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Development and preparation of expandable graphite Expandable graphite

Issuing time:2018-12-05 Popularity:709

The graphite crystal has a hexagonal mesh planar layered structure composed of carbon elements. The carbon atoms in the plane of the layer are bonded by strong covalent bonds, and the layers are bonded by van der Waals force, the bonding is very weak, and the interlayer distance is large. Therefore, under appropriate conditions, various chemicals such as acids, alkali metals, and salts can be inserted between the graphite layers and combined with carbon atoms to form a new chemical phase—Graphite Intercalation on Compounds (GIC for short). ). When heated to a suitable temperature, the interlayer compound can be rapidly and instantaneously decomposed to generate a large amount of gas, and the graphite is expanded in the axial direction into a worm-like new substance, that is, expanded graphite. This unexpanded graphite intercalation compound is expandable graphite.
The expandable graphite was first discovered by the German Schaufautl. In 1841, Schaufautl immersed the natural graphite in a mixture of concentrated HNO3 and concentrated H2SO4. After several hours, it was taken out and dried, and it was found that the graphite was swollen. In 1963, United Union Carbide first applied for a patent for expandable graphite manufacturing technology and industrialized production in 1968.

Preparation method of expandable graphite:

1. Chemical intercalation method The initial raw material for preparation is high carbon flaky graphite. The other chemical reagents such as concentrated sulfuric acid (more than 98%), hydrogen peroxide (28% or more) and potassium permanganate use industrial grade reagents. The general procedure for preparation is as follows: at a suitable temperature, different ratios of hydrogen peroxide solution, natural flake graphite and concentrated sulfuric acid are reacted under constant stirring for a certain period of time, then washed with water until neutral, and centrifuged. After dehydration, it was dried under vacuum at 60 °C.
2. Electrochemical method The graphite powder is treated in a strong acid electrolyte to form expandable graphite, hydrolyzed, washed and dried. Sulfuric acid or nitric acid is mainly used as a strong acid. The expandable graphite produced by this method has a low sulfur content.
3. Ultrasonic oxidation method In the process of preparing expandable graphite, the anodized electrolyte is subjected to ultrasonic vibration, and the ultrasonic vibration time is the same as the anodization time. Because the vibration of the electrolyte on the electrolyte is beneficial to the polarization of the anode and the anode, the speed of the anodization is accelerated and the oxidation time is shortened.
4. Gas phase diffusion method respectively introduces graphite and intercalation materials on both ends of a vacuum sealed tube, and heats at the end of the intercalation layer, and uses the temperature difference between the two ends to form the necessary reaction pressure difference, so that the intercalated material enters the scales in a small molecule state. Between the graphite layers, an expandable graphite is produced. The number of layers of expandable graphite produced by this method can be controlled, but its production cost is high.
5. Molten Salt Method Several inserts are mixed with graphite to heat and combine to form expandable graphite.
Application editing
1. Sealing material The high-carbon graphite is mixed with concentrated sulfuric acid and concentrated 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 growth nano material. Compared with traditional sealing materials such as asbestos rubber, it has excellent compressibility, resilience, self-adhesiveness, low density, etc., and can be used for a long time under severe working conditions such as high temperature and high rot. Graphite sheets and sealing elements made of it are widely used in aerospace, machinery, electronics, nuclear energy, petrochemical, electric power, shipbuilding, smelting and other industries. Because of its light weight, electrical conductivity, thermal conductivity, high temperature resistance, acid and alkali corrosion resistance, good resilience, lubricity, plasticity and chemical stability, it is known as the world's "king of seals".
2. Environmental protection field Expandable graphite obtained by high temperature expansion has a rich pore structure and thus has excellent adsorption properties, so it has a wide range of uses in environmental protection and biomedicine. The pore structure of the expandable graphite has two types: open pores and closed pores, and the pore volume accounts for about 98%, and is mainly composed of large pores, and the pore size distribution ranges from 1 to 10. 3 nm. Since it is mainly composed of large pores and mesopores, it has different adsorption characteristics from microporous materials such as activated carbon. It is suitable for liquid phase adsorption and is not suitable for gas phase adsorption. It is lipophilic and hydrophobic in liquid phase adsorption. 1 g of expandable graphite can absorb more than 80 g of heavy oil, so it is a promising environmentally friendly material to remove surface oil pollution. 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 oily organic macromolecular contamination, due to good chemical stability. Reuse and reuse, so there is a good application prospect.
3. Medicine Because expandable graphite has adsorption properties for organic and biological macromolecules, it has broad application prospects in biomedical materials.
4. High-energy battery material As a battery material, expandable graphite is converted into electric energy by utilizing the change of free energy of the reaction between layers of expandable graphite. Generally, expandable graphite is used as a cathode, lithium is used as an anode, or expandable graphite composite silver oxide is used as a cathode, and zinc is used as an anode. Expandable graphite of fluorinated graphite, graphitic acid, and metal halides such as AuCl3 and TiF4 has been applied to batteries.
5, fire retardant fire
Fireproof sealing strip Due to the expandability of expandable graphite and its high temperature resistance, expandable graphite is an excellent sealing material and is widely used in fireproof sealing strips. There are two main forms: the first one is to mix, vulcanize and form expandable graphite materials with rubber materials, inorganic flame retardants, accelerators, vulcanizing agents, reinforcing agents, fillers, etc., and to form various specifications of expansion. Sealing strips are mainly used for fire doors, fireproof glass windows, etc. This expansion seal can block the flow of smoke from the beginning to the end in normal temperature and fire. The other is to use a glass fiber ribbon as a carrier to bond the expandable graphite to the carrier with a certain adhesive. The shear resistance provided by the carbonized carbide formed at the high temperature can effectively prevent the graphite. slide. It is mainly used for fire doors, but it can not effectively block the flow of cold smoke at normal temperature or low temperature, so it must be used together with a normal temperature sealant.
Flame Retardant of Plastic Materials Expandable graphite is a good flame retardant for plastic materials. It is non-toxic and non-polluting. It can achieve the desired flame retardant effect when used alone or in combination with other flame retardants. When the expandable graphite achieves the same flame retardant effect, the amount is much smaller than that of the ordinary flame retardant. The principle of action is: at high temperature, the expandable graphite expands rapidly and suffocates the flame, and the graphite bulk material produced by the graphite expands on the surface of the substrate to isolate the contact between thermal energy and oxygen; the acid radical inside the interlayer is inflated. The release also promotes the carbonization of the substrate, thereby achieving good results by various flame retardant methods.
Fireproof package, plastic fireproof blocking material and fire retardant ring. Because expandable graphite has anti-destructive ability at high temperature and has high expansion ratio, it can be used as a fireproof package, plastic fireproof blocking material and fire retardant ring component. Intumescent flame retardant material for fireproof sealing in buildings (for example: sealing of building pipes, cables, wires, gas, gas pipes, holes through which ducts pass, etc.).
Application on coatings Fine particles of expandable graphite are added to ordinary coatings to produce flame retardant antistatic coatings with better effects and high temperature and fire resistance. The large amount of light non-combustible carbon layer formed in the fire can effectively block the radiation to the substrate and effectively protect the substrate. In addition, since graphite is a good electrical conductor, the prepared coating can prevent the accumulation of static charges and is used in petroleum storage tanks to achieve the dual effects of fireproof and antistatic.
Fireproof board, fireproof paper Corrosion resistant high temperature resistant board: The metal base layer is lined with an expandable graphite layer, a carbonized adhesive layer is formed between the expandable graphite layer and the metal base layer, and the expandable graphite layer is covered with a carbonized protective layer. It has corrosion resistance, high temperature resistance and high pressure performance. At the same time, it has good thermal shock resistance and can be used normally at low temperature. It is not afraid of rapid cooling and rapid heat, and has excellent heat transfer coefficient. The operating temperature is -100~2 000 °C. Wide application range, easy to manufacture and low cost. In addition, after the high-expansion of expandable graphite, the graphite paper produced by compression is also used in a place for fireproof insulation.

Keywords in this article:Expandable graphite
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