Graphite is a natural solid lubricant, rich in resources, cheap in price and widely used. Graphite has laminar structure. Alkali metal, halogen metal halide, strong oxidizing oxic acid can be embedded into interlaminar and interlaminar compounds. Expanded graphite is a kind of graphite interlaminar compound product produced by chemical or electrochemical treatment with natural flake graphite as raw material. In this paper, the excellent properties of graphite and the application of expanded graphite are introduced.
The expanded graphite can expand by 150~300 times in an instant at high temperature, changing from lamellar to worm-like, so that the structure is loose, porous and curved, the surface area is enlarged, the surface can be enhanced, and the adsorption of flake graphite is enhanced. The worm-like graphite can be self-imbedded, which increases its softness, resilience and plasticity. Expanded graphite is a high quality material for flexible graphite board and various sealing parts. It is widely used in machinery, petroleum, chemical industry, metallurgy, navigation, aerospace, transportation and other industrial fields.
1.Properties of expanded graphite
Expansive graphite material, also known as flexible graphite material, is a crystalline compound that USES physical or chemical methods to insert non-carbonaceous reactants into graphite layers and combines them with the hexagon network plane of carbon elements while maintaining the graphite-laminated structure. Not only does it keep graphite high temperature resistance, corrosion resistance, can withstand the neutron flux, x rays, and gamma irradiation for a long time, low friction coefficient, good self-lubrication, conductive thermal conductivity, and the anisotropy and other excellent physical and chemical properties, but due to the interaction of insert material and graphite layer presents the original graphite and intercalated materials do not have new performance, overcoming the natural graphite brittleness and impact resistant bad faults. Graphite with the interlayer compounds in high temperature, interlayer compounds will decompose, produce a kind of the graphite layer between the C axis of thrust, thrust is greater than the adhesion strength between the layers of graphite particles, under the action of the thrust, graphite layers was away, so that the graphite particles along the C axis high inflation, the expansion of the formation of vermicular graphite. Graphite layer and layer can be "embedded" into chemical substances and have expansibility. If the graphite can be treated with sulfuric acid, the graphite expands at high temperature after drying, which is caused by the embedding of sulfuric acid molecules into the graphite layer. The expansion of the expanded graphite flake characteristics different from other expansion agent, heated to a certain temperature, due to the absorption in interlayer matrix compound decomposition, expanded graphite began to swell, referred to as the initial temperature, completely at 1000 ℃, big volume. The expansion volume can be more than 200 times as large as it was at the beginning.
2. Application of expanded graphite
1 sealing material
The development of flexible graphite seals in China has unique conditions, and its raw materials, flake graphite, have the largest reserves and quality in the world. Expanded graphite, also known as flexible graphite, is made from flake graphite, which has been specially processed. It is the preferred material for sealing in the engineering industry at present. It has the advantages of light weight, electric conduction, heat conduction, high temperature resistance, acid and alkali corrosion resistance, good elasticity, lubricity, plasticity and chemical stability.
Graphite sealing products are mainly divided into two categories: soft sealing parts and hard sealing parts. The soft seal, also known as the flexible graphite seal, is made of phosphorous flake graphite through chemical treatment and high temperature expansion to obtain the "graphite worm", and then further processed into the seal. The main varieties include expanded graphite disc, expanded graphite packing ring, expanded graphite metal winding pad, expanded graphite metal composite high strength gasket, expanded graphite metal composite high strength plate, impregnated carbon-fiber - expanded graphite composite disc. Hard seals are made of porous graphite impregnated with resin or metal, which is then processed into rotating seals or mechanically sealed moving rings and stationary rings. In addition, graphite can be combined with macromolecular organic compounds to make special seals. For example, combined with polyurethane elastomer, it can be used for sealing ring of oil field pumping unit. Combined with polytetrafluoroethylene, it can be used for the upper and lower sealing rings of the new type plunger valve.
2. Environmental protection
The expanded graphite obtained from high temperature expansion has rich pore structure and good adsorption performance, so it is widely used in environmental protection and biomedicine. The pore structure of expanded graphite consists of two types: open hole and closed hole. The pore volume accounts for about 98%, and the pore size is mainly large, with the pore diameter ranging from 1 nm to 10.3nm and the peak value of 10.3nm. It is also different from activated carbon, molecular sieve and other microporous materials in adsorption characteristics because it is mainly composed of large and medium pores. It is suitable for liquid adsorption, but not for gas adsorption. It is hydrophilic and hydrophobic in liquid phase adsorption, and 1g expanded graphite can absorb more than 80g heavy oil, so it is a promising environmental material to remove oil pollution from water surface. By pressing method or suction filtration method, 80% or 50%~70% of the heavy oil can be recovered from the expanded graphite of A heavy oil or crude oil with its own weight of about 80 times. At the same time, it is found that if the method of suction filtration is adopted, the B heavy oil and C heavy oil with high viscosity are almost not recovered, and the specific turbidity of expanded graphite is not damaged and can be re-adsorbed and used. However, with repeated recycling operations, the amount of sorption and recycling will gradually decrease.
Because of the adsorption properties of expanded graphite to organic and biological macromolecules, it has wide application prospects in biomedical materials. Expanded graphite is a new material with high carbon content. Its physical and chemical properties have strong adsorption performance. It is permeable to water and air. Carbon and carbon materials have been used in human engineering research since world war ii. Subsequently, internal medicine began to treat diarrhea. Blood filtration; Carbon fiber and carbon/carbon composite materials are used to make skull, tooth root, heart valve, etc. The former Soviet union also used activated carbon fiber for advanced breast cancer, cervical cancer postoperative erosion on the wound, it has a good adsorption anti-inflammation effect. The application of various carbon and carbon materials in human body shows that it has good biosolubility, no repulsion, no toxicity, no stimulation and no sensitization.
4 Flame retardant of plastic material
Expandable graphite is a good flame retardant for plastic materials. It has the characteristics of halogen free, non-toxic and pollution-free. The amount of expandable graphite is much smaller than that of common flame retardant when it reaches the same flame retardant effect. Its action principle is: at high temperature, expandable graphite expands rapidly and suffocates the flame. Meanwhile, the graphite-bulk material produced by it covers the surface of the substrate and insulates the contact between thermal radiation and oxygen. The acid inside the interlayer is released when it expands, which also promotes the carbonization of the base material, so as to achieve a good effect through various flame-retardant methods. At present, expanded graphite has been widely used in flame retardant polyurethane, flame retardant polyolefin, flame retardant rubber, flame retardant polystyrene plastics, flame retardant carbonate oligomers, flame retardant asphalt and flame retardant cables.
5 Fire protection package, plastic fire retardant, fire retardant ring
Because has the ability to resist damage in the expanded graphite under high temperature and high inflation rate, can be used as fire protection package, plastic type fireproof plugging material, composition of fire collars in the expansion of the effective flame retardant material, used in construction of fireproof plugging (such as sealing building pipelines, cables, wires, gas, gas pipe, the pipe through the hole, etc).
6 applications in paint
The fine particles of expandable graphite can be added into the common coatings, which can produce a good flame retardant antistatic coating and improve its resistance to high temperature and fire. A large number of lightweight non-combustible carbon layers formed in fire can effectively block the radiation of heat from the base material and effectively protect the base material. In addition, because graphite is a good electric conductor, the coating can prevent the accumulation of static charge and be used in oil storage tank, so as to achieve the dual effect of fire prevention and static electricity.