Expandable graphite: a new type of lightweight and high-performance material
Expandable graphite was first discovered by German Schaufautl. In 1841, Schaufautl soaked natural graphite in a mixture of concentrated HNO3 and H2SO4 and, after a few hours, took it out for drying. It was found that the graphite had undergone expansion.
United Carbide Corporation of the United States first applied for a patent for expandable graphite manufacturing technology in 1963 and began industrial production in 1968.
Graphite crystals have a hexagonal network planar layered structure composed of carbon elements. The carbon atoms on the layer plane bind with strong covalent bonds, while the bonding between layers is very weak due to van der Waals forces, and the interlayer distance is large. Therefore, under appropriate conditions, various chemical substances such as acids, alkali metals, and salts can be inserted between graphite layers and combined with carbon atoms to form a new chemical phase - Graphite Intercalation on Compounds (GIC). When heated to an appropriate temperature, this interlayer compound can rapidly decompose and produce a large amount of gas, causing graphite to expand along the axis into a new wormlike substance, namely expanded graphite. This unexpanded graphite interlayer compound is expandable graphite.
Common preparation methods
1. The initial raw material for the chemical intercalation method is high carbon flake graphite. At the same time, industrial grade reagents are used for other chemical reagents, such as concentrated sulfuric acid (over 98%), hydrogen peroxide (over 28%), potassium permanganate, etc. The general steps of preparation are as follows: at an appropriate temperature, different ratios of hydrogen peroxide solution, natural flake graphite, and concentrated sulfuric acid are added in different procedures, stirred continuously for a certain time, washed to neutral, centrifuged, dehydrated, and vacuum dried at 60 ℃.
2. Electrochemical treatment of graphite powder in a strong acid electrolyte to produce expandable graphite, hydrolysis, cleaning, and drying. As a strong acid, sulfuric acid or nitric acid are mainly used. The expandable graphite produced by this method has a low sulfur content.
3. In the process of preparing expandable graphite using the ultrasonic oxidation method, the electrolyte of anodic oxidation is subjected to ultrasonic vibration, and the time of ultrasonic vibration is the same as that of anodic oxidation. Due to the favorable polarization effect of ultrasound on the cathode and anode of the electrolyte, the speed of anodic oxidation is accelerated and the oxidation time is shortened.
4. The gas phase diffusion method places graphite and the intercalated material at both ends of a vacuum sealed tube and heats them at the end of the intercalated material. The necessary reaction pressure difference is formed by utilizing the temperature difference between the two ends, allowing the intercalated material to enter the interlayer of flake graphite in a small molecule state, thus producing expandable graphite. The number of layers of expandable graphite produced by this method can be controlled, but its production cost is high.
5. The molten salt method heats several inserts with graphite to form expandable graphite.
What are the applications of expandable graphite?
1. The sealing material mixes high carbon graphite with concentrated sulfuric acid and nitric acid for acidification treatment, heat treatment, and then compression molding. The prepared, flexible graphite is a new high-performance sealing material, which is an in-situ grown nanomaterial. Compared with traditional sealing materials such as asbestos rubber, it has excellent compressibility, resilience, self-adhesiveness, and low density. It can be used for a long time under harsh working conditions such as high temperature and high corrosion. The graphite sheets and sealing components made from them are widely used in industries such as aerospace, machinery, electronics, nuclear energy, petrochemical, power, shipbuilding, and smelting. Because it has excellent characteristics such as lightweight, conductivity, thermal conductivity, high temperature resistance, acid and alkali corrosion resistance, good resilience, lubricity, plasticity, and chemical stability, it is known as the "king of sealing" in the world.
2. Expandable graphite obtained from high-temperature expansion in the field of environmental protection has rich pore structures and excellent adsorption properties, making it widely used in environmental protection and biomedical fields. There are two types of pore structures in expandable graphite: open and closed pores, with a pore volume of about 98%, and the main pore size is large, with a pore size distribution range of 1-10 3 nm. Due to its predominance of macropores and mesopores, it differs from microporous materials such as activated carbon in terms of adsorption characteristics. It is suitable for liquid phase adsorption, not for gas phase adsorption. In liquid phase adsorption, it is hydrophilic and hydrophobic. 1 g expandable graphite can adsorb over 80 g of heavy oil, making it a promising environmentally friendly material for removing oil pollution from water surfaces. In the wastewater treatment of chemical enterprises, microbial (bacterial) treatment is often used. Expandable graphite is a good microbial carrier, especially in the treatment of water contaminated by organic macromolecules such as oils. Due to its good chemical stability and renewable reuse, it has good application prospects.
3. Due to the adsorption properties of expandable graphite on organic and biological macromolecules, it has broad application prospects in biomedical materials.
4. The high-energy battery material, expandable graphite, is used as the battery material to convert the free energy changes of interlayer reactions of expandable graphite into electrical energy. Usually, expandable graphite is used as the cathode, lithium as the anode, or expandable graphite composite silver oxide as the cathode and zinc as the anode. Expandable graphite with metal halides such as fluorinated graphite, graphite acid, and AuCl3 and TiF4 has been applied to batteries.
5.Flame retardant and fireproof
Due to the expandability and high temperature resistance of expandable graphite, fireproof sealing strips have become excellent sealing materials and are widely used in fireproof sealing strips. There are mainly two forms: the first is to mix, vulcanize, and shape expandable graphite materials with rubber materials, inorganic flame retardants, accelerators, vulcanizing agents, reinforcing agents, fillers, etc., to produce various specifications of expansion sealant strips, mainly used for fire doors, fire resistant glass windows, and other occasions. This type of expansion sealing strip can play a role in blocking the flow of smoke throughout normal temperatures and fire. Another method is to use fiberglass tape as a carrier to bond expandable graphite to the carrier with a certain adhesive. The carbonized material formed by this adhesive at high temperatures provides shear resistance, which can effectively prevent the sliding of graphite. It is mainly used for fire doors, but it cannot effectively block the flow of cold smoke at room temperature or low temperature, so it must be used in conjunction with room temperature sealing agents.
The flame retardant expandable graphite of plastic materials is a good flame retardant for plastic materials, with non-toxic and pollution-free characteristics. It can achieve ideal flame retardant effects when used alone or mixed with other flame retardants. The amount of expandable graphite used to achieve the same flame retardant effect is much smaller than that of ordinary flame retardants. Its working principle is that at high temperatures, expandable graphite rapidly expands, suffocating the flame. At the same time, the generated graphite expanded material covers the surface of the substrate, isolating thermal radiation and oxygen contact. The acid radicals inside the interlayer are released during expansion, which also promotes the carbonization of the substrate, achieving good results through various flame retardant methods.
Fireproof bags, plastic fireproof blocking materials, and fire-resistant rings can be used as effective intumescent flame-retardant materials in fireproof bags, plastic fireproof blocking materials, and fire-resistant rings due to their ability to expandable graphite to damage at high temperatures and high expansion rates. They are used for fire-resistant sealing in buildings (such as sealing holes through building pipelines, cables, wires, gas pipes, air ducts, etc.).
The application of expandable graphite fine particles in coatings can be added to ordinary coatings to produce flame retardant and anti-static coatings with good effects, improving their high temperature resistance and fire resistance performance. The large amount of lightweight non-combustible carbon layer formed in a fire can effectively block the radiation of heat to the substrate and protect the substrate. In addition, as graphite is a good electrical conductor, the coating prepared can prevent the accumulation of static charges and be used in oil storage tanks to achieve a dual effect of fire and static electricity prevention.
Fireproof board and fire-resistant paper are corrosion-resistant and high-temperature resistant. The metal substrate is lined with an expandable graphite layer, and there is a carbonization adhesive layer between the expandable graphite layer and the metal substrate. The expandable graphite layer is covered with a carbonization protective layer. It has corrosion resistance, high temperature resistance, and high pressure performance. At the same time, it is heat-resistant and can be used normally at low temperatures. It is not afraid of rapid cooling and heating and has excellent thermal conductivity, with a service temperature of -100~2000 ℃. Widely applicable, easy to manufacture, and low cost. In addition, graphite paper, which is compressed from expandable graphite after high-temperature expansion, is also used in places with fire prevention and insulation.
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