has a Mw/Mn quotient in the range from 1 to 5. 
     DE 197 47 309 describes the use of a nylon-12 powder which has increased melting point and increased enthalpy of fusion and which is obtained by reprecipitation of a polyamide previously produced through ring-opening and subsequent polycondensation of laurolactam. This is a nylon-12. 
     DE 10 2004 003 485 describes the use of particles with at least one cavity for use in processes that build layers. All of the particles comprise at least one cavity, and the particles comprising the cavity are melted by introduction of electromagnetic energy. The powder particles described have a thin surface layer. 
     DE 102 27 224 describes a granulated material which is intended for 3D binder printing and which is composed of particles provided with a surface layer comprising a non-polar external area. The surface layer of the powder particles described is, however, thin. 
     In conventionally known methods, powders such as described above are sometimes mixed with other particles for reinforcement, e.g. metal particles, glass particles or TiO 2  particles. However, a disadvantage encountered in these processes is that the handling of powder mixtures of this type often leads to demixing phenomena, and the mechanical properties that the reinforcing material is intended to achieve therefore sometimes vary. The regions where the proportion of filler is too high become very brittle and therefore unusable, and the regions comprising too little filler are softer than intended. The demixing derives from the different density of the polymer particles and of the filler, and tends to be apparent to some extent during any transport of the powder mixture and during its handling. In particular if the handling of the powder is automated in the rapid manufacturing process, it is difficult to control deviations in the properties of the components produced. 
     WO 2007/051691 describes processes for producing ultra-fine powders based on polyamides, by precipitating polyamides in the presence of inorganic particles, where a suspension is used with inorganic particles suspended in the alcoholic medium, where the d 50  median size of the inorganic particles is in the range from 0.001 to 0.8 μm. Fine polyamide powders were obtained here, and, because of their small size, the inorganic particles have uniform distribution in the composite particles here. The process was aimed at achieving colouring of the powder and of the moulding formed therefrom. The measure does not alter the mechanical properties of the moulding. 
     It was an object of the present invention to eliminate the problem of the demixing phenomenon and to achieve an improvement in the consistency of mechanical properties in the moulding, preferably strengthening of the moulding, and providing flame retardancy and/or an improvement in thermal conductivity in the moulding, where these improvements are intended to be achieved with the reinforcing material. 
     SUMMARY OF THE INVENTION 
     These and other objects have been achieved by the present invention, the first embodiment of which includes a powder, comprising composite particles: 
     wherein the composite particles, comprise: 
     an inorganic core particle having a d 50  median diameter of from 1 to 70 μm; and
         at least a partial coating of a polymer on the core;       

     wherein the inorganic core particle material does not include titanium dioxide, and a melting point of the coating polymer is obtainable when the polymer is exposed to an electromagnetic energy. 
     In a first preferred embodiment, the inorganic core particle of the composite particle is at least one material selected from the group consisting of silicon dioxide, a polyphosphate, a phosphinate, boron carbide, a mixed oxide, a spinel and a ceramic In a second preferred embodiment, the polymer of the coating of the composite particle comprises at least one polymer selected from the group consisting of a polyolefin, a polyethylene, a polypropylene, a polyvinyl chloride, a polyacetal, a polystyrene, a polyimide, a polysulphone, a poly(N-methylmethacrylimide) (PMMI), a polymethyl methacrylate (PMMA), a polyvinylidene fluoride (PVDF), an ionomer, a polyether ketone, a polyaryl ether ketone, a polyamide, and a copolyamide. 
     In another embodiment a d 50  median diameter of the composite particles is from 20 to 150 μm. 
     In a highly preferred embodiment, a ratio of a d 50  median diameter of the composite particles to a d 50  median diameter of the core particles is from 1.15 to 30. 
     In a further embodiment, the invention includes a process for producing the composite particles, the process comprising: 
     at least partially dissolving a polymer for the coating in a medium comprising a solvent which at least partially dissolves the polymer; 
     adding the inorganic core particles to the medium, before, during or after at least partially dissolving the polymer; 
     suspending the core particles in the medium; and then 
     precipitating the polymer from the at least partial solution onto the core particles to obtain the composite particles; wherein the d 50  median diameter of the core particles is from 1 to 70 μm. 
     In another embodiment, the invention includes a process for producing a moulded article, the process comprising: 
     applying a layer of the composite powder according to the present invention; 
     selectively melting at least one region of the layer by introduction of electromagnetic energy; 
     allowing the melted region to solidify; 
     applying another layer of composite powder and repeating the melting and solidification to perform a layer-by-layer process in which a molding having a structure according to the selective treatment is obtained; 
     wherein the melting selectivity is achieved by applying susceptors, inhibitors, or absorbers to each applied layer or by applying a mask to the applied layer. 
     Throughout the following description of the invention numerical ranges and values provided include all values and subvalues therebetween as well as all intermediate ranges within the stated range values. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One technical object of the invention may be achieved by provision of a powder for use in a layer-by-layer process for producing mouldings by selectively melting regions of the respective powder layer through introduction of electromagnetic energy, comprising composite particles which comprise an inorganic core particle having a d 50  median diameter of from 1 to 70 μm; and at least a partial coating of a polymer on the core; wherein the inorganic core particle material is at least one selected from the group consisting of silicon dioxide, a polyphosphate, a phosphinate, boron carbonate, a mixed oxide, a spinel and a ceramic, and a melting point of the coating polymer is obtainable when the polymer is exposed to an electromagnetic energy. 
     The core particles are inorganic core particles, with the exception of titanium dioxide, and where the d 50  median diameter of the inorganic core particles is from 1 to 70 μm. The d 50  median diameter of the core particles in all three spatial directions here is from 1 to 70 μm. The dimensions in each spatial direction may be different. It may be preferable that the d 50  median diameter of the core particles in all three spatial directions independently of one another is from 1 to 70 μm. The data for the diameters of the core particles is based on the particles which provide the core in the composite particle to be formed. 
     The layer-by-layer process for producing mouldings is preferably selective laser sintering. 
     Because of the firm bond between polymer and filler, the powder according to the present invention is no longer subject to the problems of demixing, and this leads to an improvement in consistency of mechanical properties in the moulding produced from the powder. Since demixing no longer occurs in the powder according to the invention, it may be possible to use the powder according to the invention in construction processes to produce uniform components and components with uniform quality. The durably uniform constitution resulting from the firm bond between polymer and core particle significantly improves the recyclability of the powder, even when a plurality of stages are involved. There may also be advantages in the use of the powders according to the invention: the powders according to the invention may be stored, transported and used in larger packaging units without any possibility of demixing. Feed quantities of the product may therefore also be greater during the laser sintering process, i.e. more powder can be charged to the sample feed container, and/or the dimensions of the sample feed container can be greater, without any resultant adverse effect on the quality of the resultant components. Furthermore, fluidization in the feed does not lead to the demixing that is relatively frequently observed in conventionally known systems. Because the powders of the present invention have an exterior shell made of polymer, the introduction of energy by the laser may also be more uniform. In conventional powders as described above, the laser sometimes encounters a polymer particle and sometimes encounters a filler particle. As a function of filler type, the result can vary in extreme cases from almost complete absorption to almost complete reflection of the energy. Powders according to the present invention advantageously avoid these problems. 
     Surprisingly, it has now been found that, by using core particles made of an inorganic material with d 50  median diameter of greater than 1 to 70 μm as reinforcing material firmly bonded to the polymer (composite particles), it may be possible, through a layer-by-layer process (in which regions of the respective powder layer are selectively melted) to produce mouldings which have advantages in relation to density and susceptibility to warpage and with this have better properties in relation to consistency of processing than those made of the conventionally known reinforced polymer powders. According to preferred embodiments of the invention reinforced mouldings having improved thermal conductivity, and/or improved flame retardancy may be obtained. 
     In one preferred embodiment, the inorganic core particles to be coated with the precipitatable polymer may be selected from the group consisting of silicon dioxide, polyphosphates, phosphinates, metal nitrides, semimetal nitrides, aluminium nitrides, boron nitride, boron carbide, metal oxides, for example Al 2 O 3 , mixed oxides, spinets, metal, ceramic and mixtures thereof. It may be further preferable that the core particles to be coated with the precipitatable polymer have been selected from the group of silicon dioxide, polyphosphates, phosphinates, boron carbide, metal oxides, for example Al 2 O 3 , mixed oxides, spinels and mixtures thereof. It is moreover possible that further fillers or filler mixtures are present alongside the core particles mentioned in the composite particle. 
     The respective core particles may be of spherical, lamellar or elongate form. The respective core particles may moreover be sharp-edged, rounded or smooth. The core particles mentioned may optionally be coated with sizes prior to application of the polymer coating. 
     The core particles provide the core in the composite particle. The powder according to the present invention preferably has a core-shell structure. 
     In another preferred embodiment, the average thickness of the coating made of the precipitated polymer may be 1.5 μm or more, preferably 2, 3, 5, 10, 15, 20, 25, 30, 50, or 75 μm or more. 
     In another preferred embodiment, the d 50  median diameter of the core particles (core of the composite particle) may be from 1 to 60 μm, preferably from 1 to 50 μm, with preference from 1 to 40 μm, more preferably from 1 to 30 μm, still more preferably from 1 to 25 μm, particularly preferably from 1 to 20 μm and very particularly preferably from 1 to 10 μm. Suitable particle size distributions can be ensured by known methods, e.g. sieving or sifting. 
     In an alternative embodiment, the d 50  median diameter of the core particles (core of the composite particle) may be from 10 to 60 μm, preferably from 10 to 50 μm, with preference from 10 to 40 μm, more preferably from 10 to 30 μm, still more preferably from 10 to 25 μm and particularly preferably from 10 to 20 μm. Again in this preferred embodiment, the core particles to be coated with the precipitatable polymer have been selected from the group of silicon dioxide, polyphosphates, phosphinates, aluminium nitride, boron nitride, boron carbide, metal oxides, e.g. Al 2 O 3 , mixed oxides, spinels, metal and ceramic and mixtures thereof. 
     It is moreover preferable that the d 50  median diameter of the composite particles may be from 20 to 150 μm, with preference from 20 to 120 μm, preferably from 20 to 100 μm, more preferably from 25 to 80 μm and particularly preferably from 25 to 70 μm. 
     The ratio of the d 50  median diameter of the composite particles to the d 50  median diameter of the core particles may preferably be from 1.15 to 30, with preference from 1.2 to 30, more preferably from 1.5 to 25; still more preferably from 1.5 to 15, particularly preferably from 1.5 to 12 and very particularly preferably from 1.5 to 10. 
     The ratio by weight of the polymer to the core particles, based on the entirety of the composite particles, is preferably from 0.1 to 30, with preference from 1.0 to 20.0 and more preferably from 1.3 to 10.0. 
     In another preferred embodiment, the BET specific surface area of the powder according to the invention may be in the range from 1 to 60 m 2 /g, with preference from 3 to 50 m 2 /g, more preferably from 3 to 40 m 2 /g; particularly preferably from 3 to 30 m 2 /g, still more preferably from 3 to 20 m 2 /g and very particularly preferably from 3 to 12 m 2 /g. The bulk density (BD) of the powder according to the invention may be in the range from 120 to 700 g/l, with preference from 250 to 450 g/l. 
     The precipitated or precipitatable polymer is a polymer which can be dissolved in a liquid medium comprising a solvent and which precipitates in the form of a completely or partially insoluble deposit in the form of flakes or droplets, or in crystalline form, as a result of changes of certain parameters, e.g. temperature, pressure, solvent content, non-solvents, anti-solvents, or precipitants. The type of solvent and the solvent content depend on the properties of the polymer, as also do the other parameters for dissolving or precipitating the polymer. 
     The precipitatable polymer may preferably be selected from polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide and mixtures thereof, in particular mixtures of homo- and copolyamide. 
     In another embodiment, the precipitatable polymer for coating the core particles may be obtained through precipitation of at least one polyamide of the AABB type or through joint precipitation of at least one polyamide of the AB type and at least one polyamide of the AABB type. Preference may be given here to co-precipitated polyamides, where at least nylon-11 or nylon-12 and at least one polyamide based on PA1010, PA1012, PA1212 or PA1013 is present. 
     The following precipitatable polymer and solvent combinations may be mentioned as examples. Polyolefins and polyethylene may be dissolved, for example in toluene, xylene and/or 1,2,4-trichlorobenzene. Polypropylene may be dissolved in toluene and/or xylene. Polyvinyl chloride may be dissolved in acetone. Polyacetal may be dissolved in DMF, DMAc and/or NMP. Polystyrene may be dissolved in toluene. Polyimides may be dissolved in NMP. Polysulphones may be dissolved in sulpholane. Poly(N-methylmethacrylimides) (PMMI) may be dissolved in DMAc and/or NMP. Polymethyl methacrylate (PMMA) may be dissolved in acetone. Polyvinylidene fluorides may be dissolved in N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc) and/or cyclohexanone. Polyether ketones and polyaryl ether ketones may be dissolved in diphenyl sulphone and/or in sulpholane. Polyamides may be dissolved in an alcoholic medium, preferably an ethanol-water mixture. As explained above, it may sometimes also necessary to adjust parameters such as temperature and pressure in order to dissolve a given polymer. 
     Once the relevant polymer has been dissolved, the dissolved polymer is precipitated in the presence of the core particles, in order to coat the core particles completely or partially with the relevant precipitated polymer. The precipitation of the polymer may be initiated and/or accelerated by changing the pressure, changing the temperature, changing (reducing) the concentration of the solvent, and optionally adding a non-solvent, anti-solvent and/or precipitant. In the case of amorphous polymers, such as polystyrene, sulphones, PMMI, PMMA, and ionomer, it may be necessary to add a non-solvent to precipitate the relevant polymer. 
     The precipitatable polymer may preferably be a polyamide which has at least 8 carbon atoms per carbonamide group. The polymer may particularly preferably be a polyamide which has 10 or more carbon atoms per carbonamide group. The polymer may very particularly preferably be a polyamide selected from nylon-6,12 (PA 612), nylon-11 (PA 11) and nylon-12 (PA 12). The production process for the polyamides that can be used in the sinter powders according to the invention is well-known and, for the production of PA 12, can be found for example, in the documents DE 29 06 647, DE 35 10 687, DE 35 10 691 and DE 44 21 454. The granulated polyamide material required may be purchased from various producers, for example, granulated nylon-12 material is available with trade name VESTAMID from Evonik Industries AG. 
     In a highly preferred embodiment according to the invention the coating polymer is nylon-12. 
     It may be moreover possible to use the corresponding copolyamides or mixtures of homo- and copolyamides which comprise at least 70 percent by weight of the units mentioned. Accordingly, they may comprise, as comonomers, from 0 to 30 percent by weight of one or more comonomers, such as caprolactam, hexamethylenediamine, 2-methyl-1,5-pentanediamine, 1,8-octamethylenediamine, dodecamethylenediamine, isophoronediamine, trimethylhexamethylenediamine, adipic acid, suberic acid, azeleic acid, sebacic acid, dodecanedioic acid, aminoundecanoic acid. The homo- and copolyamides mentioned, termed polyamides hereinafter, may be used in the form of granulated materials or ground material, where the relative solution viscosity of these is from 1.5 to 2.0 (measured in 0.5% m-cresol solution at 25° C. in accordance with DIN 53 727), preferably from 1.70 to 1.95. They may be produced by conventionally known methods, including polycondensation, or hydrolytic or acidolytic or activated anionic polymerization. It may be preferable to use unregulated polyamides having NH 2 /COOH end group ratios of from 40/60 to 60/40. However, it may also be advantageous to use regulated polyamides and specifically preferably those in which the NH 2 /COOH end group ratio is 90:10 and 80:20 or 10:90 and 20:80. 
     In another preferred embodiment, the density of the core particles is greater than the density of the solvent or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% less than the density of the solvent used for the precipitation of the polymer. 
     It may be particularly preferable to use an alkanol (for example: methanol, ethanol, propanol or butanol), preferably ethanol, as solvent for the precipitation of the polymer in the presence of the core particles, where the density of the core particles is greater than the density of the alkanol or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% less than the density of the alkanol, preferably of ethanol. 
     The powder may comprise the composite particles mentioned alone or together with, admixed therewith in uncompacted form, (dry-blend) fillers, and/or auxiliaries. The proportion of the composite particles in the powder may be at least 50% by weight, with preference at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably at least 99% by weight. 
     The powders according to the invention may optionally comprise auxiliaries and/or other organic or inorganic pigments. These auxiliaries may be a powder-flow aid, e.g. precipitated and/or fumed silicas. Precipitated silicas are available by way of example with product name AEROSIL® with various specifications from Evonik Industries AG. It may be preferable that the powder according to the invention comprises less than 3% by weight of these auxiliaries, with preference from 0.001 to 2% by weight and very particularly preferably from 0.025 to 1% by weight, based on the entirety of the polymers present. 
     In order to improve processability or for further modification of the powder according to the invention, inorganic foreign pigments, e.g. transition metal oxides, stabilizers, e.g. phenols, in particular sterically hindered phenols, flow aids and powder-flow aids, e.g. fumed silicas, may be added. The amount of the substances added to the polymers, based on the total weight of polymers in the polymer powder, is preferably such as to provide compliance with the concentrations stated for auxiliaries for the powder according to the invention. 
     Ideal properties in the further processing of the powder may be achieved when the melting point of the polymer in the first heating procedure is greater than in the second heating procedure, measured by differential scanning calorimetry (DSC); and when the enthalpy of fusion of the polymer in the first heating procedure is at least 50% greater than in the second heating procedure, measured by differential scanning calorimetry (DSC). When such conditions are met, the polymer content (of the shell or of the coating) of the composite particles has higher crystallinity when compared with other powders which can be produced by processes other than co-precipitation of a dissolved polymer with particles. A particularly suitable material for the laser sintering process is a nylon-12 which has a melting point of from 185 to 189° C., with preference from 186 to 188° C., an enthalpy of fusion of 112+/−17 kJ/mol, with preference from 100 to 125 kJ/mol, and a freezing point of from 138 to 143° C., preferably from 140 to 142° C. 
     An embodiment of the invention also includes a process for producing the composite particles, which comprises: at least partially dissolving a polymer for the coating in a medium comprising a solvent which at least partially dissolves the polymer; adding the inorganic core particles to the medium, before, during or after at least partially dissolving the polymer; suspending the core particles in the medium; and then precipitating the polymer from the at least partial solution onto the core particles to obtain the composite particles; wherein the d 50  median diameter of the core particles is from 1 to 70 μm and the inorganic core particle material is at least one selected from the group consisting of silicon dioxide, a polyphosphate, a phosphinate, boron carbide, a mixed oxide, a spinel and a ceramic. 
     In order to produce an at least partial solution, a polymer may be brought into contact, in the presence of the inorganic core particles, with exposure to pressure and/or heat, with a medium comprising solvent which dissolves the polymer, and then the polymer is precipitated from the at least partial solution, and composite particles according to the invention are obtained which are produced by core particles coated entirely or partially with a precipitated polymer. According to the invention, titanium dioxide is excluded as an inorganic material of the core particle. 
     In one preferred process, the thickness of the coating made of the precipitated polymer may be 1.5 μm or more, preferably 2, 3, 5, 10, 15, 20, 25 or 30 μm or more. 
     In another preferred process, the d 50  median diameter of the core particles (core of the composite particle) may be from 1 to 60 μm, preferably from 1 to 50 μm, with preference from 1 to 40 μm, more preferably from 1 to 30 μm, still more preferably from 1 to 25 μm, particularly preferably from 1 to 20 μm and very particularly preferably from 1 to 10 μm. Suitable particle size distributions can be ensured by known processes, e.g. sieving or sifting. 
     The use of inorganic core particles which are in suspension in the solvent for the precipitatable polymer is particularly important. In this manner, the inorganic particles provide surface upon which the polymer precipitates. A feature of one preferred variant of the process of the invention is that a suspension of inorganic core particles suspended in the alcoholic medium is used, where the (d 50 ) median size of the core particles is the size stated above. 
     The d 50  median diameter of the composite particles produced by the production process may preferably be from 20 to 150 μm, with preference from 20 to 120 μm, preferably from 20 to 100 μm, more preferably from 25 to 80 μm and particularly preferably from 25 to 70 μm. 
     In the composite particles produced by the production process, the ratio of the d 50  median diameter of the composite particles to the d 50  median diameter of the core particles may preferably be from 1.15 to 30, preferably from 1.2 to 30, with preference from 1.5 to 25; more preferably from 1.5 to 15, more preferably from 1.5 to 12 and particularly preferably from 1.5 to 10. 
     An advantage of the process according to the invention may be provided by, where appropriate, saving an operation during the production of the powder, because there is no longer any need for the dry-blend mixing of polymer particles and auxiliary particles and/or filler particles. 
     In one preferred process, the core particles are selected from silicon dioxide, polyphosphates, phosphinates, boron nitride, boron carbide, mixed oxides, spinels, and ceramic. Particularly preferred core particles are of at least one material selected from the group consisting of ammonium polyphosphate, silicon dioxide and a phosphinate. 
     Depending on intended utility and properties, the core particles may be solid beads, hollow beads or porous beads. The shapes of the respective core particles may be spherical, lamellar or elongate. The respective core particles may optionally, be sharp-edged, rounded-off or smooth particles. The core particles may optionally be coated with sizes prior to coating via polymer precipitation. 
     In preferred embodiments of the invention, the precipitatable polymer may be selected from the group consisting of polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide, and mixtures thereof, in particular mixtures of homo- and copolyamide. 
     In another embodiment, the polymer for coating of the core particles may be obtained through precipitation of at least one polyamide of AABB type or through joint precipitation of at least one polyamide of AB type and of at least one polyamide of AABB type. Preference may be given to co-precipitated polyamides, according to this embodiment, where at least nylon-11 or nylon-12 and at least one polyamide based on PA1010, PA1012, PA1212 or PA1013 are co-precipitated in forming the coating on the core particles. 
     The type of solvent and the content of solvent, and also the other parameters for dissolving and reprecipitating the appropriate polymer, is dependent on the polymer and is described above. 
     The following description relates to polymers which can be dissolved in the alcoholic medium, in particular polyamides. For the coating of core particles with polymers for which other solvents must be, or are, used, the parameters and solvents must be modified appropriately, as recognized by one of ordinary skill in the art. 
     A feature of a preferred embodiment of the process of the invention is that a suspension is used which is obtainable by suspending the inorganic core particles in the medium comprising a solvent which dissolves the polymer, for example an alcoholic medium, with introduction of energy input greater than 1000 kJ/m 3 . This generally already produces very useful suspensions of the core particles in the medium. The energy input mentioned may be achieved through known assemblies. Suitable assemblies include, but are not limited to, planetary-gear kneaders, rotor-stator machinery, an agitated ball mill, a roll mill or the like. 
     The suspensions that are useful for the invention are produced in a medium comprising solvents which dissolve the precipitatable polymer, for example an alcoholic medium. In the case of an alcoholic medium, the solvent may be a pure alcohol, a mixture of a plurality of alcohols, or alcohols with a content of water or of other substances which do not in essence have any disadvantageous effect on the desired reprecipitation of the polyamides. The alcoholic medium of the suspensions preferably has less than 50% by weight content of non-alcoholic substances (preferably water), particularly preferably comprising less than 30% by weight, and particularly advantageously less than 10% by weight, of foreign non-alcoholic substances. The invention may optionally use any of the types of alcohols or mixtures thereof which permit reprecipitation of polymers, preferably polyamides, under the desired conditions (pressure and temperature). In any particular case, it is relatively easy for the person skilled in the art to modify the system to meet specific requirements. The process of the invention may preferably use as alcoholic medium for the precipitation of the polyamide and/or the suspension of the core particles, one or more alcohols which have a numeric ratio of oxygen atoms to carbon atoms in the range from 1:1 to 1:5. 
     Suitable alcohols for producing the suspension of the core particles may be those where the ratio of oxygen to carbon is 1:1, 1:2, 1:3, 1:4 or 1:5, preferably those where the ratio of oxygen to carbon is 1:2 or 1:3, particularly preferably where the oxygen to carbon ratio is 1:2. It is very particularly advantageous to use ethanol for producing a suspension of the core particles, and also for the reprecipitation of the precipitatable polymer, preferably of the polyamides. 
     As previously described, the precipitatable polymer may preferably be selected from the group consisting of polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide and mixtures thereof, in particular mixtures of homo- and copolyamide. The precipitatable polyamide is dissolved in an appropriate solvent so that it can be reprecipitated on the surface of the core particles, thereby coating the particle. 
     In a highly preferred embodiment, the reprecipitatable polymer may be a polyamide, preferably a polyamide which has at least 8 carbon atoms per carbonamide group. It is particularly preferable that the polymer is a polyamide which has 10 or more carbon atoms per carbonamide group. Polyamides which may preferably be used as starting material for the process of the invention comprise inter alia nylon-11, nylon-12 and polyamides having more than 12 aliphatically bonded carbon atoms per carbonamide group, preferably nylon-12. It is moreover possible to use the corresponding copolyamides or a mixture of homo- and copolyamides where these comprise at least 70 percent by weight of the polymer. They may accordingly comprise from 0 to 30 percent by weight of one or more comonomers, such as caprolactam, hexamethylenediamine, 2-methyl-1,5-pentanediamine, 1,8-octamethylenediamine, dodecamethylenediamine, isophoronediamine, trimethylhexamethylenediamine, adipic acid, suberic acid, azeleic acid, sebacic acid, dodecanedioic acid or aminoundecanoic acid. The homo- and copolyamides, termed polyamides hereinafter, may be used in the form of granulated materials or ground material, where the relative solution viscosity of these is from 1.5 to 2.3 (measured in 0.5% m-cresol solution at 25° C. in accordance with DIN 53 727), preferably from 1.55 to 1.95. They may be produced by polycondensation, or hydrolytic or acidolytic or activated anionic polymerization, by conventionally known processes. In a preferred embodiment, unregulated polyamides with NH 2 /COOH end group ratios of from 40/60 to 60/40 may be used. However, it may also be advantageous to use regulated polyamides and specifically preferably those in which the NH 2 /COOH end group ratio is 90:10 and 80:20 or 10:90 and 20:80. 
     Any of methods known to one of skill in the art may be used to produce the solution of the precipitatable polymers, preferably the polyamides, for the reprecipitation process. It may be advantageous to achieve maximum completeness of dissolution of the precipitatable polymers, preferably of the polyamide, in the appropriate medium, preferably an alcoholic medium, in the presence of the core particles suspended therein. Dissolution may be promoted by use of pressure and/or heat. In an advantageous procedure, the precipitatable polymer, may initially be present in the alcoholic medium and may be dissolved with exposure to elevated temperature for the required time. The suspension of the core particles may be added prior to, during or after the dissolution of the precipitatable polymer. The suspension of the core particles may advantageously be present together with the precipitatable polymer, in the starting mixture. The dissolution procedure may advantageously be assisted by the use of appropriate agitation assemblies. The precipitation of the precipitatable polymer may be assisted by using pressure and/or heat, preferably using a temperature reduction and/or removal of the solvent, by distillation (preferably under reduced pressure) to precipitate the precipitatable polymer. However, it may also be possible to assist the precipitation process by adding an anti-solvent (precipitant). 
     In an optionally preferred process, after formation of the composite particles, a post-treatment may be carried out in a mixer with high shear. The temperature here may particularly preferably be above the glass transition temperature of the respective polymer. This measure may serve to round the grains and improve powder-flowability. 
     The abovementioned parameters may be determined as follows: 
     BET surface area was determined in accordance with DIN ISO 9277: 2003-05 with gas-adsorption equipment from Micromeritics for determining specific surface area by the BET method (Micromeritics TriStar 3000 V6.03: V6.03 refers to the software version of the Win3000 Software). BET surface area was determined by means of nitrogen gas adsorption by the discontinuous volumetric method (DIN ISO 9277:2003-05, Section 6.3.1.). For this, a number (seven) of measurement points were determined at relative pressures P/P0 from 0.05 to 0.20. He (purity at least 4.6 [99.996%] according to operating instructions, or at least 4.0 [99.99%] according to standard; this also applies to N 2 ) was used for dead volume calibration. The samples were devolatilized respectively for 1 hour at room temperature (21° C.) and 16 hours at 80° C. in vacuuo. The specific surface area was based on the devolatilized specimen. The evaluation used multipoint determination (DIN ISO 9277:2003-05, Section 7.2). The temperature during the measurement was 77 K. 
     The particle size (d 50  fineness) was determined by means of laser scattering. The measurements were carried out with a Malvern Mastersizer 2000. A dry measurement is involved here. For the measurement, in each case from 20 to 40 g of powder were metered into the system with the aid of Scirocco dry-dispersion equipment. The feed rate used to operate the vibrating trough was 70%. The pressure of the dispersion air was 3 bar. Each measurement involved a background measurement (10 seconds/10 000 individual measurements). The measurement time for the sample was 5 seconds (5000 individual measurements). The refractive index, and also the blue-light value, was defined as 1.52. Evaluation was based on the Mie theory. 
     Bulk density is calculated in accordance with DIN EN ISO 60. 
     Particle content is determined by ash/ignition residue determination in accordance with DIN EN ISO 3451 Part 1 and Part 4. 
     Solution viscosity was determined in 0.5% meta-cresol solution in accordance with ISO 307. 
     In another embodiment, the present invention includes a process for producing a moulded article, comprising: 
     applying a layer of the composite powder according to the present invention; 
     selectively melting at least one region of the layer by introduction of electromagnetic energy; 
     allowing the melted region to solidify; 
     applying another layer of composite powder and repeating the melting and solidification to perform a layer-by-layer process in which a moulding having a structure according to the selective treatment is obtained; 
     wherein the melting selectivity is achieved by applying susceptors, inhibitors, or absorbers to each applied layer or by applying a mask to the applied layer. 
     Accordingly, regions of the respective powder layer are selectively melted through introduction of electromagnetic energy, where the selectivity is achieved by applying susceptors, inhibitors, or absorbers or by masks, where the powder comprises composite particles which are produced by core particles coated entirely or partially with a precipitated polymer, where the d 50  median diameter of the core particles, preferably at least one material selected from the group consisting of ammonium polyphosphate, silicon dioxide and a phosphinate, is 1 to 70 μm. 
     The present invention also provides mouldings obtained from the powder according to the invention by the abovementioned process. The moulding thus produced here comprises (a) polymer(s) preferably selected from polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide and mixtures thereof, in particular mixtures of homo- and copolyamide. In another embodiment, the polymer may be at least one polyamide of AABB type or a mixture of at least one polyamide of AB type and of at least one polyamide of AABB type. Preference is given here to mixtures of polyamides where at least nylon-11 or nylon-12 and at least one polyamide based on PA1010, PA1012, PA1212 or PA1013 may be present. 
     The process with use of the powder according to the invention provides advantages in that the powder no longer demixes, fewer cavities are produced in the component, and recyclability is better, and the components have higher density and uniform quality, and also in that there is clear separation between molten and non-molten regions, and in that the components have low warpage. 
     The energy may be introduced through electromagnetic radiation, and the selectivity may be introduced for example, by masks, or application of inhibitors, absorbers or susceptors, or else by focussing of the radiation, for example by lasers. The electromagnetic radiation may comprise the range from 100 nm to 10 cm, preferably from 400 nm to 10 600 nm or from 800 to 1060 nm. The source of the radiation may be a microwave generator, a suitable laser, a fibre laser, a radiant heat source or a lamp, or else a combination thereof. After cooling of all of the layers, the moulding can be removed. 
     The following examples of processes of this type serve for illustration, but there is no intention that the invention be restricted thereto. 
     Laser sintering processes are based on the selective sintering of polymer particles, where layers of polymer particles are briefly exposed to laser light and the polymer particles exposed to the laser light bond to one another. The successive sintering of layers of polymer particles produces three-dimensional objects. Details concerning the selective laser sintering process may be found in U.S. Pat. No. 6,136,948 and WO 96/06881. 
     Other processes which may be suitability employed include the SIB processes described in WO 01/38061, and the process described in EP 1 015 214. Both processes operate with large-surface-area infrared heating for melting of the powder. 
     The selectivity of the melting process is achieved in the first case by applying an inhibitor, and in the second process it is achieved by a mask. DE 103 11 438 describes another process in which the energy required for the fusion process is introduced through a microwave generator, and the selectivity is achieved by applying a susceptor. 
     Other suitable processes are those operating with an absorber which is either present in the powder or is applied by ink-jet processes, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6 and DE 10 2004 020 452.7. 
     A feature of the mouldings according to the present invention which are produced by a layer-by-layer process in which regions are selectively melted is that the density of the moulding component may be reduced in comparison to a component produced from conventionally known powder as described above. Susceptibility to warpage is moreover reduced, and an improvement is achieved in the reproducibility of mechanical properties in the moulding. 
     The mouldings may moreover comprise auxiliaries as described as optional components for the powder, e.g. heat stabilizers, e.g. sterically hindered phenol derivatives. The mouldings preferably comprise less than 3% by weight of these auxiliaries, based on the entirety of the polymers present, particularly preferably from 0.001 to 2% by weight and very particularly preferably from 0.05 to 1% by weight. 
     Application sectors for the said mouldings may be found both in rapid prototyping and in rapid manufacturing. The latter certainly also means small runs, i.e. the production of more than one identical part, where however production by means of an injection mould is not economic. Examples include parts for high specification cars of which only small numbers of units are produced, or replacement parts for motorsport, where availability time is important, as well as the small numbers of units. Sectors in which the parts are used include but are not limited to the aerospace industry, medical technology, mechanical engineering, automobile construction, the sports industry, the household goods industry, the electrical industry, and the lifestyle sector. 
     The invention also provides the use of the powder according to the invention in a process for producing mouldings by a layer-by-layer process in which regions of the respective powder layer are selectively melted through introduction of electromagnetic energy, where the selectivity is achieved by applying susceptors, inhibitors, or absorbers or by masks, where at least one powder is used which comprises composite particles which are coated entirely or partially with a precipitated polymer, where the core particles are inorganic core particles, with the exception of titanium dioxide, and where the d 50  median diameter of the inorganic core particles is from 1 to 70 μm. 
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
     The values measured for bulk density were determined by an apparatus in accordance with DIN EN ISO 60. 
     EXAMPLES 
     Example 1 
     Reprecipitation of Nylon-12 (PA 12) (not According to the Invention) 
     400 kg of unregulated PA 12 produced by hydrolytic polymerization and having a relative solution viscosity of 1.62 and 75 mmol/kg of COOH end group content and 69 mmol/kg of NH 2  end group content were heated to 145° C. with 2500 l of ethanol denatured with 2-butanone and 1% water content, within a period of 5 hours in a 3 m 3  stirred tank (a=160 cm) and held at the said temperature for 1 hour, with stirring (blade stirrer, x=80 cm, rotation rate=49 rpm). The jacket temperature was then reduced to 124° C. and the internal temperature was brought to 125° C. at the same stirrer rotation rate with continuous removal of the ethanol by distillation, with a cooling rate of 25 K/h. From this juncture onwards, with the same cooling rate, the jacket temperature was held at from 2K to 3 K below the internal temperature. The internal temperature was brought to 117° C., with the same cooling rate, and was then held constant for 60 minutes. Material was then removed by distillation with a cooling rate of 40 K/h, and the internal temperature was thus brought to 111° C. At the said temperature, the precipitation process began, discernible from the evolution of heat. The distillation rate was increased in such a way that the internal temperature did not rise beyond 111.3° C. After 25 minutes, the internal temperature fell, indicating the end of the precipitation process. The temperature of the suspension was brought to 45° C. by further removal of material by distillation and cooling by way of the jacket, and then the suspension was transferred to a paddle dryer. The ethanol was removed by distillation at 70° C./400 mbar, and the residue was then further dried for 3 hours at 20 mbar/86° C. 
     This gave a precipitated PA 12 with an average grain diameter of 55 μm. Bulk density was 435 g/l. 
     By analogy with the procedure indicated in Example 1 or in accordance with DE 19708146, a powder was produced with particles as core and with a shell made of PA12, PA 10.12, PA10.10, PA6.12, PA6.13, PA10.13, PA6.18 and PA12.18. 
     Example 2 
     Single-Stage Reprecipitation of PA12 with Particles (According to the Invention) 
     As in Example 1, 250-375 kg of a PA 12 produced by hydrolytic polymerization with a relative solution viscosity (η rel ) of 1.62 and with 75 mmol/kg of COOH end group content and 66 mmol/kg of NH 2  end group content were reprecipitated in the presence of 162.5-250 kg of particles with the properties set out in Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Properties of the various core particles used in Example 2: 
               
            
           
           
               
               
               
            
               
                   
                 Particle 
                 d 50   
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Al 2 O 3  (Martoxid ® MN/Y 216) 
                 &gt;10 
                 μm 
               
               
                   
                 Al 2 O 3  (Martoxid ® DN 206) 
                 5-7 
                 μm 
               
               
                   
                 Al 2 O 3  (Martoxid ® MDLS-6) 
                 3-4 
                 μm 
               
               
                   
                 Al 2 O 3  (Martoxid ® MZS-1) 
                 1.5-1.9 
                 μm 
               
               
                   
                 Ammonium polyphosphate (Exolit ® AP 422) 
                 18.33 
                 μm 
               
               
                   
                 Phosphinate (Exolit ® OP1230) 
                 11 
                 μm 
               
               
                   
                 SiO 2  (Aeroperl ® 300/30) 
                 8 
                 μm 
               
               
                   
                 Stainless steel flakes 
                 31 
                 μm 
               
               
                   
                 AS081 aluminium powder 
                 28 
                 μm 
               
               
                   
                   
               
            
           
         
       
     
     In this example, the precipitation conditions were altered in the following way in comparison with Example 1: 
     Precipitation temperature: 108° C. 
     Precipitation time: 150 min 
     Stirrer rotation rate: from 39 to 82 rpm 
     Table 2 collates the characterization (bulk density, diameter and BET surface area) of the powders produced in accordance with Example 2. Alongside this, Table 2 also gives the amounts used of polyamide, core particles and ethanol, and also the stirrer rotation rate used in the process. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Characterization of the powders produced 
               
               
                 in accordance with Example 2 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Core 
               
               
                 RD 
                 SD 
                 d 50   
                 BET 
                 EtOH 
                 PA 
                 particles 
               
               
                 rpm 
                 g/L 
                 μm 
                 m 2 /g 
                 L 
                 kg 
                 kg 
               
               
                   
               
            
           
           
               
            
               
                 Exolit ® AP 422 in PA12/10.13 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 77 
                 419 
                 45 
                 6.3 
                 2500 
                 156/156 
                 94 
               
               
                 77 
                 425 
                 46 
                 7.2 
                 2500 
                 156/156 
                 94 
               
            
           
           
               
            
               
                 Exolit ® AP 422 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 39 
                 529 
                 66 
                 7.4 
                 2500 
                 348 
                 43 
               
               
                 39 
                 388 
                 61 
                 11.5 
                 2500 
                 261 
                 87 
               
               
                 39 
                 319 
                 56 
                 11.9 
                 2500 
                 243.5 
                 104.5 
               
            
           
           
               
            
               
                 Exolit ® OP 1230 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 40 
                 340 
                 42.8 
                 10 
                 2500 
                 313 
                 138 
               
               
                 40 
                 403 
                 31.8 
                 4.8 
                 2500 
                 313 
                 213 
               
               
                 39 
                 135 
                 69 
                 4.1 
                 2500 
                 348 
                 35 
               
               
                 39 
                 370 
                 61 
                 3.4 
                 2500 
                 348 
                 104 
               
            
           
           
               
            
               
                 Aeroperl ® 300/30 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 44 
                 321 
                 54.7 
                 31.6 
                 2500 
                 375 
                 44 
               
               
                 44 
                 336 
                 49 
                 32 
                 2500 
                 375 
                 44 
               
               
                 39 
                 272 
                 42 
                 30.2 
                 2500 
                 348 
                 35 
               
               
                 39 
                 296 
                 26 
                 58.4 
                 2500 
                 348 
                 105 
               
            
           
           
               
            
               
                 Martoxid ® MN/Y 216 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 53 
                 411 
                 67 
                 4.5 
                 2500 
                 375 
                 44 
               
               
                 53 
                 422 
                 65.2 
                 3.3 
                 2500 
                 375 
                 44 
               
               
                 39 
                 371 
                 67 
                 5.1 
                 2500 
                 348 
                 87 
               
               
                 39 
                 407 
                 67 
                 5.6 
                 2500 
                 348 
                 174 
               
               
                 82 
                 340 
                 37 
                 10 
                 2500 
                 348 
                 232 
               
               
                 39 
                 423 
                 55 
                 5.7 
                 2500 
                 348 
                 348 
               
            
           
           
               
            
               
                 Martoxid ® DN 206 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 39 
                 370 
                 56 
                 6 
                 2500 
                 348 
                 174 
               
            
           
           
               
            
               
                 Martoxid ® MDLS-6 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 39 
                 388 
                 43 
                 8.1 
                 2500 
                 348 
                 174 
               
            
           
           
               
            
               
                 Martoxid ® MZS-1 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 39 
                 321 
                 36 
                 10.1 
                 2500 
                 348 
                 174 
               
            
           
           
               
            
               
                 Stainless steel flakes 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 52 
                 312 
                 74 
                 8.3 
                 2500 
                 348 
                 87 
               
               
                 52 
                 297 
                 68 
                 8.4 
                 2500 
                 348 
                 150 
               
               
                 52 
                 298 
                 63 
                 8.6 
                 2500 
                 348 
                 232 
               
               
                 39 
                 327 
                 83 
                 7.3 
                 2500 
                 348 
                 150 
               
               
                 65 
                 277 
                 59 
                 10.1 
                 2500 
                 348 
                 150 
               
               
                 78 
                 339 
                 66 
                 7.7 
                 2500 
                 348 
                 39 
               
               
                 78 
                 352 
                 68 
                 6.4 
                 2500 
                 348 
                 17.5 
               
            
           
           
               
            
               
                 Aluminium powder 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 65 
                 381 
                 57 
                 4.6 
                 3480 
                 348 
                 87.5 
               
               
                   
               
               
                 SR = stirrer rotation rate; BD = bulk density