RU2693755C1 - Method of producing graphene material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to nanotechnology and chemical industry and can be used in making polymer composite materials. First, graphite is treated with ammonium persulphate solution in sulphuric acid free of free water. Intercalated graphite is heat treated until expansion, after which expanded graphite compound is treated with gaseous ammonia, and then subjected to thermal treatment for 2 to 8 hours at temperature from 280 to 420 °C. Before treating the expanded graphite compound with ammonia, it can be evacuated or extracted from it with excess sulphuric acid by liquid sulphur dioxide. Obtained product is dispersed and washed with water until removal of sulphate and ammonium bisulphate.

EFFECT: high electrical conductivity of the polymer composite materials containing the obtained graphene material, the electroconductive properties of which are less degraded during machining.

3 cl, 2 dwg, 1 tbl, 9 ex

Description

The invention relates to the technology of graphene materials, specifically, to the technology of multilayer graphene, with high electrical conductivity in the composition of the polymer compositions.

Giving polymeric materials electrical conductivity is an important technical problem, which is solved by introducing into the composition of polymers metallic (copper, silver, nickel) powders or fibers, carbon fibers, carbon nanotubes, various types of graphene.

However, the use of metal electrically conductive additives in many cases is unacceptable due to the ability of metals to corrosion, a large proportion, biological incompatibility, and for other reasons. Carbon electrically conductive additives are free from these disadvantages, however, they have lower electrical conductivity compared to metal ones. In addition, such widely used electrically conductive filler as soot of different grades, when introduced into polymers in large quantities, significantly deteriorates the mechanical properties of polymers. Carbon nanotubes in an excessively large amount also deteriorate the mechanical properties of polymers, since they give the composite material porosity and friability.

, Konrad Hilarius, Marco Liebscher, Dirk Lei-linger, Ingo Alig, Petra

. Effect of Graphite Nanoplate Morphology on the Dispersion and Physical Properties of Polycarbonate Based Composites // Materials 2017, 10, 545; dot: 10.3390/ma10050545] приведены данные по электропроводности полимерных композиций, наполненных различными видами дисперсных углеродных материалов. Согласно приведенным в этой работе данным, для поликарбонатных композиционных материалов наибольшая удельная объемная электропроводность составляет (данные приблизительные, поскольку оцифрованы нами из приведенного в цитируемой публикации рисунка):Theoretically, carbon nanotubes or graphene with an ideal structure may have very high electrical conductivity, however, in practice, such indicators are not achieved. When introduced into the composition of polymers, another factor appears that strongly influences the properties of polymer compositions - aggregation of filler particles and the formation of insulating layers on the surface of carbon particles due to adsorption of polymer molecules. For the known carbon electrically conductive fillers, such as electrically conductive carbon black, carbon nanotubes, graphene nano-plates, due to these reasons, it is often not possible to achieve the desired conductivity. So, in [Michael Thomas

, Konrad Hilarius, Marco Liebscher, Dirk Lei-linger, Ingo Alig, Petra

. Effects of Graphite Nanoplate for Polycarbonate Based Composites // Materials 2017, 10, 545; dot: 10.3390 / ma10050545] shows data on the electrical conductivity of polymer compositions filled with various types of dispersed carbon materials. According to the data presented in this paper, for polycarbonate composite materials, the highest specific volume electrical conductivity is (the data are approximate, since we digitized it from the figure given in the cited publication):

Carbon nanotubes - 0.2 Siemens / cm with a CNT content of 9%;

Electrically conductive soot Printex XE2 B - 0.01 Siemens / cm with a carbon black content of 11%;

Graphene nanoplates (GNP) of a layered structure - 0.0002 Siemens / cm with a content of GNP 10%;

Graphene nanoplates of compact structure - 0.00002 Siemens / cm with a content of GNP 20%;

Graphene nanoplates of fragmented structure - 0.0000003 Siemens / cm with a content of GNP 20%.

It should be noted that the data from the cited work are taken by us for compositions with the highest content of carbon filler. Our experience with similar materials shows that to achieve greater electrical conductivity by further increasing the filler content is almost impossible, because it leads to a sharp deterioration in the mechanical properties of polymers up to cracking.

The properties of graphene materials are determined by the method of their preparation. Closest to the claimed method is the patent of Ukraine No. 4707, MKL. COJB 31/04, published 12/28/1994 in Bull. № 7-1 Ukrpatent (prototype). This patent describes a method for producing “flaked graphite” (MHG), which is called modern multilayer graphene by modern terminology. The prototype method includes the processing of natural graphite fuel-1 with a solution of ammonium persulfate in anhydrous sulfuric acid at a mass ratio of components of graphite: ammonium persulfate: sulfuric acid = 1: 4: 9.5.

The reaction mixture was kept for 10 min at room temperature and 20 min at 100 ° C. As a result of intercalation of graphite and subsequent reactions, an expanded graphite compound was obtained in the form of a foul brown-green mass with an apparent volume of 80 cm ³ / g of the initial graphite. Gaseous ammonia was passed through this mass until neutralization of sulfuric acid was completed, as a result of which there was still some increase in the apparent volume and the color of the reaction mass changed from the initial brown-green to gray. At the same time, due to the heat of reaction of ammonia with sulfuric acid, a spontaneous increase in temperature to 200-300 ° C occurred. The process was completed within 5 minutes, counting from the beginning of the transmission of ammonia. The resulting product was a mixture of MHG, ammonium sulfate and ammonium hydrogensulfate. This product was subjected to dispersion in the device, which is a PTFE cylindrical rotor with an outer diameter of 19.6 mm, rotating in a glass test tube with an inner diameter of 20.0 mm. When the rotor was raised and lowered, the product passed through the gap between the rotor and the walls of the glass test tube and experienced shear deformation, which resulted in grinding. In order to allow the passage of the product through the gap, an organic solvent (for example, n-butanol) was added to it. That is, in fact, the suspension in the described device was subjected to suspension or paste of the product in an organic solvent. It is impossible to process a dry product in such a device. The product suspension treated with a dispersing device in an organic solvent was filtered, washed with water to remove ammonium sulfate and ammonium hydrosulfate, and then washed again with a water-miscible organic solvent to replace the water with an organic solvent. Next, the resulting MHG was introduced into the composition of the polymer compositions in the form of a suspension or paste in a suitable organic solvent, for example, dioxane.

In the prototype patent, the characteristics of the received MHG are given:

The transverse size of the flakes is 2-10 microns, the thickness is 0.005-0.1 microns (= 5-100 nm), i.e., it is a typical multilayer graphene according to modern terminology. In connection with the terminology, you can cite a patent application [United States Patent Application US 2015/0360956. Production of graphene nanoplatelets by oxidative anhydrous acidic media. - Published Dec. 17, 2015], which describes exactly the same method of processing graphite by intercalating with ammonium persulfate in anhydrous sulfuric acid, as in the prototype method in question, and they call the resulting product graphene nano-plates.

In the method prototype also provides data on the specific electrical resistance of acrylic coatings filled with MCG, which are:

1.1 Ω cm at a MHT content of 4.8% by weight in the coating;

0.11 Ω cm at a MHG content of 9.1% by weight in the coating;

0.06 Ω cm at a MHG content of 33% by weight in the coating.

In this case, according to the cited patent, the introduction of even a large number of MHG did not lead to cracking of the coating. It can be seen that the specific resistance value is 0.11 Ohm.cm at 9.1 mass% filling, which is 9 Siemens / cm in terms, exceeds by many orders the known graphene materials and significantly exceeds all known electrically conductive carbon dispersions, the data for which were cited in the publication cited above.

The likely reason for the high efficiency of this graphene material as an electrically conductive filler is the high coefficient of particle shape (ratio of transverse size to thickness) and the special nature of the surface, which provides minimal contact resistance between particles dispersed in the polymer.

Common essential features of the prototype method and the claimed invention are:

- processing (oxidative intercalation) of natural graphite with a solution of ammonium persulfate in sulfuric acid that does not contain free water;

- extract of intercalated graphite at a certain temperature before expansion;

- treatment of the expanded graphite compound with ammonia gas;

- dispersion of the product obtained after treatment with gaseous ammonia;

- washing the dispersed product with water to remove the sulfate and ammonium hydrosulfate to obtain a water paste of multilayer graphene;

- if necessary, replace the water in graphene paste with an organic solvent.

The disadvantage of graphene material obtained by the method of the prototype, is its too great sensitivity to mechanical or ultrasonic dispersion, which leads to a significant loss of electrical conductivity. In the description of the prototype method this moment is not reflected in any way, however, our experiments showed this effect.

This phenomenon interferes with practical application, since any mixing of graphene paste with a melt or polymer solution requires quite intensive mechanical mixing, grinding or ultrasonic processing in order to evenly distribute graphene. Even when the initial graphene paste is obtained, it is necessary to limit the dispersion intensity of the product in order not to lose electrical conductivity. A lack of intensive dispersion gives a product with coarse particles or aggregates of particles. Paint coatings containing coarse graphene are uneven, with a rough surface, and unsatisfactory mechanical properties. The probable cause of such phenomena is that the multilayer graphene material according to the prototype method contains not only individual graphene nano-plates, but also their conglomerates in the form of honeycomb-like structures built of graphene nano-plates and similar to fragments of thermally expanded graphite particles. Such particles are easily crushed by mechanical action. At the same time, mechanical dispersion in a centrifugal field not only leads to the separation of graphene nano-plates, but also to a crush of conglomerates with the formation of coarser particles that do not have a high shape factor.

The disadvantage of the method described in example 1 of the invention of the prototype is also the use of a sufficiently large amount of organic solvent to disperse the ammonia-treated material. This creates a problem, since the organic solvent needs to be regenerated. However, in the claims of the prototype does not declare the use of organic solvent at the stage of dispersion as an essential feature of the invention, it is said only about dispersion in a centrifugal field.

The basis of the claimed invention, the task, by introducing additional technological operations and the choice of technological modes, to ensure the production of electrically conductive graphene filler, the electrically conductive properties of which are less degraded during machining.

The task is solved by the fact that in the method of obtaining graphene material, including processing of graphite with ammonium persulphate solution in sulfuric acid, not containing free water, heat treatment of intercalated graphite before expansion, processing the expanded graphite compound with gaseous ammonia, dispersing and washing with water before removing sulfate and ammonium hydrosulfate According to the invention, the product after treatment with gaseous ammonia and before dispersing is additionally subjected to heat treatment from 2 d about 8 hours at a temperature of 280 to 420 ° C.

As an embodiment of the invention, useful in large-scale production, a reactor with an expanded graphite compound is evacuated before intake of ammonia, then gaseous ammonia is admitted to the reactor. Evacuation of expanded intercalated graphite prior to treatment with ammonia gas provides accelerated penetration of ammonia into the porous mass of expanded intercalated graphite and thereby shortens the process time.

As an embodiment of the invention, before processing the expanded intercalated graphite with ammonia, excess sulfuric acid is extracted with liquid sulfur dioxide. Extraction of excess sulfuric acid with liquid sulfur dioxide before treating expanded intercalated graphite with ammonia reduces the consumption of ammonia.

Additional heat treatment with a duration of 2 to 8 hours at a temperature of from 280 to 420 ° C. after treatment of the product with gaseous ammonia and before dispersing, it reduces the sensitivity of the finished product to mechanical stress during the production of composite materials. The upper temperature of the claimed range is due to technological considerations. Ammonium hydrosulfate should not evaporate, since this significantly complicates the operation of the equipment; about 420 ° C and higher, the evaporation of ammonium hydrosulfate at atmospheric pressure becomes noticeable. The lower temperature of the claimed range was found experimentally.

Based on the prior art, it was impossible to predict the effect of the heat treatment operation at 380-420 ° C or select the heat treatment mode. However, for a better understanding of the essence of the invention, we present a tentative explanation.

The crushing of conglomerates of graphene nano-plates with intensive mechanical action occurs because in the graphite material after its treatment with ammonia there are voids between the graphene flakes. And it is easy to understand, because such voids exist in the expanded graphite compound before treatment with ammonia - its apparent volume increases and decreases, respectively, when pumping in vacuum and air intake precisely due to the presence of closed cavities filled with gas (oxygen). When treating with gaseous ammonia, the reaction of ammonia with sulfuric acid, which is present in excess and covers the surface of intercalated graphite, occurs first. Since this reaction is highly exothermic, the expanded compound of graphite partially decomposes, which, as was shown in our experiments, comes to sintering graphene flakes with a loss of dispersibility. As a result, in the finished product there are strong conglomerates of graphene flakes, which are easily crushed under mechanical action with a loss of shape factor. In order to eliminate or reduce this negative effect, it is necessary to fill all the slit-like or cell-like pores between graphene nano-plates with some substance that prevents the compaction of graphene layers back into a graphite-like structure. Presumably, in the claimed technical solution such a substance is the melt of ammonium hydrosulfate, which is formed when the normal ammonium sulfate is heated:

According to Wikipedia:

https: /en.wikipedia.org/wiki/%D0%93%D0%B8%D0%B4%D1%80%D0%BE%D1% 81% D1% 83% D0% BB% D1% 8C% D1% 84% D0% B0% D1% 82% D0% B0% D0% BC% D0% BC% D0% BE% D0% BD% D0% B8% D1% 8F reaction (1) takes place at a temperature of 235-360 ° C. The melting point of ammonium hydrosulfate is 251 ° C (data spread, this figure is the most reliable), and the boiling point with decomposition is 490 ° C:

With a decrease in temperature, the reaction (2) is reversed, and the condensation of the gas mixture of the indicated composition leads to the precipitation of ammonium hydrosulfate on the cold parts of the reactor and pipelines. Presumably, for the most efficient filling of the pores in the expanded graphite material, heat treatment should be carried out at a temperature sufficient for the reaction (1) to proceed, and the resulting ammonium hydrosulfate should be in the molten state. Theoretically, according to the given data, the minimum temperature of heat treatment should be about 250 ° C. Practically, as shown by our experiments, it is necessary to choose a temperature range not lower than 280 ° C for a sufficiently fast reaction. The upper temperature of the claimed range is due to technological considerations. It should not be allowed to “pairing ammonium hydrosulfate, as this significantly complicates the operation of the equipment. As shown by our experiments, one should not exceed a temperature of about 420 ° C - higher evaporation of ammonium hydrosulfate at atmospheric pressure becomes noticeable. Thus, it is possible to substantiate the claimed temperature range of heat treatment, from 280 to 420 ° C. In principle, the technical result is achieved outside this range, but this either takes too long (if the temperature is less than 280 ° C), or it is necessary to complicate the equipment for trapping the evaporating ammonium hydrogen sulfate, if the temperature is above 420 ° C. In addition, it should be noted that at a temperature of about 400 ° C and above, the reduction of carbonate sulfate groups begins at a noticeable rate, which leads to carbon loss and the formation of sulfur dioxide. Above 420 ° C these processes become significant.

Of course, the above explanations are conjectural, since the properties of intercalated sulfuric acid and sulfate groups between graphene layers may differ from those of pure substances. The following are data proving the possibility of implementing the claimed invention.

To implement the invention, the following starting materials were used:

Special low-ash graphite fuel-2 according to GOST 17022-81, containing up to 0.5% ash. It should be noted that in the method of the prototype used graphite fuel-1 with a purity of more than 99.9% of carbon, which differs from the fuel-2 by a larger size of crystals, a more perfect crystalline structure. When using fuel-1 graphene is obtained with better electrical conductivity, due to the larger shape factor of graphene particles. Unfortunately, at present, the graphite of the GSM-1 is not being produced and we have not been able to find a graphite of similar quality. Therefore, to compare the claimed invention with the prototype, we repeated the synthesis of MHG according to the prototype method using graphite fuels and lubricants-2 and compared the electrical conductivity of coatings prepared under the same conditions.

Ammonium persulphate

Anhydrous sulfuric acid was prepared by fixing concentrated sulfuric acid of the grade KhC by oleum of the mark KhCh. The equivalence point (completeness of water binding) was controlled by the smoke of sulfur trioxide, as described in the book [A. Gordon, R. Ford. Satellite chemist. M., "Peace", 1976 - p. 442].

Gaseous ammonia was obtained by adding 25% aqueous ammonia to an excess of granulated potassium hydroxide. Obtaining ammonia and processing them expanded intercalated graphite was performed under exhaust ventilation.

Heat treatment of the product after treatment with ammonia was carried out on a stainless steel tray in a muffle furnace under exhaust ventilation.

For grinding the dry product after treatment with ammonia, a shock type disintegrator was used - 1000g Multifunctional Disintegrator WF-20B, 3000 W, 25000 rpm, made in China.

To test the electrical properties of graphene-derived materials, water paste of graphene material was mixed with a water-dispersion acrylic varnish: Acrylic varnish for wood ORION transparent silky-matte, TU 2313-082-51218143-2011, colorless, gloss degree 4 of 6, manufactured by Descartes Moscow Region, Khimki, 31 Leningradskaya Str. Www.dekart.ru. The dry residue of this varnish after drying at 110 ° C to constant weight is determined by us = 23.38%.

Acrylic varnish with the addition of graphene material was applied in an even layer onto a glass plate (microscope slide) 25 × 75 mm in size, dried first for at least 1 day at room temperature and then 3 hours at 110 ° C. In most experiments, the mass of graphene material was calculated so that the content of graphene in the coating dried at 110 ° C was 20%. In a number of experiments, the dependence of the specific electrical resistance of coatings on the mass content of graphene was determined, for which samples of coatings with variable graphene content were prepared. The electrical resistivity of the coatings was measured by the 2-electrode method using an M-838 multimeter. To eliminate the contact resistance, the measuring electrodes were made of Graflex elastic graphite foil and pressed against the surface of the test coating through a rubber gasket under pressure. Our experiments showed that with this method of measurement the contact resistance is practically absent due to the tight fit of the Graflex electrodes to the surface of the coating. Under these conditions, metal (copper or steel) electrodes were impossible to apply because, because of the asperities of the surface of the tested coatings, it was impossible to ensure their close contact with the measuring electrodes, which resulted in significant contact resistance. If the width of the coating on the glass plate was 25 mm and the distance between the edges of the Graflex electrodes was also 25 mm, then it is obvious that the measured resistance equals the surface resistance per unit area of the coating. For conversion to specific volume electrical resistance, the surface resistance was multiplied by the coating thickness. The exact thickness of the coatings was determined by weighing them, taking into account the density of the cured varnish.

The invention is illustrated by the figures of the graphic images of FIG. 1 and FIG. 2, which show the dependence of the specific electrical resistance of graphene-acrylic coatings on the time of grinding the graphene material.

FIG. 1. The dependence of the electrical resistivity (R, Ω.cm) of graphene-acrylic coatings on the time of grinding the graphene material in the disintegrator (t, min). Mass content of graphene in coatings of 20%.

1 - Example 1, heat treatment of graphene material 2 hours 300 ° C.

2 - Example 2 (according to the method prototype, without heat treatment).

4 - Example 4, heat treatment of graphene material for 2 hours at 400 ° C.

FIG. 2. The dependence of the electrical resistivity (R, Ω.cm) of graphene-acrylic coatings on the time of grinding the graphene material in the disintegrator (t, min). Mass content of graphene in coatings of 20%.

1 - Example 1, heat treatment of graphene material 2 hours 300 ° C.

2 - Example 2 (according to the method prototype, without heat treatment).

4 - Example 4, heat treatment of graphene material for 2 hours at 400 ° C.

The following are examples of the implementation of the invention.

Example 1

In a 2-liter glass of heat-resistant glass was placed 20 g of ammonium persulfate and added 26 ml of anhydrous sulfuric acid. To protect from moisture in the air, the beaker was closed with plastic wrap. The mixture was stirred for 20 minutes at room temperature until the ammonium persulfate was dissolved, then 5 g of graphite gcm-2 was added and mixed until a uniform suspension was formed. The suspension was stirred for 10 minutes until it thickened, after which the glass with the reaction mixture, covered with film, was placed for 1 hour in an oven with a temperature of 60 ° C for expansion. Got a loose mass consisting of worm-like particles of brown-green color, with an apparent volume of about 0.5 DM ³ .

Note: as shown by our experiments and in agreement with previously published literature data [Melezhik A.V., Chuyko A.A. Cold expansion of graphite - the path to new technologies // Chemical Technology (Kiev), 1992, №2, p. 3-11], a change in the temperature of expansion in the range of 40-100 ° C does not affect the quality of the product, only the time required for full expansion changes. In this example, the expansion was carried out at 60 ° C, since at this temperature it takes a not too long time - 1 hour. If the expansion is carried out at 40 ° C, it takes 3 hours. At 100 ° C (as was done in the prototype method) 15-20 minutes is enough for full expansion, however, too short time is inconvenient with an increased scale of synthesis, because with a large mass of charge and glass, the reaction mixture does not have time to warm up evenly if too short.

The expanded brown-green reaction mass was overloaded into another 2-liter glass, while using a spatula it was crushed into pieces of 1-3 cm. This was done to improve the access of ammonia into the volume of the expanded mass. Then a glass tube for supplying ammonia gas, a thermocouple was inserted into the glass, the glass was closed with a fluoroplastic film, the glass was placed on a heat-resistant substrate of fibrous silica material Supersil and ammonia was fed. This operation was performed under exhaust ventilation. Over the course of the reaction was monitored by changing the color of the reaction mass and temperature. Gradually, the brown-green color turned gray, and the temperature rose to about 300 ° C. Approximately in 15-20 minutes, the entire reaction mass became sulfur, and the temperature began to subside, which indicated the completion of the neutralization of sulfuric acid and the decomposition of intercalated graphite with ammonia. The apparent volume of the gray reaction mass after completion of the reaction was about 1 dm ³ . The beaker with the cooled reaction mixture and the atmosphere of ammonia was sealed with food-grade plastic wrap and held for 16 hours. After opening the glass, there was a smell of ammonia, which was evidence of the complete neutralization of sulfuric acid. The gray reaction mixture from two syntheses, carried out as described above, was loaded into a stainless steel container and placed in a muffle furnace. The oven was turned on and after reaching a temperature of 300 ° C, it was kept for 2 hours at this temperature, then the container was removed from the oven (there was an ammonia smell), covered with a lid from air moisture and allowed to cool to room temperature.

The gray reaction mixture from two syntheses (that is, a total of 10 g of the original graphite), heat-treated as described above, was loaded into a disintegrator and grinded for 15 seconds. A sample of product was taken of 10 g, and grinding continued (with a break every 30 seconds to cool the disintegrator), taking samples of 10 g after each grinding time. Samples were taken after 15 seconds, 30 seconds, 1 minute, 2 minutes, and 4 minutes of grinding. Thus, samples were obtained to determine the effect of grinding time on the electrical properties of the resulting graphene material.

Samples of graphene material in the form of an aqueous paste were mixed with a water-dispersion acrylic lacquer, samples of coatings were prepared and electrical resistivity was measured as described above. For each graphene material (each grinding time), 5 samples of coatings were prepared and the average value was taken when calculating the electrical resistivity. As a rule, a good match was observed.

Example 2 (according to the method - the prototype).

This example was made as described in example 1, but the heat treatment operation at 300 ° C was not performed, the product after treatment with ammonia was immediately ground in a disintegrator. Further processing and testing as described in example 1.

Examples 3-5

Performed as in example 1, but the temperature of the heat treatment was chosen 280, 400 or 420 ° C.

Example 6

Performed as in example 1, but the heat treatment time at 300 ° C was chosen 4 hours.

Example 7

Performed as in example 1, but the heat treatment time at 300 ° C was chosen 8 hours.

Example 8

Performed as in example 1, but the processing of expanded intercalated graphite with ammonia was carried out in a round bottom flask, from which air was evacuated with a vacuum pump before the intake of ammonia. It should be noted that during evacuation, the apparent volume of expanded intercalated graphite increased markedly, which proves the presence of closed pores filled with gas (in this case, oxygen).

We also obtained samples of films and coatings, in which epoxy resin, ABS plastic, KO-08 silicone lacquer, curable silicone rubber were used as a polymer binder. In order to combine graphene material with these polymers, it was necessary to obtain it in the form of a paste in organic solvents. Acetone paste was obtained by washing the aqueous paste on the filter with acetone to remove water. To obtain a toluene paste, the aqueous paste was mixed with an excess of toluene and with stirring, the toluene was distilled off as an azeotrope with water.

The table shows the electrical resistivity of samples of acrylic coatings obtained in examples 1-8, as well as, as reference information, data for some other graphene-polymer films and coatings obtained by us.

The data of examples 1, 2, 4 are depicted in FIG. 1 For clarity, the initial part of these dependencies is shown in FIG. 2

As can be seen from the table and figures, with an increase in the grinding time of graphene material in the disintegrator, the electrical resistivity of graphene-containing coatings increases sharply. However, the graphene material obtained according to the claimed invention is significantly less sensitive to the duration or intensity of grinding. Our experiments showed that the same behavior is observed when mixing the finished graphene material (in the form of an aqueous, acetone or toluene paste) with aqueous or organic polymeric binders. Intensive mechanical or ultrasonic dispersion of such mixtures leads to a sharp increase in the specific electrical resistance of the polymer coated. However, for the graphene material obtained according to the claimed invention, this effect is much less pronounced than for the graphene material obtained according to the prototype method. This is important for the production of electrically conductive graphene-polymer composite materials, since, as a rule, intensive mechanical processing is used for their preparation (high-speed mixer, disintegrator, disk mill, extruder, cavitation equipment).

Comparing the data of the claimed invention with the method of the prototype, it should be borne in mind that with graphite fuel-2 it is impossible to obtain such a small specific electrical resistance of coatings, as was published in the method of the prototype, which used coarse-crystalline, high-purity graphite fuel-1, currently not manufactured Therefore, we compare the data on electrical conductivity obtained using the same graphite fuel-2.

The disadvantage of the invention of the prototype is also the fact that for the preparation of expanded intercalated graphite used a large amount of sulfuric acid, in the specific examples given, 9.5 wt. parts of sulfuric acid on 1 mass. part of graphite. It also entails an increased consumption of ammonia and the formation of large amounts of ammonium sulfate in the form of waste production.

To eliminate this disadvantage, the present invention proposes, as an option, the extraction of excess sulfuric acid with liquid sulfur dioxide. The feasibility of such a technological option depends on the scale of production and design of the equipment used. Probably, for laboratory scale to apply washing with liquid sulfur dioxide is not advisable. However, with large-scale production, this option may be beneficial.

Example 9

A laboratory experiment on the extraction of excess sulfuric acid was carried out as follows (using protective equipment in case of rupture of the device from excessive pressure of sulfur dioxide — exhaust ventilation, a protective screen, canvas gloves, a mask). The laboratory instrument was made of a Pyrex glass tube with a diameter of 10 mm, which withstands a pressure of at least 1.0 MPa. The bottom of the tube was sealed like a test tube. Then, at a height of 120 mm above the bottom, the waist was filled and a tampon of quartz fiber was placed on it. Then the tube over the tampon was filled with expanded intercalated graphite to a height of 50 mm, the walls were carefully wiped, the substance was covered with another quartz tampon from above and above the tampon the second waist was completed. Finally, at a height of 150 mm above the second waist, a tee was soldered from a tube with a diameter of 6 mm. One free end of the tee through the valve attached to the foreline pump, the other end through the blocked valve attached to the supply system of gaseous sulfur dioxide. The vacuum pump was turned on and the instrument was pumped out. Then shut off the valve leading to the vacuum pump, and opened the sulfur dioxide inlet. The lower segment of the device was cooled with solid carbon dioxide, thus, it was freezing liquid sulfur dioxide. When the segment was half full, the sulfur dioxide supply was cut off, and the lower segment was frozen with liquid nitrogen. The valve to the vacuum pump was opened and pumped out in a vacuum, which was necessary to remove non-condensable impurities. Then they soldered the tee tube and separated it from the device. Now remove the device from liquid nitrogen and place the lower segment in the hole in the foam tile. Thus, with slow heating, sulfur dioxide was thawed, starting from the top, and this prevented from breaking the tube. When sulfur dioxide reached room temperature, the device was turned upside down and now the lower part was immersed for cooling in solid carbon dioxide. Liquid sulfur dioxide passed through a layer of expanded graphite compound and washed sulfuric acid from it. Then the segment with sulfur dioxide condensate was frozen again with liquid nitrogen, the soldered process of the tube was opened, argon was admitted into the device and the segment with washed expanded intercalated graphite was cut to the waist. After that, let the flow of ammonia. The product treated with ammonia, held for 2 hours at 300 ° C in a muffle furnace, and then processed for 15 seconds in a miniature grinder, working from a high-speed mini-drive. The ground material was washed on a microfilter with water and the resulting aqueous paste of graphene material was mixed in an acrylic lacquer, as described above. Since there was little material, they applied the mixture onto a glass plate 10 mm wide and the distance between the measuring electrodes was also made 10 mm. Got a coating with a specific volume resistance of 0.08 Ohm.cm. Unfortunately, when working with such small quantities, one cannot be sure that the results are fully comparable, since the temperature regime of synthesis and the grinding mode are different. Currently, we are not able to implement this technological option on a large scale. Nevertheless, a laboratory experiment proves the possibility of obtaining a graphene material using the extraction of excess sulfuric acid with liquid sulfur dioxide. This option can save sulfuric acid and ammonia in large-scale production. With the appropriate hardware design, there is no need to freeze sulfur dioxide with liquid nitrogen, as in a laboratory experiment. Also, in this technological variant, the quality of graphene material can also be improved. In a system containing an excess of sulfuric acid and intercalated graphite, excess sulfuric acid reacts with ammonia in the first place with ammonia treatment. Intercalated graphite either reacts after neutralization of sulfuric acid with ammonia, or thermally decomposes before the completion of this reaction. Extraction of excess sulfuric acid with ammonia and the direct interaction of ammonia with intercalated graphite can change the chemistry of the processes taking place, in particular, make exfoliation of intercalated graphite more efficient and, consequently, improve the quality of the graphene material.

Thus, the claimed invention allows to obtain graphene material, characterized by high electrical conductivity in the composition of polymer composite materials and thus better processability (less degradation of electrical conductivity in the processes of dispersion and mixing) compared with the method of the prototype. The resulting graphene material can be widely used in engineering.

Claims (3)

1. A method of producing graphene material, which includes the treatment of graphite with ammonium persulfate solution in sulfuric acid containing no free water, the heat treatment of intercalated graphite before expansion, the treatment of the expanded graphite compound with gaseous ammonia, dispersion and washing with water before removing sulfate and ammonium hydrosulfate, characterized in that the product after treatment with gaseous ammonia and before dispersing is additionally subjected to heat treatment from 2 to 8 hours at a temperature of from 280 to 420 ° C. 2. The method according to p. 1, characterized in that the expanded intercalated graphite is evacuated before treatment with gaseous ammonia. 3. The method according to paragraphs. 1 and 2, characterized in that, prior to the treatment of expanded intercalated graphite with ammonia, excess sulfuric acid is extracted with liquid sulfur dioxide.

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