CN118638338A - A polyurethane sheet based on modified graphene - Google Patents

Abstract

The invention discloses a polyurethane sheet based on modified graphene, belonging to the technical field of building materials. Each raw material is calculated by weight as follows: 5-10 parts of modified graphene oxide, 150-200 parts of N, N-dimethylformamide, 30-40 parts of diphenylmethane diisocyanate, 50-60 parts of polytetrahydrofuran ether glycol, 4-8 parts of 1,4-butanediol, and 0.2-0.4 parts of catalyst. Among them, modified graphene oxide has good compatibility with the matrix compared with ordinary graphene oxide, and the agglomeration phenomenon is reduced, which can significantly enhance the mechanical properties of the sheet; and the organic molecular chain on the modified graphene oxide can also greatly improve the flame retardancy, smoke suppression and a certain degree of mechanical properties of the sheet, and the performance is long-term and stable; except for the modified graphene oxide, there is no other addition, which ensures the homogeneity of the matrix and does not destroy the mechanical properties of the sheet.

Description

A polyurethane sheet based on modified graphene

Technical Field

The invention belongs to the technical field of building materials, and in particular relates to a polyurethane plate based on modified graphene.

Background Art

With the continuous development of society, people's requirements for buildings are getting higher and higher. In addition to the requirements of building quality and aesthetics, the thermal insulation performance of buildings is also one of the important requirements. Because the energy consumption of buildings accounts for a considerable proportion of the total energy consumption of society. And the energy consumption of buildings is mainly the energy consumption for temperature regulation. Nowadays, energy conservation and emission reduction have become the mainstream of world economic development. It is of great significance to improve the thermal insulation performance of buildings and reduce the energy consumption of building temperature regulation.

At present, the insulation materials used in construction mainly include rock wool, polystyrene foam board, extruded polystyrene foam insulation board and polyurethane. Among them, polyurethane is the full name of polyurethane, which is formed by polyol and polyisocyanate through polycondensation reaction. It is a polymer material with excellent performance. The thermal conductivity of polyurethane rigid foam board made of polyurethane is the lowest among all insulation materials at present, which is only 0.018-0.023W/(m.K). In addition, it has low water absorption and high compression strength. It also has good electrical properties, acoustic properties and chemical resistance. It is easy to shape and has high production efficiency. It can achieve industrialized production, standardized product specifications and dimensions, and assembled construction. It is easy to control product quality and engineering quality, high construction efficiency, and beautiful appearance. However, the fatal weakness of polyurethane foam is that it has extremely poor flame retardancy, is easy to ignite, and releases harmful gases during the combustion process, which has potential safety risks, limiting its application in the field of building materials technology.

Some common flame-retardant modified polyurethane foam sheets on the market have significantly improved their flame retardancy due to the addition of a large amount of flame-retardant fillers, but these inorganic fillers have poor compatibility with polyurethane, which destroys the homogeneity of the material and leads to a significant decrease in the mechanical properties of the polyurethane foam sheet. In summary, it is urgent to invent a polyurethane foam sheet with good flame retardancy and high mechanical strength to meet the higher demands in the field of building materials technology.

Summary of the invention

The purpose of the present invention is to overcome the defects of the prior art and provide a polyurethane sheet based on modified graphene.

The purpose of the present invention can be achieved through the following technical solutions:

A polyurethane sheet based on modified graphene is prepared by the following steps:

A1. In a three-necked flask, the modified graphene oxide was mixed with N,N-dimethylformamide (DMF), mechanically stirred for 1-2 hours, and ultrasonicated for 30-45 minutes to uniformly disperse the modified graphene oxide to obtain a modified graphene oxide dispersion;

A2, adding diphenylmethane diisocyanate (MDI) to the modified graphene oxide dispersion obtained in step A1, placing it on a constant temperature magnetic stirrer, and introducing nitrogen, stirring and reacting at 70-80° C. for 2 hours, then adding polytetrahydrofuran ether diol, stirring at 80-90° C. for 2 hours to cause a prepolymerization reaction, evacuating to remove bubbles, maintaining the temperature unchanged, then adding 1,4-butanediol (chain extender) and a catalyst, stirring for 2 hours to react, and then placing it in an oven to dry to obtain a polyurethane composite material;

A3. The polyurethane composite material obtained in step A2 is placed in a microcellular foaming injection molding machine, CO ₂ is injected through a supercritical fluid device, and injection molding is performed in a mold to form microcellular foams to obtain a polyurethane sheet based on modified graphene.

Furthermore, the raw materials are calculated in parts by weight as follows: 5-10 parts of modified graphene oxide, 150-200 parts of N,N-dimethylformamide, 30-40 parts of diphenylmethane diisocyanate, 50-60 parts of polytetramethylene ether glycol, 4-8 parts of 1,4-butanediol, and 0.2-0.4 parts of catalyst.

Furthermore, in step A2, the catalyst is one of stannous octoate and dibutyltin dilaurate.

Furthermore, in step A3, the pressure of supercritical _CO2 injected is 10-12 MPa and the speed is 200-220 mm/s.

First, the modified graphene oxide is reacted with diphenylmethane diisocyanate, and then the polyurethane sheet is obtained through prepolymerization, chain extension, and microporous foaming; the use of supercritical _CO2 microporous foaming technology reduces the amount of material used, saves costs, and does not reduce the mechanical properties of the material. Not only that, the modified graphene oxide can also be used as a nucleating agent to reduce the activation energy required for nucleation, so that the pores change from homogeneous nucleation to heterogeneous nucleation, making the pore size smaller, the pores more numerous, and the distribution more uniform, which greatly improves the thermal insulation performance of the board.

Furthermore, the modified graphene oxide is prepared by the following steps:

S1. After phosphorus oxychloride, pentaerythritol and toluene are uniformly stirred in a three-necked flask equipped with a stirring reflux device, nitrogen is introduced, and the temperature is raised to 65° C. After reacting for 75 minutes, the temperature is raised to 108° C. and refluxed until no hydrogen chloride gas is generated. After the reaction is completed, the mixture is cooled to room temperature, washed with ether, benzene and dichloromethane in sequence, and the solvent is removed by distillation under reduced pressure. The mixture is dried in vacuo to obtain intermediate 1; the ratio of phosphorus oxychloride, pentaerythritol and toluene is 35.4 g:13.6 g:80 mL;

Phosphorus oxychloride and pentaerythritol undergo esterification reaction, and the phosphorus oxychloride is in excess to obtain intermediate 1; the specific reaction process is as follows:

S2. At room temperature, under nitrogen protection, in a three-necked flask equipped with a stirring reflux device, the intermediate 1, 1-nonylamine, triethylamine and toluene were mixed, stirred until the solid was completely dissolved, and the temperature was raised to 70°C for 7 hours of insulation reaction. After the reaction was completed, the mixture was filtered and distilled under reduced pressure to obtain the intermediate 2; the ratio of the intermediate 1, 1-nonylamine, triethylamine and toluene was 30.7 g:14.3 g:15 mL:75 mL;

Intermediate 1 and 1-nonylamine undergo nucleophilic substitution, and by controlling the molar ratio of the two to be close to 1:1 and a slight excess of intermediate 1, only one chlorine group on intermediate 1 participates in the reaction, and triethylamine removes the hydrogen chloride generated by the reaction to obtain intermediate 2; the specific reaction process is as follows:

S3. At room temperature, under nitrogen protection, melamine, intermediate 2, triethylamine and toluene were mixed in a three-necked flask equipped with a stirring reflux device, stirred until the solid was completely dissolved, and the temperature was raised to 80°C for 8 hours of insulation. After the reaction was completed, the mixture was filtered, and part of the solvent was removed by rotary evaporation. The mixture was purified by column chromatography (the eluent was a mixed solvent of cyclohexane/ethyl acetate in a volume ratio of 1:1), and the eluent was removed by rotary evaporation to obtain intermediate 3. The ratio of melamine, intermediate 2, triethylamine and toluene was 13.4 g:80.6 g:30 mL:125 mL.

Melamine and intermediate 2 undergo nucleophilic substitution. By controlling the molar ratio of the two to be close to 1:2 and a slight excess of melamine, only two amino groups on melamine participate in the reaction. Triethylamine removes the hydrogen chloride generated by the reaction to obtain intermediate 3. The specific reaction process is as follows:

S4. In a three-necked flask equipped with a stirring device, graphene oxide and N,N-dimethylformamide were mixed, mechanically stirred for 30 min, ultrasonically treated for 45 min, intermediate 3 and dicyclohexylcarbodiimide (DCC, dehydrating agent) were added, mixed and stirred evenly, and kept warm at 55° C. for 8 h, stirring was continued during the reaction. After the reaction was completed, vacuum distillation, washing, and freeze-drying were performed to obtain modified graphene oxide; the ratio of graphene oxide, N,N-dimethylformamide, intermediate 3, and dicyclohexylcarbodiimide was 10 g:100 mL:15 g:23.4 g;

The carboxyl group on graphene oxide reacts with the amino group on intermediate 3, and under the action of DCC dehydrating agent, it can occur without very high conditions; the specific reaction process is as follows:

Graphene oxide has extremely high tensile strength and tensile modulus, and is an excellent reinforcement material. It can also be grafted with organic molecular chains through chemical bonding, that is, an organic layer is formed on its surface, which can change the surface of graphene oxide from hydrophilic to hydrophobic, reduce the agglomeration of modified graphene oxide, and promote the uniform dispersion of modified graphene oxide in the matrix, so that its performance can be fully exerted and the mechanical properties of the matrix can be greatly enhanced. In addition, the organic molecular chains grafted with graphene oxide can effectively prevent the migration and seepage of organic molecules, and ensure the durability of various properties. In addition, the organic molecular chains also contain spirocyclic phosphates, triazine structures and long Carbon chain structure; spirocyclic phosphate has good flame retardant effect and has a stable six-membered heterocyclic structure. When burning, the pentaerythritol skeleton will form a coke protective layer to inhibit further combustion of the matrix. In addition, the introduction of triazine structure will produce nitrogen-containing gas during combustion, which will dilute and reduce the smoke density. It will self-condense at high temperature to form melem to make the carbon layer tight, thereby improving the flame retardant and smoke suppression performance of the matrix. It plays a synergistic role with spirocyclic phosphate and significantly enhances the flame retardant performance of the matrix. Finally, the long carbon chain belongs to the methylene chain segment, which can further improve the mechanical properties of the matrix to a certain extent, and can be interspersed in the molecular chain of the matrix to further improve the stability.

It should be added that the use of graphene oxide has better performance than graphene, and the surface of graphene oxide contains a large number of hydroxyl groups, which can produce chemical bonds with diphenylmethane diisocyanate, further improving the compatibility of graphene oxide with the matrix.

Beneficial effects of the present invention:

1. The present invention first reacts modified graphene oxide with diphenylmethane diisocyanate, and then obtains a plate through prepolymerization, chain extension, and microporous foaming. The microporous foaming technology not only saves costs, but also greatly improves the thermal insulation performance of the plate;

2. Compared with ordinary graphene oxide, the modified graphene oxide has good compatibility with the matrix, reduced agglomeration, and can significantly enhance the mechanical properties of the board; and the organic molecular chains on the modified graphene oxide can also greatly improve the flame retardancy, smoke suppression and mechanical properties of the board to a certain extent, and the performance is long-term and stable;

3. Except for modified graphene oxide, there is no other additive, which ensures the homogeneity of the matrix and does not damage the mechanical properties of the sheet;

Therefore, the board prepared by the present invention has stable and efficient flame retardant, smoke suppression, thermal insulation and mechanical properties, and has good homogeneity, realizes halogen-free flame retardancy, is environmentally friendly, and has important application value in the field of building materials technology.

DETAILED DESCRIPTION

The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Example 1

Preparation of modified graphene oxide:

S1. In a three-necked flask equipped with a stirring reflux device, 35.4 g of phosphorus oxychloride, 13.6 g of pentaerythritol and 80 mL of toluene were stirred evenly, nitrogen was introduced, and the temperature was raised to 65° C. After reacting for 75 min, the temperature was raised to 108° C. and refluxed until no hydrogen chloride gas was generated. After the reaction was completed, the mixture was cooled to room temperature, washed with ether, benzene and dichloromethane in sequence, the solvent was removed by distillation under reduced pressure, and vacuum dried to obtain intermediate 1;

S2. At room temperature, under nitrogen protection, 30.7 g of intermediate 1, 14.3 g of 1-nonylamine, 15 mL of triethylamine and 75 mL of toluene were mixed in a three-necked flask equipped with a stirring reflux device, and stirred until the solid was completely dissolved. The temperature was raised to 70°C and kept for reaction for 7 h. After the reaction was completed, the mixture was filtered and distilled under reduced pressure to obtain intermediate 2.

S3, at room temperature, under nitrogen protection, in a three-necked flask equipped with a stirring reflux device, 13.4 g of melamine, 80.6 g of intermediate 2, 30 mL of triethylamine and 125 mL of toluene were mixed, stirred until the solid was completely dissolved, and the temperature was raised to 80°C for 8 h. After the reaction was completed, the mixture was filtered, and part of the solvent was removed by rotary evaporation. The mixture was then purified by column chromatography (the eluent was a mixed solvent of cyclohexane/ethyl acetate in a volume ratio of 1:1), and the eluent was removed by rotary evaporation to obtain intermediate 3;

S4. In a three-necked flask equipped with a stirring device, 10 g of graphene oxide was mixed with 100 mL of N,N-dimethylformamide, mechanically stirred for 30 min, ultrasonically treated for 45 min, 15 g of intermediate 3 and 23.4 g of dicyclohexylcarbodiimide were added and mixed and stirred evenly, and the mixture was kept warm at 55°C for 8 h with continuous stirring. After the reaction was completed, the mixture was distilled under reduced pressure, washed, and freeze-dried to obtain modified graphene oxide.

Example 2

Preparation of modified graphene oxide:

S1. In a three-necked flask equipped with a stirring reflux device, 70.8 g of phosphorus oxychloride, 27.2 g of pentaerythritol and 160 mL of toluene were stirred evenly, nitrogen was introduced, and the temperature was raised to 65° C. After reacting for 75 min, the temperature was raised to 108° C. and refluxed until no hydrogen chloride gas was generated. After the reaction was completed, the mixture was cooled to room temperature, washed with ether, benzene and dichloromethane in sequence, the solvent was removed by distillation under reduced pressure, and vacuum dried to obtain intermediate 1;

S2. At room temperature, under nitrogen protection, in a three-necked flask equipped with a stirring reflux device, 61.4 g of intermediate 1, 28.6 g of 1-nonylamine, 30 mL of triethylamine and 150 mL of toluene were mixed, and the mixture was stirred until the solid was completely dissolved. The mixture was heated to 70°C and kept for 7 h. After the reaction was completed, the mixture was filtered and distilled under reduced pressure to obtain intermediate 2.

S3, at room temperature, under nitrogen protection, in a three-necked flask equipped with a stirring reflux device, 26.8 g of melamine, 161.2 g of intermediate 2, 60 mL of triethylamine and 250 mL of toluene were mixed, stirred until the solid was completely dissolved, and the temperature was raised to 80°C for 8 h. After the reaction was completed, the mixture was filtered, and part of the solvent was removed by rotary evaporation. The mixture was then purified by column chromatography (the eluent was a mixed solvent of cyclohexane/ethyl acetate in a volume ratio of 1:1), and the eluent was removed by rotary evaporation to obtain intermediate 3;

S4. In a three-necked flask equipped with a stirring device, 20 g of graphene oxide was mixed with 200 mL of N,N-dimethylformamide, mechanically stirred for 30 min, ultrasonically treated for 45 min, 30 g of intermediate 3 and 46.8 g of dicyclohexylcarbodiimide were added and mixed and stirred evenly, and the mixture was kept warm at 55°C for 8 h with continuous stirring. After the reaction was completed, the mixture was distilled under reduced pressure, washed, and freeze-dried to obtain modified graphene oxide.

Example 3

A1. In a three-necked flask, 5 g of the modified graphene oxide prepared in Example 2 was mixed with 150 g of N,N-dimethylformamide, mechanically stirred for 1 h, and ultrasonicated for 30 min to uniformly disperse the modified graphene oxide to obtain a modified graphene oxide dispersion;

A2, to the modified graphene oxide dispersion obtained in step A1, 30g of diphenylmethane diisocyanate was added, placed on a constant temperature magnetic stirrer, and nitrogen was introduced. After stirring and reacting at 70°C for 2h, 50g of polytetrahydrofuran ether diol (relative molecular weight 1000) was added, and the mixture was stirred at 80°C for 2h for prepolymerization reaction. Vacuuming was performed to remove bubbles, and the temperature was maintained constant. Then, 4g of 1,4-butanediol and 0.2g of stannous octoate were added, and the mixture was stirred for 2h for reaction, and then dried in an oven to obtain a polyurethane composite material;

A3. The polyurethane composite material obtained in step A2 is placed in a microcellular foaming injection molding machine, CO ₂ is injected through a supercritical fluid device at a pressure of 10 MPa and a speed of 200 mm/s, and injection molding is performed in a mold to form microcellular foams to obtain a polyurethane sheet based on modified graphene.

Example 4

A1. In a three-necked flask, 7.5 g of the modified graphene oxide obtained in Example 2 was mixed with 175 g of N,N-dimethylformamide, mechanically stirred for 1.5 h, and ultrasonicated for 40 min to uniformly disperse the modified graphene oxide to obtain a modified graphene oxide dispersion;

A2, to the modified graphene oxide dispersion obtained in step A1, 35g of diphenylmethane diisocyanate was added, placed on a constant temperature magnetic stirrer, and nitrogen was introduced. After stirring and reacting at 75°C for 2h, 55g of polytetrahydrofuran ether diol (relative molecular weight 1000) was added, and the mixture was stirred at 85°C for 2h for prepolymerization reaction. Vacuuming was performed to remove bubbles, and the temperature was maintained unchanged. 6g of 1,4-butanediol and 0.3g of dibutyltin dilaurate were added. After stirring and reacting for 2h, the mixture was placed in an oven for drying to obtain a polyurethane composite material;

A3. The polyurethane composite material obtained in step A2 is placed in a microcellular foaming injection molding machine, CO ₂ is injected through a supercritical fluid device at a pressure of 11 MPa and a speed of 210 mm/s, and injection molding is performed in a mold to form microcellular foams to obtain a polyurethane sheet based on modified graphene.

Example 5

A1. In a three-necked flask, 10 g of the modified graphene oxide prepared in Example 2 was mixed with 200 g of N,N-dimethylformamide, mechanically stirred for 2 h, and ultrasonicated for 45 min to uniformly disperse the modified graphene oxide to obtain a modified graphene oxide dispersion;

A2, to the modified graphene oxide dispersion obtained in step A1, 40g of diphenylmethane diisocyanate was added, placed on a constant temperature magnetic stirrer, and nitrogen was introduced. After stirring and reacting at 80°C for 2h, 60g of polytetrahydrofuran ether diol (relative molecular weight 1000) was added, and the mixture was stirred at 90°C for 2h for prepolymerization reaction. Vacuuming was performed to remove bubbles, and the temperature was maintained unchanged. Then, 8g of 1,4-butanediol and 0.4g of dibutyltin dilaurate were added. After stirring for 2h, the mixture was dried in an oven to obtain a polyurethane composite material;

A3. The polyurethane composite material obtained in step A2 is placed in a microcellular foaming injection molding machine, CO ₂ is injected through a supercritical fluid device at a pressure of 12 MPa and a speed of 220 mm/s, and injection molding is performed in a mold to form microcellular foams to obtain a polyurethane sheet based on modified graphene.

Comparative Example 1

Ordinary graphene oxide of the same mass was used to replace the graphene oxide in Example 5, and the remaining steps were the same as in Example 5.

Comparative Example 2

Commercially available rigid polyurethane foam insulation boards produced by Hebei Huana New Building Materials Co., Ltd. were used.

Examples 3-5 and comparative examples 1-2 were made into corresponding shapes according to different test standards, and the following performance tests were performed:

The tensile strength was determined using the national standard GB 9641-1988 "Test method for tensile properties of rigid foam plastics";

The oxygen index is determined using the national standard GB/2406.1-2008 "Determination of combustion behavior of plastics by oxygen index method";

The smoke density is measured using the national standard GB/T 8627-2007 "Test method for smoke density of burning or decomposition of building materials";

In the sealed insulation room, use the sample plate to separate the sealed insulation room into two rooms AB. The volumes of the two rooms AB are the same. The initial temperature of room A is 100℃, and the initial temperature of room B is 25℃. After 2h, measure the temperature of room B.

The measured results are shown in the following table:

Test items Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Tensile strength/MPa 0.33 0.36 0.38 0.19 0.22 Oxygen index/% 28.4 28.9 29.3 21.3 23.6 Smoke density 36.2 34.1 32.9 54.7 48.8 Room B temperature/℃ 28 28 27 29 35

It can be seen from the above table that the flame retardant, smoke suppression and mechanical properties of the board prepared in the embodiment of the present invention are higher than those of the control example, and the board has excellent thermal insulation performance, is halogen-free and flame-retardant, and is environmentally friendly, and has important application value in the field of building materials technology.

In the description of the specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims (8)

1. A polyurethane sheet based on modified graphene, characterized in that it is prepared by the following steps: A1. Mixing the modified graphene oxide with N,N-dimethylformamide, mechanically stirring for 1-2 hours, and ultrasonicating for 30-45 minutes to obtain a modified graphene oxide dispersion; A2, adding diphenylmethane diisocyanate to the modified graphene oxide dispersion obtained in step A1, and introducing nitrogen, reacting under magnetic stirring at 70-80° C. for 2 hours, adding polytetrahydrofuran ether diol, stirring at 80-90° C. for 2 hours, evacuating to remove bubbles, maintaining the temperature unchanged, adding 1,4-butanediol and a catalyst, stirring for 2 hours to react, and drying to obtain a polyurethane composite material; A3. The polyurethane composite material obtained in step A2 is placed in a microcellular foaming injection molding machine, CO ₂ is injected through a supercritical fluid device, and injection molding is performed in a mold to form microcellular foams to obtain a polyurethane sheet based on modified graphene. 2. A polyurethane sheet based on modified graphene according to claim 1, characterized in that the modified graphene oxide is prepared by the following steps: S1. After phosphorus oxychloride, pentaerythritol and toluene are stirred evenly, nitrogen is introduced, and the temperature is raised to 65°C. After reacting for 75 minutes, the temperature is raised to 108°C, and reflux reaction is carried out until no hydrogen chloride gas is generated. After the reaction is completed, the mixture is cooled to room temperature, washed with ether, benzene and dichloromethane in sequence, distilled under reduced pressure, and dried in vacuo to obtain intermediate 1; S2, at room temperature, under nitrogen protection, mix the intermediate 1, 1-nonylamine, triethylamine and toluene, stir until the solid is completely dissolved, react at 70°C for 7h, filter and distill under reduced pressure to obtain intermediate 2; S3, at room temperature, under nitrogen protection, melamine, intermediate 2, triethylamine and toluene were mixed, stirred until the solid was completely dissolved, reacted at 80°C for 8h, and after the reaction was completed, filtered, rotary evaporated, purified by column chromatography, and the eluent was removed by rotary evaporation to obtain intermediate 3; S4. Graphene oxide was mixed with N,N-dimethylformamide, mechanically stirred for 30 min, ultrasonically treated for 45 min, intermediate 3 and dicyclohexylcarbodiimide were added and mixed and stirred evenly, reacted at 55° C. for 8 h with continuous stirring. After the reaction was completed, vacuum distillation, washing, and freeze-drying were performed to obtain modified graphene oxide. 3. A polyurethane sheet based on modified graphene according to claim 2, characterized in that the ratio of phosphorus oxychloride, pentaerythritol and toluene used in step S1 is 35.4g:13.6g:80mL. 4. A polyurethane sheet based on modified graphene according to claim 2, characterized in that the ratio of the amount of intermediate 1, 1-nonylamine, triethylamine and toluene used in step S2 is 30.7g:14.3g:15mL:75mL. 5. A polyurethane sheet based on modified graphene according to claim 2, characterized in that the ratio of melamine, intermediate 2, triethylamine and toluene used in step S3 is 13.4g:80.6g:30mL:125mL. 6. A polyurethane sheet based on modified graphene according to claim 2, characterized in that the ratio of graphene oxide, N,N-dimethylformamide, intermediate 3 and dicyclohexylcarbodiimide used in step S4 is 10g:100mL:15g:23.4g. 7. A polyurethane sheet based on modified graphene according to claim 1, characterized in that the raw materials are calculated in parts by weight as follows: 5-10 parts of modified graphene oxide, 150-200 parts of N,N-dimethylformamide, 30-40 parts of diphenylmethane diisocyanate, 50-60 parts of polytetramethylene ether glycol, 4-8 parts of 1,4-butanediol, and 0.2-0.4 parts of catalyst. 8. A polyurethane sheet based on modified graphene according to claim 1, characterized in that the catalyst in step A2 is one of stannous octoate and dibutyltin dilaurate.