Supraparticles composed of dry-coated graphene particles, process for production thereof and use thereof

Abstract

The present invention relates to a process for producing supraparticles composed of dry-coated graphene particles by dry-coating graphene material, dispersing the particles obtained in a solvent, adding an additive to the dispersion, and at least partly removing the solvent from the dispersion thus obtained by spray drying to give supraparticles composed of dry-coated graphene particles

Description

[0001] Supraparticles composed of drv-coated qraphene particles, process for production thereof and use thereof

[0002] The present invention relates to a process for producing supraparticles composed of dry-coated graphene particles, to said supraparticles composed of dry-coated graphene particles themselves, and to the use thereof.

[0003] There are various discussions in the technical literature about graphene and the production, properties and uses thereof, for example in Rompp online (ROMPP editorial team, Balgar T, Graphene, RD-07-02758 (2010) in Bockler F., Dill B., Eisenbrand G., Faupel F., Fugmann B., Gamse T., Matissek R., Pohnert G., Ruhling A., Schmidt S., Sprenger G., ROMPP [Online], Stuttgart, Georg Thieme Verlag, [September 2022] https: / / roempp.thieme.de / lexicon / RD-07- 02758).

[0004] Just as in graphite, each carbon atom in graphene is linked covalently to three neighbouring atoms by way of a sigma bond. The C-C bond length is 142 pm. The atoms are sp2hybridized, and the sigma bonds lie within a single plane. Graphite accordingly has a planar structure. A partially filled pzorbital remains on each atom. These pzorbitals are orthogonal to the plane of the bonds and form a delocalized pi electronic system which is of primary importance in determining the electronic properties of graphene.

[0005] In crystallographic terms, graphene can be described via two equivalent sublattices with unit cell vectors a = b = 0.246 nm, the angle between these being 60°. The unit cell consists of two carbon atoms at the respective positions (0, 0) and (a / 3, 2b / 3). Atomic density is therefore 38.2 nm-2.

[0006] The process presented below can be performed with all graphenes specified hereinafter. The definition outlined above is applicable to these graphenes.

[0007] What is meant by “graphene material” in the context of the invention is material or materials according to ISO / TS 80004-13, namely graphene, graphenic carbon materials, mono-, bi- and trilayer graphene, epitaxial graphene, exfoliated graphene, few layer graphene, multilayer graphene, few layered nanoribbons, graphene nanoplate, graphene nanoplatelet, graphene nanosheet, graphene microsheet, graphene nanoflakes, graphene nanoribbon, graphene oxide, graphene oxide nanosheet, multilayer graphene oxide, graphene quantum dot, graphite, graphite nanoplate, graphite nanosheet, graphite nanoflakes, graphite oxide, reduced graphene oxide, or a mixture of these materials.

[0008] Fillers in the context of the invention are understood to mean boron nitride, nitrides in general, carbon-based materials (diamonds), aluminium-based materials, metal particles and alloys, TiO2, ZnO, MgO, precious metals and alloys thereof and metal salts, silica and silicates, SiC materials, layered materials (WS2, vanadium-based, silicon-based), composite components: fibres etc., retardants, impact modifiers, pigments and UV stabilizers. Graphene materials are used in a large number of technical fields. This is elucidated, for example, in application EP 21150690. Materials composed of graphene materials, for example what are called composite materials, likewise form part of the prior art.

[0009] WO 2016078664 A1 discloses composite materials in which a given macroscopic surface is endowed with structure-imparting substances using appropriately selected graphenes and with the aid of binder materials.

[0010] The polynorbornene / graphene oxide composite described in KR 1190014 B1 serves to adjust gas barrier properties and various mechanical parameters in layer structures.

[0011] KR 20190048574 outlines, by way of example, the production of a composite material including graphene oxide using a dispersion of 10 g / l in distilled water. Accordingly, only a proportion by weight of 1% by weight of graphene material is achieved. The graphene oxide particles have been partly agglomerated to particles having a remotely spherical shape and having diameters below 30 pm. Another portion is in the form of flakes of different topology. No free-flowing composite material has been obtained here.

[0012] In the paper by S. Wintzheimer et al., "Supraparticles: Functionality from Uniform Structural Motifs", ACS NANO, 12 (2018), 5093-5120, nanoparticles form the building block for supraparticles. Using the controlled agglomeration of exclusively such individual nanoparticles, supraparticles having certain electronic, plasmonic, magnetic and / or photonic properties are obtained. In order to bring about said agglomeration, van der Waals forces or electrostatic forces that exist between the nanoparticles are utilized with the aid of spray drying, or alternatively nanoparticles are combined in an environment in which the nanoparticles are formed by means of sol-gel processes, or sonochemical methods are implemented. Especially in the case of spray drying, there is no discussion either of geometric peculiarities of the apparatus used for the purpose or of any associations with the properties of the products thus agglomerated.

[0013] Graphene materials are obtainable commercially as powders and often have very low bulk densities, for example in the range between 2 and 400 g / l. Alongside the low bulk densities, most graphene materials also have poor flowability and / or generate a lot of dust during transfer by gravity-driven flow. This leads to poor handling properties and to problems during weighing-out and metering, and must also be considered critical in relation to aspects involving protection of the environment and safety of operators.

[0014] Poor handling properties are apparent by way of example when the powders are incorporated in elastomeric systems, as is the case during the kneading of rubber. Production of a good filled rubber compound is contingent on incorporation of pulverulent fillers at the correct juncture and over the correct duration. These are poured into the mixing chamber by means of a hopper and then pushed in the direction of rotating rollers by a pneumatic piston. The shear forces in play during such a mixing process break up agglomerates of the filler and thus contribute to its distribution. The maximum filler level that can be achieved is therefore substantially determined by the shear forces acting on it. The creation of fine graphite or graphene agglomerates in electrode production, for example, is known. However, this conventional material, because of the size of the agglomerate particles, is not sufficiently free-flowing to enable use thereof in conventional compounders or extruders.

[0015] It is likewise known that spray drying process can be utilized for optimization of powder systems in that the spray drying of graphene dispersions, for example, leads to round agglomerates. However, powders composed of graphene materials having improved flowability have not been found to date.

[0016] In order to characterize the flowability of a material, a number of test methods are employed in the context of the invention, namely the measurement or determination of angle of repose according to ISO 4324, dynamic avalanche angle, and the Hausner factor according to ASTM 527.

[0017] A further measurement value in the context of the invention is the dust value according to DIN 55992.

[0018] The abovementioned parameters are well known to the person skilled in the art and have been ascertained by the test methods outlined below. Angle of repose and avalanche angle are expressed here in degrees (°).

[0019] The better the values of at least two, and even better three, of these values for the material, which means low angle of repose, low dynamic avalanche angle, low Hausner factor, the better the flowability of the material. The smaller the dust value, moreover, the less dust is obtained in the processing of the material used in the context of the invention, i.e. that presented and claimed here.

[0020] The terms “dust value” and “dust evolution” are synonymous in the context of the invention. Conventionally produced graphene powders are cohesive, which is synonymous with high measurement values, and hence have only low flowability and high evolution of dust. Such powders are processible with difficulty or only with onerous technical safety precautions, and only low bulk densities are achieved when dust values are high.

[0021] There is thus a need for a technical approach that provides pulverulent graphene material with improved flowability.

[0022] There is likewise a continuing need for reduction in the amounts of dust formed. This is because, firstly, dusts lead to contamination and make it difficult to measure the quantity of filler that has actually been introduced into the matrix system. Secondly, for reasons relating to occupational hygiene and to protection of the environment, it is essential that dusts be removed by suction or by other means.

[0023] Furthermore, the dusts and the generally low bulk density of the graphene material increase the cost of introducing the said material into the desired matrix system. Corresponding problems arise by way of example when a thermoplastic compounded material filled with a graphene material is produced in an extruder. For the said reasons it is difficult or impossible to ensure compliance with essential chronological requirements in the introduction of pulverulent graphene material during the process for production of thermoplastic systems.

[0024] Moreover, the electrical conductivity of graphene, which is known to the person skilled in the art, results in limitation of the use of graphene materials in electrically insulating composite materials. For this purpose, in addition, an electrically insulating material is required. However, the use of such a material has the disadvantage that the thermal conductivity of graphene is additionally limited. There is thus a need to efficiently connect a graphene material to an insulator in order to achieve a complete electrical insulating effect with high thermal conductivity.

[0025] It was therefore an object of the invention to improve the method of spray drying for the production of a powder including free-flowing graphene material, wherein the graphene material, or the compound or masterbatch thereof, has improved flowability and / or less dust is obtained in the processing thereof. The resultant graphene material was additionally to be thermally conductive and simultaneously electrically insulating.

[0026] The object was achieved by the providing of a process for producing supraparticles composed of dry-coated graphene particles by

[0027] (a) dry-coating at least one graphene material with a material selected from SiO2, aluminium oxide, TiO2, MgO, ZnO, SbO, organic filler, or selected from polymers, and subsequently

[0028] (b) dispersing the dry-coated graphene material obtained in step (a) in a solvent, and simultaneously or subsequently

[0029] (c) adding to the dispersion obtained in step (b) 0.1-150% by weight, preferably 0.1-100% by weight based on the mass of graphene material used dispersing aids or emulsifiers or wetting aids and / or defoamers, and subsequently

[0030] (d) at least partly removing the solvent from the dispersion obtained in step (c) by spray drying, to obtain supraparticles composed of dry-coated graphene particles.

[0031] It has been found that, surprisingly, dispersions of a dry-coated graphene material which have been dry-coated with a material selected from SiO2, aluminium oxide, TiO2, MgO, ZnO, SbO, organic filler, or selected from polymers, and preferably contain dispersing aids or emulsifiers, or wetting agents and / or defoamers, can be spray-dried, with achievement of improved flowability, obtaining of the dry-coated graphene material, and the desired properties of thermal conductivity and electrical insulation of the spray-dried material.

[0032] During the drying operation, the surface of the particles of the dry-coated graphene material is or has been wetted and stabilized with the additives. The spray drying, by means of agglomeration, affords supraparticles of a dry-coated graphene material.

[0033] The agglomerated dry-coated graphene particles obtained after step (d) are the supraparticles composed of dry-coated graphene particles. These are called “supraparticles composed of dry- coated graphene particles” in the context of the invention. These supraparticles composed of dry- coated graphene particles preferably have a spherical or nearly spherical shape and a particle diameter dso of 1 pm to 500 pm, preferably of 5 pm to 250 pm, more preferably of 50 pm to 100 pm.

[0034] In the context of the invention, particle diameter is measured using a Partica Laser Scattering particle size analyzer LA-950V2 from Rentsch Technology. All particles were analysed in water. The pumped circulation rate was set here at 6 and the stirrer speed at 6. For analysis of the starting materials, ultrasound was set at level 6 for 1 min. Supraparticles were measured in a 0.001 % by weight soap solution in order to stabilize them.

[0035] The process according to the invention has the advantage that the supraparticles composed of dry- coated graphene particles thus obtained have improved flowability over conventional graphene particles. It has now been found that supraparticles composed of dry-coated graphene particles have good flowability only over and above particle diameters dso of at least 40 pm. Supraparticles composed of dry-coated graphene particles having particle diameters dso of at least 50 pm, preferably at least 70 pm, have particularly good flowability.

[0036] In this context, the additive used, such as dispersing aids, emulsifiers, wetting agents and / or defoamers, facilitates the handling of the dispersion without affecting the intrinsic properties of the coated graphene material.

[0037] It has been found that electrical insulation can be achieved by the selection of suitable materials for dry-coating.

[0038] As well as improved flowability, the bulk density of the supraparticles composed of dry-coated graphene particles obtained is likewise elevated. Moreover, the material obtained by the process can be easily metered and added to any conventional matrix material. In the processing of the supraparticles composed of dry-coated graphene particles obtained in accordance with the invention, the dust value is additionally relatively low. Operational safety is thus increased. Furthermore, the usability of the further-processed material is maintained. For example, in a conventional compounder, the supraparticles composed of dry-coated graphene particles can be broken down into the dry-coated graphene materials using the active shear forces in a matrix material, preferably a polymer, a monomer or a solvent, which maintains the original physical properties.

[0039] The invention likewise provides supraparticles composed of dry-coated graphene particles which are characterized in that they have an angle of repose according to ISO 4324 of 50° to 20°, preferably of 40° to 25°, further preferably of 37.5° to 30°, especially preferably of 37° to 27.5°, and a dynamic avalanche angle of 65° to 30°, preferably of 55° to 35°, most preferably of 45° to 40°, and / or a Hausner factor according to ASTM 527 of 1 .5 to 1 , preferably of 1 .4 to 1 .1 , more preferably of 1 .3 to 1 .2, or a dust value according to DIN 55992 type I of 10 to 0.001 , preferably of 5 to 0.01 , more preferably of 3 to 0.1 .

[0040] The supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention provide a free-flowing but nevertheless non-dusting powder.

[0041] The supraparticles composed of dry-coated graphene particles according to the invention preferably have the features claimed simultaneously. Thus, the supraparticles composed of dry- coated graphene particles have an angle of repose according to ISO 4324 of 50° to 20° and a dynamic avalanche angle of 65° to 30°, and a Hausner factor according to ASTM 527 of 1 .5 to 1 , and a dust value according to DIN 55992 type I of 10 to 0.001 . These supraparticles preferably have diameters dso of 1 pm to 500 pm.

[0042] The supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention, for the purpose of classification, can be collected in three different vessels and accordingly divided into three sizes.

[0043] What is called the “coarse matter"’, after step (d) of the process according to the invention, can be collected vertically in a collecting vessel downstream of the drying chamber. The powder including the supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention experiences a correspondingly low level of shear forces during the process, which means that few fragments are obtained.

[0044] Downstream of the drying chamber, the powder can be introduced into a cyclone. What is called the “fines fraction” is separated here from what is called the “cyclone product’. The cyclone product was subjected to a high shear force by virtue of the centrifugal force, as a result of which the supraparticles composed of dry-coated graphene particles in particular were prone to fall apart.

[0045] The fines fraction can be collected in a filter mat downstream of the cyclone. The invention also provides for the use of the supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention in the following applications:

[0046] - thermally conductive materials and thermally conductive adhesives such as films, underflow materials, castable electronic materials, materials having phase transitions, thermal pastes, encapsulating compounds, especially in the field of batteries, sensors, ICs and LEDs,

[0047] - composites based on thermoplastics, thermosets and / or elastomers having elevated electrical conductivity and / or thermal conductivity or for EMI shielding, especially in the field of housings of electrical components, motors, battery packs and pipelines,

[0048] - coatings and varnishes having thermal and / or electrical conductivity, or for applications in the EMI shielding sector,

[0049] - thermally and / or electrically conductive oils, cooling liquids or inks.

[0050] In particular, the supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention are suitable for use in thermoplastics selected from standard thermoplastics, preferably PE, PP, PS, PVC, alpha-olefins, butadiene derivatives and / or Vestenamer®, industrial thermoplastics, preferably PET, PMMA, PC, POM, PA, PBT, PEBA, TPU, PU and / or TPE, high-performance thermoplastics, preferably PPS, PEEK, PES, PI and / or PEI, copolymers, elastomers, preferably silicones, further preferably room temperature crosslinking (RTV) silicone, high temperature crosslinking (HTV) silicone, liquid silicone rubber (LSR), heat crosslinking rubber (HCR), acrylates and / or pastes containing poly- and oligosiloxanes, polyurethanes, rubbers, preferably styrene-butadiene rubber (SBR), butadiene rubber (BR) and / or natural rubber, thermosets, preferably polyurethanes, polyester resins, phenolic resins, epoxy resins, acrylate resins and / or silicone resins, solvents, preferably aprotic-nonpolar, aprotic-polar and / or protic solvents, oils, preferably mineral oils, silicone oils and / or process oils.

[0051] More preferably, the supraparticles composed of dry-coated graphene particles according to the invention or obtained in accordance with the invention find use as additives in the plastics processing industry. They are preferably used in the compounding, the extruding or the injection moulding of plastics.

[0052] The advantages of the uses claimed are that the compounding of graphenes is in some cases actually enabled. Furthermore, the inventive use permits the simple and reliable processing of graphenes, and also what is called “Rapid injection molding” (RIM).

[0053] The use enables the efficient use of high-performance fillers such as graphenes without impairing the high-performance characteristics of the matrix, for example in the case of PA12.

[0054] When used as additives, the supraparticles composed of dry-coated graphene particles enable an improvement in thermal, electrical and / or mechanical properties, for example in the degree of extrusion in the case of high-performance polymers with high-performance applications in which the filler content has to be kept low to maintain the characteristics of the matrix.

[0055] The use of the supraparticles composed of dry-coated graphene particles according to the invention also achieves an advantageous lubricating effect for simple compounding and extrusion that would otherwise be difficult with fillers. In addition, electrical conductivity is improved.

[0056] Further advantages will arise in the course of handling in masterbatching for thermoplastics, epoxies, elastomers and / or quite generally in the handling of concentrates called masterbatches.

[0057] When used in gas membranes or gas-conducting systems, there is likewise a decrease in proneness to leaks.

[0058] The invention is more particularly elucidated hereinbelow.

[0059] The process according to the invention is shown schematically in Fig. 2.

[0060] In step (a) of the process according to the invention, graphene material is dry-coated with a further material selected from SiO2, aluminium oxide, TiO2, MgO, ZnO, SbO, organic filler, or selected from polymers. In the context of the present invention, any process known to those skilled in the art is suitable for dry-coating, for example coating by means of a high-energy mixer. The starting material used may, for example, be aluminium oxide in powder form.

[0061] The particle dispersion obtained after step (b) preferably have a particle diameter dso of 0.1 pm to 100 pm, further preferably 0.5 pm to 60 pm, more preferably of 2 pm to 40 pm.

[0062] The graphene material in step (a) having particle size diameter dso equal to the graphene material obtained in step (b).

[0063] The graphene particles at least one order of magnitude larger than the aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers.

[0064] The diameter dso of the aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers used in step (a) of the process according to the invention is smaller than the diameter dso of the initial graphene particles, where the diameter dso of the aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers used in step a of the process according to the invention is further preferably smaller at least by a factor of 10, even further preferably at least by a factor of 20, most preferably at least by a factor of 50, than the diameter dso of the graphene particles used for dry-coating. The aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers used may advantageously have a diameter dso of 500 nm or less.

[0065] Preferably in accordance with the invention, the aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers used are hydrophobic material. The aluminium oxide particles, SiO2 particles, TiO2 particles, MgO particles, ZnO particles, SbO particles, polymer particles or organic fillers used in accordance with the invention thus preferably, by contrast with hydrophilic material, do not have (surface) modifications that increase the water solubility thereof.

[0066] Because of the improved material properties, it is advantageously possible to break down the supraparticles composed of dry-coated graphene particles obtained in step (d) in a compounder into the dry-coated graphene materials using the active shear forces in a matrix material, preferably a polymer, a monomer or a solvent, while maintaining the original physical properties.

[0067] In step (b), organic fillers used may be polymers selected from PE, PP, PS, PVC, alpha-olefins, butadiene derivatives, Vestenamer® (rubber additive from Evonik, Essen, Germany), industrial thermoplastics, preferably PET, PMMA, PC, POM, PA, PBT, PEBA, TPU, PU, TPE, high- performance thermoplastics, preferably PPS, PEEK, PES, PI, PEI.

[0068] In the context of the invention, inorganic fillers are likewise considered to include salts that are insoluble in the solvent used in step (a).

[0069] It may further be advantageous to select the solvent in step (b) from water, from distilled water, from alkanol, preferably ethanol.

[0070] The solvent may be selected from hexane, chlorobenzene, toluene, tetrachloromethane, dichloromethane, water, distilled water, ethanol or a mixture of these solvents.

[0071] These and other solvents that are suitable for the graphene material and the additive are known to the person skilled in the art.

[0072] It may be advantageous in the process according to the invention to produce the dispersion batchwise or continuously. In the case of a batchwise formulation in step (b), the graphene material may be weighed out and added gradually to the solvent. It is possible here to use a suitable stirrer unit, for example an ultrasound finger or an Ultraturrax. It is observed that the viscosity of the dispersion rises with increasing proportion by mass of graphene material.

[0073] More preferably, a homogeneous dispersion is obtained in step (b). The properties of the dispersion are very important in the performance of step (d) of the process. This is because the size of the supraparticles composed of dry-coated graphene particles is affected by the viscosity, surface tension, density and proportion by mass of the dry-coated graphene material, and the geometry of the drying chamber.

[0074] As described above, in step (d) of the process according to the invention, the dispersion obtained in step (c) is subjected to spray-dryingThe drying method in step (d) preferably comprises the following steps: spraying the dispersion obtained in step (c) by means of a spraying unit into an inert gas stream, with at least partial evaporation of the solvent of the droplets formed by the spraying.

[0075] In the spray-drying method the dispersion composed of graphene material and additive or additives is atomized via the application of shear forces. In the course of this, there is initial film formation and subsequent separation of the film into numerous droplets. The droplets here surround a hot gas stream, preferably inert gas, more preferably nitrogen, and at a temperature preferably within the range from 80 to 400°C. The temperature may be adjusted according to the solvent.

[0076] In the context of the invention, the dry-coated graphene material particles according to the invention or obtained in accordance with the invention are called “supraparticles composed of dry- coated graphene particles” when they are in agglomerated form.

[0077] In step (d), the solvent of the droplets evaporates and is taken up by the gas stream. This cools the gas. The evaporation of the solvent results in a continuous decrease in volume of the droplets. Accordingly, there is a rise in the proportion by mass of graphene material and of the additive(s) in the droplet as it passes through the gas stream. The drying of the droplets and hence the decreasing volume is linear at first, until what is called the “locking point’ is attained. At the locking point, a hard crust is formed, while the core of the supraparticle composed of dry-coated graphene particles that forms is moist, meaning that residues of solvent are present in the core of the supraparticle composed of dry-coated graphene particles as well as the additive. The further drying is slower since the moisture has to get from the core to the surface. The drying rate can be adjusted via the temperature of the inert gas stream during step (d) of the process.

[0078] If this is very high, it is possible to obtain a hollow supraparticle composed of dry-coated graphene particles.

[0079] If the drying rate is low, the result is generally a solid supraparticle composed of dry-coated graphene particles.

[0080] In this second case, it is necessary to pay attention to the required residence time in the drying chamber since drying will otherwise be incomplete.

[0081] In addition, the particle diameter of the supraparticles composed of dry-coated graphene particles can be adjusted via the drying rate. Further means of influencing the size of the supraparticles composed of dry-coated graphene particles are that of the selection of the spraying unit, which is preferably a two-phase nozzle, and the selection of the ratio of the mass flows of the gas to the sprayed dispersion, called the air-to-liquid ratio.

[0082] Without being bound to a particular theory, there may be various routes 1-4 for the formation of the supraparticles composed of dry-coated graphene particles during the performance of the process according to the invention, shown schematically in Fig. 3. The routes depend upon factors including the temperature of the inert gas stream. According to the route that has been followed after the performance of step (c), supraparticles composed of dry-coated graphene particles have a respectively different shape F, J, P with properties that differ in pairs.

[0083] The performance of steps (a) and (b) of the process according to the invention creates dry-coated graphene particles in dispersion. As a result, the process according to the invention in step (c) yields supraparticles composed of dry-coated graphene particles which surprisingly do not have a core-shell structure. This contrasts noticeably with the results from conventional technology. This is because the known manufacturing processes yield supraparticles with a core-shell structure if the primary particles used have distinctly different diameters.

[0084] The dispersion can be atomized, for example, via a two-phase nozzle. In this case, the shear energy required is applied via a gas stream which is greatly accelerated in a constriction. Depending on the gas volume flow rate and hence gas velocity at the nozzle outlet, higher or lower shear forces are applied. This alters the droplet size and droplet size distribution of the dispersion.

[0085] It is possible to use proportions by mass of 5% to 50% by weight of graphene material based on the solvent. The proportion by mass of additive used in % by weight is based on the mass of graphene material used.

[0086] The additives may be dispersed with the aid of a suitable stirrer unit. In order to prevent demixing, the dispersion is preferably stirred continuously.

[0087] In the case of continuous preparation in step (b), the graphene material may be added via a suitable machine, preferably by conveying screw, to a mixture of solvent and additive. If two phases are obtained, these can be pumped through a dispersing chamber. A corresponding stirrer unit is required for the necessary energy input. The selection of dispersion chambers and the energy input to be established are known to the person skilled in the art. The dispersion is then fed continuously to the stirrer unit.

[0088] In step (c) of the process according to the invention, the at least one additive may be selected from the group of dispersing aids, emulsifiers or wetting agents and / or defoamers.

[0089] At least one additive may be used in a proportion by mass of the respective additive of 0.1% to 150% by weight, preferably of 0.1% to 100% by weight, more preferably of 0.1% to 10% by weight based on the mass of graphene material used.

[0090] The at least one wetting or dispersing aid may preferably be selected from copolymers based on oxyalkylene glycol alkenyl ethers or polyalkylene oxide alkenyl ethers and unsaturated dicarboxylic acid derivatives. Such copolymers are disclosed in patent specification EP 114292 B1 and assure, for example, the production of binder-, cosolvent- or VOC-free pigment concentrates based on transparent iron oxides.

[0091] In step (d) of the process according to the invention, the spraying unit may be an apparatus having at least one two-phase nozzle and / or the inert gas used may be nitrogen, preferably dried nitrogen, the temperature of the inert gas stream may further preferably be set at from above the cooling limit temperature of the solvent to 200°C to 400°C, further preferably within a range from 220 to 300°C, and / or the spray droplets may have a size of 30 to 1000 pm.

[0092] The size of the sprayed droplets results from the atomizer force which is known to the person skilled in the art, and which can be adjusted with the shear effect by the nitrogen flow rate or by the atomizer gas flow rate. The invention is elucidated in detail by examples without restricting the subject-matter of the invention.

[0093] SEM images shown below were evaluated with the aid of the JEOL NeoScope by scattering the pulverulent material to be examined onto an adhesive graphene film and cautiously removing excess material with a set of bellows. The SEM images shown in the context of the invention were recorded at 10 kV.

[0094] Hausner factor according to ASTM 527

[0095] The Hausner factor was determined according to ASTM 527. This is also a measure of the compactability of a powder.

[0096] In the determination of the Hausner factor, the ratio between the bulk density and tamped density is formed. This is done by introducing 100 g of the powder material into a cylinder. Depending on the bulk density, it is possible to use a 100 ml or 250 ml cylinder. The powder is introduced gradually and cautiously. The volume of the loose bed is read off, and this is the bulk volume. The quotient of the mass of powder introduced and the bulk volume is the bulk density.

[0097] Subsequently, the powder is tamped until no change in volume can be seen. The volume which is then present is the tamped volume, which is read off.

[0098] The quotient of the mass of powder introduced and the tamped volume is the tamped density.

[0099] The Hausner factor calculated from bulk density and tamped density can be classified into various classes of assessment, shown in Table 1.

[0100] Table 1. Classification of Hausner factors

[0101] Dynamic avalanche angle, measured with the Revolution Powder Analyzer

[0102] Dynamic avalanche angle is likewise a measure of the flowability of a powder and can be measured directly. In the context of the invention, the Revolution Powder Analyzer, model Rev2015, from PS Prozesstechnik GmbH was used. This involved, as shown by Figure 4a, charging a defined quantity into a rotating drum, and using a camera to determine the angle at which the material began to form avalanches. A smaller avalanche angle (ava,h) between the surface formed by the particles of the horizontal ( / ?) at the juncture of the avalanches indicates better flowability of the material. The avalanche angle was measured between (ava) and the horizontal ( / ?) in the mathematically positive direction. The mode of operation of the Powder Analyzer is known to the person skilled in the art, as also are the variables measured therewith. Details can be found for example in the following article by Amado: "Advances in SLS powder characterization", 22nd Annual International Solid Freeform Fabrication Symposium - An Additive Manufacturing Conference, SFF, 2011 , pp. 438-452.

[0103] 100 ml of powder material was weighed out and introduced into a glass chamber. The chamber rotated at 0.5 rpm, which formed an angle. On attainment of the maximum of this angle, the dynamic avalanche angle, the powder began to flow down the slope. A camera recorded the powder movement, and hence the dynamic avalanche angle was ascertained continuously. In addition, the avalanche energy of the powder flowing downward was calculated.

[0104] The dynamic avalanche angle may vary from 70° for very poorly flowing powder to 30° for very efficiently flowing powder.

[0105] The measurement parameters of the flowability program corresponded to the standard settings for dark powder:

[0106] • 0.5 rpm

[0107] • 150 avalanche onsets

[0108] • avalanche threshold 0.65%

[0109] • Camera: shutter speed 6 ms, gain 6 dB (black powder), 10 frames per second

[0110] Homogeneity and flow properties can be calculated directly via the standard deviation of the dynamic avalanche angle and the avalanche energy.

[0111] Slope angle according to ISO 4324

[0112] The measurement of the slope angle gives a further statement as to the flowability of a powder.

[0113] In the context of the invention, 100 ml of powder material was introduced into a funnel having a 10 mm outlet. The outlet was closed. The funnel was secured at a distance of 7.5 cm from a plate having a diameter of 10 cm and a height of 25 cm. The funnel outlet was opened, and the powder flowed onto the plate. The powder thus formed a cone having the slope angle of interest.

[0114] The slope angle can be classified into various classes of assessment, shown in Table 2. Table 2. Slope angle and assessment of flowability

[0115] Scanning electron microscope (SEM)

[0116] For the SEM images disclosed here, the FlexSEM 1000 II VP-SEM was used. For this purpose, the powder to be examined was scattered onto an adhesive graphene film. Excess material was cautiously removed with a set of bellows. The images were recorded at 10 kV.

[0117] Electrical insulation

[0118] Electrical insulation was measured with an FE50 ring electrode and a Mili-TO 3 ohmmeter (Fischer Elektronik) according to ISO 62631 .

[0119] Thermal conductivity

[0120] Thermal conductivity was determined in the equilibrium state according to ASTM E1530-19 with a guarded hot plate (Thermtest GHFM-02) using two-dimensional samples having a thickness of about 10 mm that were contacted with a thermal joint compound of the 120 silicone type (Wakefield-Vette). The upper plate was set to 35°C, while the lower plate was set to 15°C. The measurements were commenced after an equilibration time of 60 min.

[0121] Devices used in the examples:

[0122] Somakon MP-GL laboratory mixer, Buchi B290 spray dryer, UltraTurrax I KA T50. The spray dryer used is shown in schematic form in Fig. 4b.

[0123] Example 1. First Graphene Pure Graph 20

[0124] The graphene material used was First Graphene pure graph 20 (from First Graphene, Henderson, Australia).

[0125] Fig. 5 shows SEM images of the graphene material. This is in the form of platelets, with the platelets partly in the form of agglomerates.

[0126] Fig. 6 shows the corresponding particle size distribution.

[0127] The following characteristic diameters are measured here: dio 13 pm, dso 22 pm, dgo 40 pm. Dry-coating:

[0128] First Graphene pure graph 20 and hydrophobic nanoparticulate aluminium oxide are first introduced into a high-energy mixer (Somakon) in a mass ratio of 80 / 20 and dry-coated for 20 min at 2000 rpm.

[0129] Production of the dispersion:

[0130] The dispersion consisting of water, graphene-based material and nanoparticulate aluminium oxide is produced as follows.

[0131] A suitable additive, TEGO® Antifoam KS 53 (from Evonik, Essen, Germany), is dissolved in water. The dry-coated powder is gradually dispersed in the solution. Care is taken here to ensure that no lumps are formed. Subsequently, the dispersion is dispersed with the aid of the H7 ultrasound finger. An amplitude of 100% and a cycle of 0.75 are set on the device. Dispersion is first effected for 5 min. Then the vessel is closed and shaken vigorously for 30 s. The procedure is repeated a total of 3 times.

[0132] Once a homogeneous dispersion is attained, it is stirred to prevent segregation. This is done using a stirrer plate and a football-shaped stirrer bar at 600 rpm.

[0133] Even during step (c), the dispersion was stirred to prevent separation of the dispersion. The process parameters during step (c) were: inlet temperature: 200°C, outlet temperature: 92°C, nozzle gas flow rate: 244 l / min, aspirator power 50% corresponding to about 20 m3 / h, pump power 25% corresponding to about 6.5 ml / min, air-to-liquid ratio about 0.78.

[0134] Since the supraparticles composed of dry-coated graphene particles obtained occurred in the form of loose agglomerates that were held together only by van der Waals forces, these could have been destroyed in the case of moderate shear forces. Such forces occurred, for example, in the cyclone of the spray dryer. Therefore, the supraparticles composed of dry-coated graphene particles were drawn off directly beneath the spray chamber, and any particles downstream of the cyclone were discarded. These were only fragments of supraparticles.

[0135] Figs. 7-9 show SEM images of the supraparticles.

[0136] Fig. 7 shows the surface structure of the supraparticles composed of dry-coated graphene particles. It is clear here that supraparticles consist of many dry-coated graphene particles which are agglomerated in a controlled manner during the spraying process to form round supraparticles composed of dry-coated graphene particles. Fig. 8 shows that there is a broad range of particle size, which does not have an adverse effect in respect of the processibility of the graphene material. Fig. 9 shows the internal structure of the supraparticles composed of dry-coated graphene particles. It can be seen here that there is no separation of the particles according to their size in the spraying process.

[0137] The particle size distribution of the supraparticles composed of dry-coated graphene particles produced in accordance with the invention is apparent from Figs. 7 and 8. The supraparticles composed of dry-coated graphene particles have diameters in the order of magnitude of more than 40 pm.

[0138] Example 2: Flowability and dust content

[0139] Flowability

[0140] For quantitative evaluation of the flowability of the supraparticles composed of dry-coated graphene particles obtained in Example 1 , the following were measured in each case: Hausner factor in accordance with ASTM 527, dynamic avalanche angle and slope angle according to ISO 4324.

[0141] The tamped densities and bulk densities of relevance to the Hausner factor from Example 1 are shown by Table 3.

[0142] Table 3: Tamped density, bulk density and resulting Hausner factor of the graphene material and of the supraparticles composed of dry-coated graphene particles produced in accordance with the invention

[0143] Comparison of the bulk densities and tamped densities for the supraparticles composed of dry- coated graphene particles produced in accordance with the invention with graphene-based material showed a distinct rise after spray drying.

[0144] In the case of the supraparticles composed of dry-coated graphene particles produced in accordance with the invention, a lower Hausner factor and hence lower compatibility was found. Table 4 shows the slope angle according to ISO 4324 and the dynamic avalanche angle for the graphene material used by comparison with the values for the supraparticles composed of dry-coated graphene particles.

[0145] Table 4: Slope angle and dynamic cone angle

[0146] Graphene-based material showed such poor flow properties, and significant smearing in the chamber, that optical evaluation of the dynamic avalanche angle was not possible.

[0147] It has thus become clear that the process according to the invention gives supraparticles composed of dry-coated graphene particles having better flowability.

[0148] Determination of dust value according to DIN 55992-1 (June 2006 version)

[0149] Dust value in the context of the invention is determined with a Heubach Dustmeter type I dust evolution device by the rotation method according to DIN 55992 (June 2006 version), shown in schematic form in Fig. 1. The construction details of this device are known to the person skilled in the art.

[0150] For the determination, 5 g of powder material as sample weight was introduced into the chamber of the Dustmeter. The chamber was equipped with paddles, which kept the powder in motion. The filter unit was weighed before the start of the measurement and after the measurement. A constant gas flow rate of 20 l / min was run across the powder. The chamber rotated at 30 revolutions per minute.

[0151] The dust value is calculated from the ratio of the mass of powder in the filter after the experiment to the initially charged mass of the powder in the chamber, meaning the mass of dust released from the sample weight by the dust evolution device with standard settings.

[0152] The mass of dust released from the sample weight by the dust evolution device with standard settings is based on the sample weight and reported in % by weight.

[0153] Table 5 shows the measured dust value results. Table 5: Dust value of the graphene material or of the supraparticles composed of dry-coated graphene particles produced in accordance with the invention therefrom

[0154] It was found that the supraparticles composed of dry-coated graphene particles according to the invention had a lower dust value than untreated graphene-based material. In the case of the particles according to the invention, the stability of the supraparticles composed of dry-coated graphene particles was increased, especially at the surface, since interstices of smaller particles of the graphene materials used were filled.

[0155] Example 3: Effect of the aluminium oxide used

[0156] Supraparticles composed of dry-coated graphene particles were produced in accordance with Example 1 , except that a different commercially available aluminium oxide was used. The measurements for tamped density, bulk density, Hausner factor, slope angle and dynamic avalanche angle of the supraparticles composed of dry-coated graphene particles according to Example 2 that were conducted according to Example 1 did not show any significant variance from the measurement values for the supraparticles composed of dry-coated graphene particles according to Example 1 .

[0157] Example 4: Thermal conductivity and electrical insulation

[0158] Inventive supraparticles composed of dry-coated graphene particles according to Example 1 and particles of First graphene pure graph 20 and the aluminium oxide used in Example 3 were each introduced separately in an amount of 20% by weight into an epoxy resin matrix, and the product obtained was examined by the test methods described above for thermal conductivity and electrical insulation. The results are shown in Fig. 10. As can be seen from this figure, the supraparticles composed of dry-coated graphene particles according to the invention have a higher sheet resistance by several orders of magnitude compared to uncoated First graphene pure graph 20, whereas no significant decrease in thermal conductivity was observed. By comparison with pure aluminium oxide, a distinct improvement in thermal conductivity was achieved. It has additionally been found that, surprisingly, electrical insulation was additionally increased compared to pure aluminium oxide.

[0159] Supraparticles composed of dry-coated graphene particles produced by the process according to the invention thus have improved electrical insulation without having poorer thermal conductivity. Brief Description of the Drawings

[0160] Fig. 1 : Heubach Dustmeter type I dust evolution device.

[0161] Fig. 2: Schematic diagram of the preferred process according to the invention.

[0162] Fig. 3: Schematic diagram of hypothetical formation of the supraparticles composed of dr<-coated graphene particles F, J, P according to the invention after performance of step c.

[0163] Fig. 4a: Principle of function of the Revolution Powder Analyzer (schematic).

[0164] Fig. 4b: Schematic diagram of the spray-dryer used with preference.

[0165] Fig. 5: First Graphene pure graph 20 starting material.

[0166] Fig. 6: Particle size distribution of: First Graphene pure graph 20 starting material measured in water, stirrer speed 6, ultrasound for 1 min, circulation speed 6.

[0167] Fig. 7: Surface structure of spray-dried First Graphene pure graph 20 (80 wt%) + aluminium oxide (20 wt%) + 5 wt% additive supraparticles composed of dry-coated graphene particles.

[0168] Fig. 8: Spray-dried First Graphene pure graph 20 (80 wt%) + aluminium oxide (20 wt%) + 5 wt% additive supraparticles composed of dry-coated graphene particles.

[0169] Fig. 9: Internal structure of spray-dried First Graphene pure graph 20 (80 wt%) + aluminium oxide (20 wt%) + 5 wt% additive supraparticles composed of dry-coated graphene particles.

[0170] Fig. 10: Comparison of sheet resistance to thermal conductivity of inventive supraparticles composed of dry-coated graphene particles (■) with primary particles of pure First Graphene pure graph 20 (♦) and pure aluminium oxide (A), in each case 20% by weight of particles in epoxy resin.

Claims

Claims1 . Process for producing supraparticles composed of dry-coated graphene particles, by(a) dry-coating at least one graphene material with a material selected from SiO2, aluminium oxide, TiO2, MgO, ZnO, SbO, organic filler, or selected from polymers, and subsequently(b) dispersing the dry-coated graphene material obtained in step (a) in a solvent, and simultaneously or subsequently(c) adding to the dispersion obtained in step (b) 0.1-150% by weight, preferably 0.1-100% by weight based on the mass of graphene material used, of dispersing aids or wetting aids or emulsifiers and / or defoamers, and subsequently(d) at least partly removing the solvent from the dispersion obtained in step (c) by spray drying, to obtain supraparticles composed of dry-coated graphene particles.

2. Process according to Claim 1 , wherein the spray drying in step (d) comprises the following: spraying the dispersion obtained in step (c) by means of a spraying unit into an inert gas stream, with at least partial evaporation of the solvent of the droplets formed by the spraying.

3. Process according to Claim 1 or 2, additionally comprising step (e) which comprises(e) deagglomerating the resultant supraparticles composed of dry-coated graphene particles by the action of a shear force in a matrix material, preferably a polymer, monomer or solvent, whereby the dry-coated graphene material are homogeneously distributed in the matrix material.

4. Process according to Claim 1 , wherein the solvent in step (b) is selected from water, distilled water and alkanol, preferably ethanol.

5. Process according to any of the preceding claims, wherein the supraparticles composed of dry-coated graphene particles having a diameter dso of 1 pm to 500 pm, preferably 5 pm to 250 pm, particularly preferably of 50 pm to 100 pm, are obtained.

6. Process according to any of the preceding claims, wherein the proportion by mass of graphene in the dispersion used in step (b) is 5% to 50% by weight.

7. Process according to any of Claims 1 to 6, wherein the spraying unit in step (d) is an apparatus having at least one two-phase nozzle, and / or the inert gas used is nitrogen, preferably dried nitrogen, the temperature of the inert gas stream is further preferably set at from above the cooling limit temperature of the solvent to 200°C to 400°C, further preferably within a range from 220 to 300°C, and / or the sprayed droplets have a size of 30 to 1000 pm.

8. Supraparticles composed of dry-coated graphene particles obtained according to at least one of Claims 1-7, characterized by an angle of repose according to ISO 4324 of 50° to 20°, preferably of 40° to 25°, a dynamic avalanche angle according to ISO 4324 of 65° to 30°, a Hausner factor according to ASTM 527 of 1 .5 to 1 , and / or a dust value according to DIN 55992 type I of 10 to 0.001 .

9. Use of the supraparticles composed of dry-coated graphene particles according to Claim 8 or obtained according to at least one of Claims 1 to 7 in thermally conductive materials and thermally conductive adhesives such as films, underflow materials, castable electronic materials, materials having phase transitions, thermal pastes, encapsulating compounds, especially in the field of batteries, sensors, ICs and LEDs, composites based on thermoplastics, thermosets and / or elastomers having elevated electrical conductivity and / or thermal conductivity or for EMI shielding, especially in the field of housings of electrical components, motors, battery packs and pipelines, coatings and varnishes having thermal and / or electrical conductivity, or for applications in the EMI shielding sector, thermally and / or electrically conductive oils, cooling liquids or inks.

10. Use of the supraparticles composed of dry-coated graphene particles according to Claim 8 or obtained according to at least one of Claims 1 to 7 in thermoplastics selected from standard thermoplastics, preferably PE, PP, PS, PVC, alpha-olefins, butadiene derivatives and / or Vestenamer®, industrial thermoplastics, preferably PET, PMMA, PC, POM, PA, PBT, PEBA, TPU, PU and / or TPE, high-performance thermoplastics, preferably PPS, PEEK, PES, PI and / or PEI,copolymers, elastomers, preferably silicones, further preferably room temperature crosslinking (RTV) silicone, high temperature crosslinking (HTV) silicone, liquid silicone rubber (LSR), heat crosslinking rubber (HCR), acrylates and / or pastes containing poly- and oligosiloxanes, - polyurethanes, rubbers, preferably styrene-butadiene rubber (SVR), butadiene rubber(BR) and / or natural rubber, thermosets, preferably polyurethanes, polyester resins, phenolic resins, epoxy resins, acrylate resins and / or silicone resins, solvents, preferably aprotic-nonpolar, aprotic-polar and / or protic solvents, oils, preferably mineral oils, silicone oils and / or process oils.11 . Use of the supraparticles composed of dry-coated graphene particles according to Claim 8 or obtained according to at least one of Claims 1 to 7 as additives in the plastics processing industry, preferably in compounders, in extrusion, in injection moulding.

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