Method for synergistically preparing PVA-based carbon foam based on supercritical CO2 foaming-pre-oxidation-carbonization

Abstract

The invention provides a method for synergistically preparing PVA-based carbon foam based on supercritical CO2 foaming-pre-oxidation-carbonization, which comprises the following steps: grinding and dispersing PVA and an inorganic carbon nanofiller by a millstone-shaped solid-phase mechanochemical reactor, and adding into a synergistically crosslinked plasticizer prepared from boric acid, tannic acid and deionized water for swelling modification; and performing hot press molding, pre-oxidation treatment and carbonization treatment in sequence to obtain the PVA-based carbon foam. According to the preparation method, cross-linking plastification and supercritical CO2 are combined for foaming, the PVA-based carbon foam is prepared from PVA-based precursor foam directly through a pre-oxidation-carbonization method, and the thermal stability of PVA is improved while the form of foam holes is regulated and controlled based on a synergistic cross-linking plasticizer, so that the carbon yield is improved. The carbon yield of the prepared PVA-based carbon foam reaches 30% or above, and the PVA-based carbon foam has stable porous structure and good electromagnetic shielding efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing PVA-based carbon foam, and relates to a method for preparing PVA-based carbon foam based on the synergistic preparation of supercritical CO2 foaming-preoxidation-carbonization, and particularly relates to a method for participating in the preparation process using an industrial millstone-shaped solid-phase force chemical reactor disclosed in Chinese invention patent CN114534660B. Background Art

[0002] Carbon foam combines the advantages of carbon and porous materials, boasting properties such as high porosity, low density, and electrical conductivity. It has found widespread application in fields such as electromagnetic shielding. However, existing carbon foam preparation technologies have numerous shortcomings, particularly in the selection of precursor materials. While traditional carbon foam precursors such as asphalt and coke possess good carbon-forming properties, their complex composition makes the foaming and carbonization processes difficult to control, and they require high temperatures and high pressures, resulting in a complex and costly preparation process. Biomass precursors, while naturally porous and recyclable, have unclear chemical structures, and their environmental friendliness and cost-effectiveness remain to be evaluated over the long term. The existing technology lacks a carbon foam precursor with a well-defined chemical composition, a simple preparation process, and low cost. Synthetic polymer precursors have well-defined structures, making them easy to dope with functional components with controllable heteroatom content during the carbon foam preparation process. However, in terms of preparation technology, commonly used polymer carbon precursors (such as phenolic resins, polyimides, and polyacrylonitrile) typically require chemical foaming, which not only increases environmental burdens but also costs.

[0003] Polyvinyl alcohol (PVA) is a thermoplastic linear polymer with advantages such as excellent comprehensive performance, low cost and biodegradability. Its molecular structure contains only carbon, hydrogen and oxygen elements, and the carbon content is as high as 54.1%, so it is an ideal candidate material for carbon precursors. However, due to the close melting point and decomposition temperature, the production of PVA foam relies on complex and long-cycle solution methods, such as mechanical stirring or chemical foaming. In addition, PVA will produce a large amount of small molecular volatiles (such as water, CO, CO2, etc.) during high-temperature pyrolysis, which affects the structural stability and results in a low actual carbon yield. These are not conducive to the precise control of the pore structure during PVA foaming and carbonization. Therefore, it is necessary to solve the technical problems in the preparation process of PVA-based carbon foam to achieve the efficient preparation of high-performance carbon foam. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems in the above-mentioned prior art and to provide a method for the synergistic preparation of PVA-based carbon foam based on supercritical CO2 foaming-preoxidation-carbonization. The present invention combines cross-linking plasticization with supercritical CO2 for foaming, and realizes the preparation of PVA-based carbon foam directly from PVA-based precursor foam by the preoxidation-carbonization method. The implementation of this process is also mainly based on the use of boric acid (BA) and tannic acid (TA) as synergistic cross-linking agents to improve the thermal stability of PVA while regulating the pore morphology, thereby increasing the carbon yield, and adding inorganic carbon nanofillers to increase the strength of the pore wall. The carbon yield of the PVA-based carbon foam prepared by the present invention reaches more than 30%, and it has a stable and porous structure with good electromagnetic shielding efficiency.

[0005] To achieve the above objectives, the present invention is implemented by adopting a technical solution consisting of the following technical measures.

[0006] A method for preparing PVA-based carbon foam based on supercritical CO2 foaming-preoxidation-carbonization synergistically comprises the following steps by weight:

[0007] (1) 100 parts of PVA and 0.5 to 2.5 parts of inorganic carbon nanofiller are added to a millstone-shaped solid phase force chemical reactor for grinding and pulverization. After the grinding is completed, a mixed powder is collected; wherein the process parameters of the millstone-shaped solid phase force chemical reactor are: grinding pressure 2 to 8 MPa, grinding disc speed 40 to 50 rpm, grinding times 2 to 6 times, and -10 to 10°C circulating cooling liquid is introduced to control the grinding disc surface temperature;

[0008] (2) dissolving 3 to 15 parts of boric acid and 3 to 12 parts of tannic acid in 100 parts of deionized water to prepare a synergistic cross-linking plasticizer;

[0009] The mixed powder obtained in step (1) is fully dispersed in a synergistic cross-linking plasticizer, and swelled at a temperature of 35 to 65° C. for at least 24 hours. After the time is up, a swollen and modified composite powder is obtained;

[0010] (3) hot-pressing the swollen and modified composite powder obtained in step (2) into a PVA-based composite sheet;

[0011] (4) subjecting the PVA-based composite sheet obtained in step (3) to supercritical CO2 foaming to obtain a PVA-based precursor foam;

[0012] (5) pre-oxidizing the PVA-based precursor foam obtained in step (4) at a temperature of 210 to 250° C. for at least 2 hours to obtain a pre-oxidized PVA-based composite foam;

[0013] (6) carbonizing the pre-oxidized PVA-based composite foam obtained in step (5) to obtain a PVA-based carbon foam;

[0014] The carbonization treatment is carried out under inert atmosphere at a heating rate of 2 to 15°C / min, heating the material to 600 to 900°C and keeping the temperature for 10 to 80 minutes.

[0015] The PVA-based carbon foam prepared above has good electromagnetic shielding efficiency and can be used as an electromagnetic shielding material.

[0016] Herein, the millstone-shaped solid phase force chemical reactor in step (1) is an industrial millstone-shaped solid phase force chemical reactor disclosed in Chinese invention patent CN114534660B.

[0017] It should be noted that this industrial millstone-shaped solid-phase force chemical reactor is based on the principle of the force chemical reactor disclosed in the previously authorized patent ZL95111258.9, and is the final improved industrial equipment. It is significantly different from the structure of the laboratory prototype when ZL95111258.9 was applied for. A new millstone structure is designed for industrial high-efficiency force chemical grinding treatment. The previous vertical setting of the millstone is improved to a horizontal setting, and the size of the millstone is greatly increased. Based on the horizontal setting and large size of the millstone, the relevant fixed millstone components and hydraulic lifting system are innovatively designed, which greatly improves its three-dimensional shear force, thereby achieving industrial mass production efficiency.

[0018] Usually, the grinding times are 2 to 6 times. When the grinding times are 2 or more, the actual process operation is to grind the material through a millstone-shaped solid-phase force chemical reactor, collect the product at the discharge end, and then place it in a millstone-shaped solid-phase force chemical reactor for grinding again. The above process is regarded as grinding twice.

[0019] Herein, in step (1), the temperature of the grinding disc surface is controlled by introducing a circulating cooling liquid at a temperature of -10 to 10°C, wherein the cooling liquid is water, ethylene glycol or glycerol.

[0020] In this article, the PVA mentioned in step (1) is the abbreviation of polyvinyl alcohol, which is a conventional raw material in the field of chemical materials. Those skilled in the art can select a suitable PVA variety according to specific needs and process requirements, or refer to conventional PVA in the application field of the final product.

[0021] In one technical solution, the PVA in step (1) is preferably polyvinyl alcohol having a degree of polymerization of 1750±50 and a degree of alcoholysis of 80-99%.

[0022] Herein, the inorganic carbon nanofiller in step (1) is a conventional choice in the art for enhancing the structural strength of carbon foam, for example, graphene oxide, carbon nanotubes, carbon nanofibers, etc.

[0023] It should be noted that in step (1), the grinding and pulverization is carried out by using a millstone-shaped solid-phase force chemical reactor, mainly relying on its three-dimensional shear force to achieve sufficient mixing and compounding between PVA and the filler, thereby achieving maximum filler dispersion by mechanical means.

[0024] It should be noted that in step (2), the composite powder obtained after swelling and modification does not need to be filtered or dried, and the obtained product is directly hot-pressed.

[0025] The main point of the present invention is that during the early stage of the project investigation, the inventors found through searching the literature related to PVA-based carbon foam in the prior art that there is a common "seesaw" phenomenon of low carbon yield or overly complicated process. In the search process, it was found that boric acid is often used as a cross-linking agent in the field of PVA-based aerogels and hydrogels to improve the structural stability of PVA-based gels. However, in exploratory actual experiments, it was found that by plasticizing PVA and adding boric acid as a cross-linking agent, although the supercritical CO2 foaming performance of PVA can be effectively improved, PVA-based foam still has the main defects of easy decomposition and low carbon yield (about 10%) during the carbonization process, and the obtained carbon foam product shows the characteristics of being extremely easy to break into particles.

[0026] Based on the above experimental findings, the inventors first added inorganic carbon nanofillers during the preparation process to address the defect of being extremely easy to break. However, in the relevant experimental results, the prepared carbon foam product still had a low carbon yield and was extremely easy to break, proving that the addition of inorganic carbon nanofillers alone cannot fundamentally solve the above defects. This also indirectly reflects the rare literature reports on the preparation of PVA-based carbon foam with high carbon yield in the existing technology.

[0027] During further exploratory experiments, it was gradually concluded that the fragmentation of carbon foam is due to the significant increase in the amount of gas generated during the decomposition of PVA. These gases cannot escape quickly inside the material, resulting in increased local pressure and ultimately causing the material to fragment. After a large number of exploratory experiments and comparative experimental screening, the inventors accidentally discovered that when boric acid and tannic acid are selected as a two-component synergistic crosslinking agent, the above-mentioned defect of extremely easy fragmentation is significantly improved. It is speculated that this is because, on the one hand, the carbon layer formed by this synergistic effect can act as a physical barrier, preventing the diffusion of gas products generated by the decomposition of PVA, which leads to the fragmentation of carbon foam. On the other hand, this synergistic effect can capture free radicals that catalyze the degradation of PVA, reducing their diffusion in the matrix, thereby preventing the complete decomposition of PVA. Based on this, further single-variable comparative experimental screening found that under the process conditions specified by the present invention, the carbon yield of the prepared PVA-based carbon foam was significantly improved, and the prepared product had a certain structural stability, a complete shape and was not easy to fragment. When inorganic carbon nanofillers were added on this basis, PVA-based carbon foam with a carbon yield of more than 30% was prepared.

[0028] It is important to note that, during the comparative experiment described above, it was accidentally discovered that the pre-oxidation temperature in step (5) greatly affects the carbon foam yield, while the carbonization temperature in step (6) greatly affects the uniformity and integrity of the pore structure.

[0029] In this article, the swelled and modified composite powder in step (3) is hot-pressed into a PVA-based composite sheet. The technical details of the specific hot-pressing process method / operation / parameters can be directly referred to the relevant technical literature or conventional process methods of hot-pressing PVA into sheets in the prior art. In this step, the swelled and modified composite powder is only formed and used as a shape template for the subsequent PVA-based carbon foam. The specific process parameters usually do not affect the technical effect of the subsequent PVA-based carbon foam. Therefore, those skilled in the art can select a suitable hot-pressing process according to actual needs.

[0030] In order to better illustrate the present invention and provide a technical solution for reference, the swelled and modified composite powder in step (3) is hot-pressed into a PVA-based composite sheet, which is formed by placing the swelled and modified composite powder into a mold, maintaining the pressure at a temperature of 160-180°C and a pressure of 10-15 MPa for 5-15 minutes, and then cooling to room temperature while maintaining the pressure.

[0031] In this article, the PVA-based composite board described in step (4) is prepared by supercritical CO2 foaming to obtain a PVA-based precursor foam, wherein supercritical CO2 foaming is a currently mature foaming process. Those skilled in the art may refer to relevant technical literature or conventional process methods of supercritical CO2 foaming based on PVA materials.

[0032] In one of the preferred technical solutions, in order to obtain a PVA-based carbon foam with uniform and dense pores, the PVA-based composite sheet in step (4) is foamed by supercritical CO2 to obtain a PVA-based precursor foam. The specific process parameters include: temperature of 90-120°C, pressure of 10-16 MPa, and heat preservation and pressure maintenance for at least 30 minutes followed by rapid pressure release to obtain the PVA-based precursor foam.

[0033] In this article, the PVA-based precursor foam described in step (5) is pre-oxidized at a temperature of 210-250° C. for at least 2 hours. The specific technical details of the process method / operation / parameters of the pre-oxidation treatment can be directly referred to the relevant technical literature or conventional process methods of pre-oxidation-carbonization in the prior art. Under laboratory conditions and in the following examples, due to the small scale, the pre-oxidation treatment is directly carried out in an oven without an additional oxygen source. However, those skilled in the art should select an appropriate pre-oxidation process according to the prior art during industrial scale-up.

[0034] The present invention has the following beneficial effects:

[0035] (1) The present invention provides a method for preparing PVA-based carbon foam based on supercritical CO2 foaming-preoxidation-carbonization synergistically. First, based on the water / supercritical CO2 foaming synergistic plasticization and physical foaming technology, on this basis, by combining the doping of boric acid, tannic acid and inorganic carbon nanofillers, a PVA-based precursor foam with optimized pore structure, enhanced pore wall and improved thermal stability is successfully prepared, and the precursor foam is carbonized to prepare PVA-based carbon foam. In one of the technical solutions, the PVA-based precursor foam has a uniform small pore size (e.g., 32 μm) and a high pore density, which lays the foundation for the subsequent carbonization process.

[0036] (2) The present invention systematically studies the pre-oxidation and carbonization process of PVA-based precursor foam, explores the process parameters, and obtains PVA-based carbon foam with a stable porous structure and high carbon yield, thereby increasing the carbon yield of PVA-based carbon foam to more than 30%.

[0037] (3) Based on the variables of the pre-oxidation-carbonization process parameters, the present invention explored their effects on the yield of PVA-based carbon foam and obtained the optimal pre-oxidation temperature, time and heating rate.

[0038] (4) Based on the prepared PVA-based carbon foam, the present invention pioneered research on the effect of carbonization temperature on its electrical conductivity and electromagnetic shielding performance. In some examples, its shielding efficiency against X-band electromagnetic waves reached 36.5 dB. The results showed that the obtained PVA-based carbon foam has a stable porous structure and good electromagnetic shielding efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The following are SEM images and statistical plots of the pore size distribution of the PVA-based carbon foams prepared in Examples 1-4 of the present invention. It is clear that the average pore size decreases with increasing carbonization temperature. The pore size distribution of carbon foams obtained at 600°C, 700°C, and 800°C is more uniform than that of foam carbonized at 900°C. This is presumably due to the decomposition of pore walls at higher temperatures, severely damaging the pore structure. To achieve a complete conductive network structure, 800°C was selected as the optimal carbonization temperature.

[0040] Figure 2 This is a line graph comparing the carbon yields of the samples prepared in Example 3 of the present invention and Comparative Examples 1 to 3. PVA-BA-TA-GO refers to the PVA-based carbon foam prepared in Example 3.

[0041] Figure 3 This is a comparative curve diagram of carbon yield changes at different carbonization temperatures based on the pre-oxidation temperature of Examples 3 and Examples 5 to 8 of the present invention.

[0042] Figure 4 The SEM images of the pre-oxidized PVA-based composite foams obtained in step (5) of Examples 3 and 5 to 8 of the present invention are shown. It is clear that cracks appear in the cell walls of the pre-oxidized PVA-based composite foams obtained in Examples 7 and 8, and the cell structure collapses after pre-oxidation.

[0043] Figure 5 The following is a comparative line graph of the electrical conductivity of the PVA-based carbon foams prepared in Examples 1 to 4 of the present invention. The intermediate product, the PVA-based precursor foam, in step (4) is a polymer-based material. The small amount of conductive filler added is insufficient to form a conductive path in the matrix, so it is almost insulating. After carbonization at 600°C, the foam acquires a certain degree of conductivity, reaching a conductivity of 0.26 S / m. This is because the removal of oxygen-containing functional groups causes the PVA chains to initially carbonize, resulting in the deposition of conductive carbon particles that form a conductive path. As the carbonization temperature increases, the carbon particle content increases, and the porous structure obtained by supercritical foaming is retained. The conductivity of the carbon foam increases to 0.32 S / m for CF-700 and 2.79 S / m for CF-800, constructing a complete 3D conductive network. However, when the carbonization temperature is further increased, the destruction of the pore structure causes the conductivity to decrease.

[0044] Figure 6 The following is a comparative curve of the shielding efficiency of the PVA-based carbon foams prepared in Examples 1 to 4 of the present invention against electromagnetic waves in the 8-12 GHz band. The PVA-based precursor foam, the intermediate product in step (4), is almost transparent to electromagnetic waves. The carbonized foam has a certain shielding effect against electromagnetic waves, with electromagnetic shielding efficiencies increasing to 11.15 dB for CF-600 and 12.6 dB for CF-700. With increasing conductivity, the electromagnetic shielding efficiency of CF-800 significantly increases to 36.52 dB, while the electromagnetic shielding efficiency of CF-900 decreases slightly to 22.23 dB.

[0045] Figure 7 This is a photo of the PVA-based carbon foam prepared in Example 3 of the present invention.

[0046] Figure 8 This is a photo of the PVA-based carbon foam prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0047] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than for limiting the claims of the invention. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters for implementation. It is particularly important to point out that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention. Although it is believed that those of ordinary skill in the art fully understand the following terms, the following definitions are still stated to help illustrate the subject matter disclosed by the present invention.

[0048] A method for preparing PVA-based carbon foam based on supercritical CO2 foaming-preoxidation-carbonization synergistically comprises the following steps by weight:

[0049] (1) 100 parts of PVA and 0.5 to 2.5 parts of inorganic carbon nanofiller are added to a millstone-shaped solid phase force chemical reactor for grinding and pulverization. After the grinding is completed, a mixed powder is collected; wherein the process parameters of the millstone-shaped solid phase force chemical reactor are: grinding pressure 2 to 8 MPa, grinding disc speed 40 to 50 rpm, grinding times 2 to 6 times, and -10 to 10°C circulating cooling liquid is introduced to control the grinding disc surface temperature;

[0050] (2) dissolving 3 to 15 parts of boric acid and 3 to 12 parts of tannic acid in 100 parts of deionized water to prepare a synergistic cross-linking plasticizer;

[0051] The mixed powder obtained in step (1) is fully dispersed in a synergistic cross-linking plasticizer, and swelled at a temperature of 35 to 65° C. for at least 24 hours. After the time is up, a swollen and modified composite powder is obtained;

[0052] (3) hot-pressing the swollen and modified composite powder obtained in step (2) into a PVA-based composite sheet;

[0053] (4) subjecting the PVA-based composite sheet obtained in step (3) to supercritical CO2 foaming to obtain a PVA-based precursor foam;

[0054] (5) pre-oxidizing the PVA-based precursor foam obtained in step (4) at a temperature of 210 to 250° C. for at least 2 hours to obtain a pre-oxidized PVA-based composite foam;

[0055] (6) carbonizing the pre-oxidized PVA-based composite foam obtained in step (5) to obtain a PVA-based carbon foam;

[0056] The carbonization treatment is carried out under inert atmosphere at a heating rate of 2 to 15°C / min, heating the material to 600 to 900°C and keeping the temperature for 10 to 80 minutes.

[0057] The PVA-based carbon foam prepared above has good electromagnetic shielding efficiency and can be used as an electromagnetic shielding material.

[0058] Herein, the millstone-shaped solid phase force chemical reactor in step (1) is an industrial millstone-shaped solid phase force chemical reactor disclosed in Chinese invention patent CN114534660B.

[0059] It should be noted that this industrial millstone-shaped solid-phase force chemical reactor is based on the principle of the force chemical reactor disclosed in the previously authorized patent ZL95111258.9, and is the final improved industrial equipment. It is significantly different from the structure of the laboratory prototype when ZL95111258.9 was applied for. A new millstone structure is designed for industrial high-efficiency force chemical grinding treatment. The previous vertical setting of the millstone is improved to a horizontal setting, and the size of the millstone is greatly increased. Based on the horizontal setting and large size of the millstone, the relevant fixed millstone components and hydraulic lifting system are innovatively designed, which greatly improves its three-dimensional shear force, thereby achieving industrial mass production efficiency.

[0060] Usually, the grinding times are 2 to 6 times. When the grinding times are 2 or more, the actual process operation is to grind the material through a millstone-shaped solid-phase force chemical reactor, collect the product at the discharge end, and then place it in a millstone-shaped solid-phase force chemical reactor for grinding again. The above process is regarded as grinding twice.

[0061] Herein, in step (1), a circulating cooling liquid at -10 to 10° C. is introduced to control the temperature of the grinding disc surface. In one embodiment, the cooling liquid is water, ethylene glycol or glycerol.

[0062] In this article, the PVA mentioned in step (1) is the abbreviation of polyvinyl alcohol, which is a conventional raw material in the field of chemical materials. Those skilled in the art can select a suitable PVA variety according to specific needs and process requirements, or refer to conventional PVA in the application field of the final product.

[0063] In one embodiment, the PVA in step (1) is preferably polyvinyl alcohol having a degree of polymerization of 1750±50 and a degree of alcoholysis of 80-99%.

[0064] Herein, the inorganic carbon nanofiller in step (1) is a conventional choice in the art for enhancing the structural strength of carbon foam. In one embodiment, for example, the inorganic carbon nanofiller includes graphene oxide, carbon nanotubes, carbon nanofibers, and the like.

[0065] It should be noted that in step (1), the grinding and pulverization is carried out by using a millstone-shaped solid-phase force chemical reactor, mainly relying on its three-dimensional shear force to achieve sufficient mixing and compounding between PVA and the filler, thereby achieving maximum filler dispersion by mechanical means.

[0066] In one embodiment, the inorganic carbon nanofiller in step (1) is 0.5 to 2.5 parts, for example, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2 parts, 2.1 parts, 2.2 parts, 2.3 parts, 2.4 parts, 2.5 parts or any range or point value therebetween.

[0067] It should be noted that in step (2), the composite powder obtained after swelling and modification does not need to be filtered or dried, and the obtained product is directly hot-pressed.

[0068] In one embodiment, the amount of boric acid in step (2) is 3 to 15 parts, for example, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts or any range or point value therebetween.

[0069] In one embodiment, the tannic acid in step (2) is 3 to 12 parts, for example, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, 11 parts, 12 parts or any range or point value therebetween.

[0070] The main point of the present invention is that during the early stage of the project investigation, the inventors found through searching the literature related to PVA-based carbon foam in the prior art that there is a common "seesaw" phenomenon of low carbon yield or overly complicated process. In the search process, it was found that boric acid is often used as a cross-linking agent in the field of PVA-based aerogels and hydrogels to improve the structural stability of PVA-based gels. However, in exploratory actual experiments, it was found that by plasticizing PVA and adding boric acid as a cross-linking agent, although the supercritical CO2 foaming performance of PVA can be effectively improved, PVA-based foam still has the main defects of easy decomposition and low carbon yield (about 10%) during the carbonization process, and the obtained carbon foam product shows the characteristics of being extremely easy to break into particles.

[0071] Based on the above experimental findings, the inventors first added inorganic carbon nanofillers during the preparation process to address the defect of being extremely easy to break. However, in the relevant experimental results, the prepared carbon foam product still had a low carbon yield and was extremely easy to break, proving that the addition of inorganic carbon nanofillers alone cannot fundamentally solve the above defects. This also indirectly reflects the rare literature reports on the preparation of PVA-based carbon foam with high carbon yield in the existing technology.

[0072] During further exploratory experiments, it was gradually concluded that the fragmentation of carbon foam is due to the significant increase in the amount of gas generated during the decomposition of PVA. These gases cannot escape quickly inside the material, resulting in increased local pressure and ultimately causing the material to fragment. After a large number of exploratory experiments and comparative experimental screening, the inventors accidentally discovered that when boric acid and tannic acid are selected as a two-component synergistic crosslinking agent, the above-mentioned defect of extremely easy fragmentation is significantly improved. It is speculated that this is because, on the one hand, the carbon layer formed by this synergistic effect can act as a physical barrier, preventing the diffusion of gas products generated by the decomposition of PVA, which leads to the fragmentation of carbon foam. On the other hand, this synergistic effect can capture free radicals that catalyze the degradation of PVA, reducing their diffusion in the matrix, thereby preventing the complete decomposition of PVA. Based on this, further single-variable comparative experimental screening found that under the process conditions specified by the present invention, the carbon yield of the prepared PVA-based carbon foam was significantly improved, and the prepared product had a certain structural stability, a complete shape and was not easy to fragment. When inorganic carbon nanofillers were added on this basis, PVA-based carbon foam with a carbon yield of more than 30% was prepared.

[0073] It is important to note that, during the comparative experiment described above, it was accidentally discovered that the pre-oxidation temperature in step (5) greatly affects the carbon foam yield, while the carbonization temperature in step (6) greatly affects the uniformity and integrity of the pore structure.

[0074] In this article, the swelled and modified composite powder in step (3) is hot-pressed into a PVA-based composite sheet. The technical details of the specific hot-pressing process method / operation / parameters can be directly referred to the relevant technical literature or conventional process methods of hot-pressing PVA into sheets in the prior art. In this step, the swelled and modified composite powder is only formed and used as a shape template for the subsequent PVA-based carbon foam. The specific process parameters usually do not affect the technical effect of the subsequent PVA-based carbon foam. Therefore, those skilled in the art can select a suitable hot-pressing process according to actual needs.

[0075] In order to better illustrate the present invention and provide an embodiment for reference, the swelled and modified composite powder in step (3) is hot-pressed into a PVA-based composite sheet, which is formed by placing the swelled and modified composite powder into a mold, maintaining the pressure at a temperature of 160-180°C and a pressure of 10-15 MPa for 5-15 minutes, and then cooling to room temperature while maintaining the pressure.

[0076] In this article, the PVA-based composite board described in step (4) is prepared by supercritical CO2 foaming to obtain a PVA-based precursor foam, wherein supercritical CO2 foaming is a currently mature foaming process. Those skilled in the art may refer to relevant technical literature or conventional process methods of supercritical CO2 foaming based on PVA materials.

[0077] In one preferred embodiment, in order to obtain a PVA-based carbon foam with uniform and dense pores, the PVA-based composite sheet in step (4) is foamed by supercritical CO2 to obtain a PVA-based precursor foam, and the specific process parameters include: temperature of 90-120°C, pressure of 10-16 MPa, and heat preservation and pressure maintenance for at least 30 minutes followed by rapid pressure release to obtain the PVA-based precursor foam.

[0078] In this article, the PVA-based precursor foam described in step (5) is pre-oxidized at a temperature of 210-250° C. for at least 2 hours. The specific technical details of the process method / operation / parameters of the pre-oxidation treatment can be directly referred to the relevant technical literature or conventional process methods of pre-oxidation-carbonization in the prior art. Under laboratory conditions and in the following examples, due to the small scale, the pre-oxidation treatment is directly carried out in an oven without an additional oxygen source. However, those skilled in the art should select an appropriate pre-oxidation process according to the prior art during industrial scale-up.

[0079] In one embodiment, the heating rate in step (6) is 2 to 15°C / min, for example, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min or any range or point value therebetween.

[0080] In one embodiment, the insulation in step (6) is 10 to 80 minutes, for example, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, 70 minutes, 75 minutes, 80 minutes or any point value therebetween.

[0081] The present invention will be further explained in detail below with reference to the examples. However, it will be appreciated by those skilled in the art that these examples are provided for illustrative purposes only and are not intended to limit the present invention.

[0082] Example

[0083] The embodiments of the present application will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if no specific conditions are indicated, the conditions are carried out according to normal conditions or manufacturer recommendations. The reagents used or the instruments that are not indicated by the manufacturer are conventional products that can be obtained commercially. The application should not be construed as being limited to the specific examples described.

[0084] 1. Raw materials

[0085] Polyvinyl alcohol (PVA1799), average degree of polymerization 1750±50, alcoholysis degree 99 mol%, industrial grade, Sichuan Vinylon Plant of Sinopec Group;

[0086] Boric acid (BA), analytical grade, Chengdu Kelong Chemical Co., Ltd.;

[0087] Graphene oxide, Suzhou Tanfeng Graphene Technology Co., Ltd.;

[0088] Tannic acid (TA), analytical grade, Chengdu Zhuopu Instrument Co., Ltd.;

[0089] Carbon dioxide, purity >99.5%, Chengdu Xuyuan Chemical Gas Co., Ltd.;

[0090] Deionized water, homemade in the laboratory.

[0091] 2. Preparation method

[0092] (1) 100 parts of PVA and 1 part of graphene oxide were added to a millstone-type solid phase force chemical reactor for grinding and pulverization. After the grinding was completed, the mixed powder was collected; wherein the process parameters of the millstone-type solid phase force chemical reactor were: grinding pressure of 4 MPa, the temperature of the millstone surface was controlled by passing 5°C circulating cooling liquid, the grinding number was 4 times, and the millstone speed was 50 rpm;

[0093] (2) dissolving 10 parts of boric acid and 8 parts of tannic acid in 100 parts of deionized water to prepare a synergistic cross-linking plasticizer;

[0094] The mixed powder obtained in step (1) is fully dispersed in the synergistic cross-linking plasticizer, and swelled at 60°C for 24 hours. After the time is up, a swollen and modified composite powder is obtained;

[0095] (3) hot pressing the swelled and modified composite powder obtained in step (2) into a PVA-based composite sheet; specifically, placing the swelled and modified composite powder into a mold, maintaining the pressure at 165° C. and 10 MPa for 5 minutes, and then cooling to room temperature;

[0096] (4) subjecting the PVA-based composite sheet obtained in step (3) to supercritical CO2 foaming to obtain a PVA-based precursor foam; specific process parameters include: temperature of 110° C., pressure of 16 MPa, heat preservation and pressure maintenance for 30 minutes, and then rapid pressure release to obtain a PVA-based precursor foam, which is recorded as composite foam;

[0097] (5) placing the PVA-based precursor foam obtained in step (4) in an oven at a temperature of 210 to 250° C. for pre-oxidation treatment for 2 hours to obtain a pre-oxidized PVA-based composite foam;

[0098] (6) carbonizing the pre-oxidized PVA-based composite foam obtained in step (5) to obtain a PVA-based carbon foam;

[0099] The carbonization treatment is carried out under argon atmosphere at a heating rate of 5°C / min, heating the temperature to 600-900°C and keeping the temperature for 60 minutes.

[0100] 3. Test methods

[0101] The cell morphology and structure were observed by scanning electron microscopy (SEM, Inspect F, FEI, USA) at an accelerating voltage of 5 kV. The average cell size and cell size distribution were studied using Nano measure 1.2 software.

[0102] The carbon yield was measured by thermogravimetric analysis (TGAQ-50, TA Instruments Co. Ltd., USA) under a nitrogen atmosphere at a heating rate of 10°C / min.

[0103] The electrical conductivity of the carbon foam was tested by a four-probe resistivity tester (FT-361A, China).

[0104] The electromagnetic interference (EMI) parameters (S) of carbon foam were tested in the 8.2-12.4 GHz (X-band) band by using a vector network analyzer (N5247A, KEYSIGHT, USA). 11 ,S 21 ). Reflection (R), transmission (T) and absorption (A) coefficients and shielding effectiveness (SE A ,SE R and SE T ) is calculated using the following formula:

[0105]

[0106] A=1-RT (1-3)

[0107] SE A=-10log(T / (1-R)) (1-4)

[0108] SE R = -10log(1-R) (1-5)

[0109] SE T =SE A +SE R +SE M (1-6)

[0110] Example 1

[0111] Example 1 is in accordance with the steps of "2. Preparation method" above, wherein step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 600°C to prepare a PVA-based carbon foam as a sample, which is recorded as CF-600.

[0112] Example 2

[0113] Example 2 is in accordance with the steps of "2. Preparation method" above, wherein step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 700°C, to prepare a PVA-based carbon foam as a sample, which is recorded as CF-700.

[0114] Example 3

[0115] Example 3 is in accordance with the steps of "2. Preparation method" above, wherein step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 800°C, to prepare PVA-based carbon foam as samples, which are recorded as CF-800 and POF-230.

[0116] Example 4

[0117] Example 4 is in accordance with the steps of "2. Preparation method" above, wherein step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 900°C, to prepare a PVA-based carbon foam as a sample, which is recorded as CF-900.

[0118] Example 5

[0119] Example 5 is in accordance with the above-mentioned "2. Preparation method" steps, wherein step (5) is a pre-oxidation treatment at a temperature of 210°C, and step (6) is a carbonization treatment at a temperature of 800°C, to prepare a PVA-based carbon foam as a sample, recorded as POF-210.

[0120] Example 6

[0121] Example 6 is in accordance with the above-mentioned "2. Preparation method" steps, wherein step (5) is a pre-oxidation treatment at a temperature of 220°C, and step (6) is a carbonization treatment at a temperature of 800°C, to prepare a PVA-based carbon foam as a sample, which is recorded as POF-220.

[0122] Example 7

[0123] Example 7 is in accordance with the above-mentioned "2. Preparation method" steps, wherein step (5) is a pre-oxidation treatment at a temperature of 240°C, and step (6) is a carbonization treatment at a temperature of 800°C, to prepare a PVA-based carbon foam as a sample, which is recorded as POF-240.

[0124] Example 8

[0125] Example 8 is in accordance with the steps of "2. Preparation method" above, wherein step (5) is a pre-oxidation treatment at a temperature of 250°C, and step (6) is a carbonization treatment at a temperature of 800°C, to prepare a PVA-based carbon foam as a sample, recorded as POF-250.

[0126] Comparative Example 1

[0127] Comparative Example 1 refers to the steps of "2. Preparation method" above, but graphene oxide is not added in step (1), boric acid and tannic acid are not added in step (2) (i.e., 100 parts of deionized water are directly used as a plasticizer), step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 800°C to prepare a PVA-based carbon foam as a comparative sample, recorded as PVA.

[0128] Comparative Example 2

[0129] Comparative Example 2 refers to the steps of "2. Preparation method" above, but graphene oxide is not added in step (1), tannic acid is not added in step (2), step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 800°C to prepare a PVA-based carbon foam as a comparative sample, which is recorded as PVA-BA.

[0130] Comparative Example 3

[0131] Comparative Example 3 refers to the above-mentioned "2. Preparation method" steps, but graphene oxide is not added in step (1), step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 800°C to prepare a PVA-based carbon foam as a comparative sample, which is recorded as PVA-BA-TA.

[0132] The actual photos are as follows Figure 8 As shown, it has broken after carbonization without the action of external force.

[0133] Comparative Example 4

[0134] Comparative Example 4 refers to the above-mentioned "2. Preparation method" steps, but tannic acid is not added in step (2), step (5) is a pre-oxidation treatment at a temperature of 230°C, and step (6) is a carbonization treatment at a temperature of 800°C to prepare a PVA-based carbon foam as a comparative sample.

[0135] The physical performance is consistent with that of Comparative Example 3, and it has broken into pieces after carbonization without the action of external force.

[0136] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing PVA-based carbon foam based on supercritical CO2 foaming-preoxidation-carbonization synergistically, characterized in that The main steps are as follows: (1) 100 parts of PVA and 0.5 to 2.5 parts of inorganic carbon nanofiller are added to a millstone-shaped solid phase force chemical reactor for grinding and pulverization. After the grinding is completed, a mixed powder is collected; wherein the process parameters of the millstone-shaped solid phase force chemical reactor are: grinding pressure 2 to 8 MPa, grinding disc speed 40 to 50 rpm, grinding times 2 to 6 times, and -10 to 10°C circulating cooling liquid is introduced to control the grinding disc surface temperature; (2) dissolving 3 to 15 parts of boric acid and 3 to 12 parts of tannic acid in 100 parts of deionized water to prepare a synergistic cross-linking plasticizer; The mixed powder obtained in step (1) is fully dispersed in a synergistic cross-linking plasticizer, and swelled at a temperature of 35 to 65° C. for at least 24 hours. After the time is up, a swollen and modified composite powder is obtained; (3) hot-pressing the swollen and modified composite powder obtained in step (2) into a PVA-based composite sheet; (4) subjecting the PVA-based composite sheet obtained in step (3) to supercritical CO2 foaming to obtain a PVA-based precursor foam; (5) pre-oxidizing the PVA-based precursor foam obtained in step (4) at a temperature of 210 to 250° C. for at least 2 hours to obtain a pre-oxidized PVA-based composite foam; (6) carbonizing the pre-oxidized PVA-based composite foam obtained in step (5) to obtain a PVA-based carbon foam; The carbonization treatment is carried out under inert atmosphere at a heating rate of 2 to 15°C / min, heating the material to 600 to 900°C and keeping the temperature for 10 to 80 minutes.

2. The method according to claim 1, wherein: The inorganic carbon nanofiller in step (1) includes any one of graphene oxide, carbon nanotubes, and carbon nanofibers.

3. The method according to claim 1, wherein: The swelled and modified composite powder in step (3) is hot-pressed into a PVA-based composite sheet by placing the swelled and modified composite powder into a mold, maintaining the pressure at a temperature of 160-180° C. and a pressure of 10-15 MPa for 5-15 minutes, and then cooling to room temperature while maintaining the pressure.

4. The method according to claim 1, wherein: The PVA-based composite sheet material in step (4) is prepared by supercritical CO2 foaming to obtain a PVA-based precursor foam, and the specific process parameters include: temperature of 90-120°C, pressure of 10-16 MPa, and heat preservation and pressure maintenance for at least 30 minutes followed by rapid pressure release to obtain the PVA-based precursor foam.

5. The method according to claim 1, wherein: The PVA polymerization degree in step (1) is 1750±50 and the alcoholysis degree is 80-99%.

6. The method according to claim 1, wherein: In step (5), the pre-oxidation treatment is carried out at a temperature of 220 to 240°C.

7. The method according to claim 1, wherein: The carbonization treatment in step (6) is performed by heating the temperature to 700-800°C.

8. The PVA-based carbon foam prepared by the method for preparing PVA-based carbon foam by synergistic supercritical CO2 foaming-preoxidation-carbonization as described in claim 1.

9. Use of the PVA-based carbon foam as claimed in claim 8 as an electromagnetic shielding material.

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