Expandable graphite is a graphite having interlaminar intercalation formed by inserting a compound between graphite flake layers by an oxidative intercalation reaction of natural flake graphite. When the expandable graphite is placed at a high temperature, the interlayer intercalation material is rapidly vaporized by high heat, and the formed gas will generate a large tensile tension to the graphite sheet layer, and the expandable graphite expands, and the expansion ratio is up to several tens to hundreds of times. Even thousands of times, the apparent volume after expansion is 250 to 300 mL.g1 or more. The expanded graphite exhibits a wormlike shape with a size between a few tenths of a millimeter and a few millimeters. It has a large number of unique network-like microporous structures inside, called expanded graphite or graphite worms. Expandable graphite and expanded expanded graphite have a wide range of applications, such as flexible graphite, flame retardants, nano-conductive fillers, multi-band smokers, stealth shielding materials, oil absorbing materials, microbial carriers, medical dressings. , catalyst, immobilized carrier, solid electrolyte, etc.
Expandable graphite is now a very important class of functional graphite materials. In recent years, a large number of applications have been carried out on expandable graphite and expanded expanded graphite, especially in the field of sealing materials, which have been studied in the field of expanded graphite and expanded expanded graphite. The expansion of the application field and the widespread application of it are of great significance.
1. Application research in the field of sealing materials
Expanded graphite has a large specific surface area and high surface activity, does not require any binder, and does not need to be sintered to be compression molded. Graphite paper, coil or sheet produced by molding or rolling is called flexible graphite. Flexible graphite not only retains a series of excellent properties such as high temperature resistance, corrosion resistance and sealing properties of natural graphite, but also has softness, resilience and low density properties which natural graphite does not have. It is a very excellent sealing material. Used in the sealing of chemical, petroleum, electric power and other industries, it is known as the “king of seal”.
However, flexible graphite also has some weaknesses that cannot be ignored. For example, flexible graphite is a porous material. If it is directly used as a cylinder head gasket sealing material for automobiles, it may cause leakage problems and cannot be reused many times due to the sticking of the cylinder. Also, flexible graphite has low strength, poor wear resistance, and is not resistant to scouring, and it is not ideal as a sealing material for some pumps and valves. Therefore, in recent years, researchers engaged in flexible graphite research at home and abroad are working hard to research and develop flexible graphite composite materials. The flexible graphite composite materials studied include metal flexible graphite composite materials, polymer flexible graphite composite materials and inorganic materials-flexible graphite composite materials.
Metal-flexible graphite composites are available in both metal-plated and metal-plated versions. The former is made by pressing stainless steel or tin-plated spur steel plate or stainless steel wire mesh into flexible graphite, and can also be made into flexible graphite metal wound mat. It can have excellent high temperature resistance and pressure resistance, and is the most commonly used static sealing material for high temperature and high pressure applications. The latter is coated with a layer of chromium, molybdenum, tungsten and other metals on the surface of the flexible graphite sheet to become a special corrosion-resistant graphite product.
Polymer-flexible graphite composite materials are actively researched and developed at home and abroad. They are made of thermosetting resin, thermoplastic resin, rubber and fiber and flexible graphite. The method for preparing the thermosetting resin and the flexible graphite composite material may be direct mixing, or vacuum impregnation of the resin using a low-density graphite sheet, followed by heating and pressing. When preparing the thermoplastic resin and the flexible graphite composite material, it can be directly mixed and pressed, and can also be composited by surface coating, surface immersion, electrostatic spraying or the like. The rubber and flexible graphite composite material may be firstly impregnated with expanded graphite by liquid rubber, then dried, and then heated and pressed; the rubber liquid may also be sprayed on the surface of the expanded graphite, and then dried and hot pressed; in addition, a mixing process may also be adopted. preparation. In addition, for the polymer flexible graphite composite material, researchers also adhere the organic binder to the surface of the expandable graphite, so that it carbonizes when the expandable graphite is heated and expanded, and becomes a sticky carbon film or carbon fiber adhered to each other. To increase the strength, gas impermeability and other properties of flexible graphite.
The inorganic-flexible graphite composite is pressure-molded with an inorganic binder such as silicic acid, silicate, phosphoric acid, phosphate, boric acid, and borate. The tensile strength and compressive strength of the composite materials are greatly improved, especially the excellent heat resistance, and are ideal for high temperature sealing materials and high temperature lubricating materials. In addition, the flexible graphite composite material can also be used in combination with carbon fiber or the like.
Flexible graphite and its composite materials have excellent resistance to high and low temperature, corrosion resistance, radiation resistance, compression resilience and impermeability, so they are widely used in dynamic sealing and static sealing in various occasions in civil or military fields. The main varieties are expanded graphite packing, expanded graphite packing ring, expanded graphite metal winding mat, expanded graphite metal composite high-strength gasket, expanded graphite metal composite high-strength wrench, impregnated carbon fiber-expanded graphite composite packing. In addition, flexible graphite and its composite materials can also be used in other fields, such as electrode plates. At present, flexible graphite and its composite materials are gradually replacing some metal materials and organic materials, and their development prospects are extremely broad.
2. Application research in the field of polymer matrix composites
2.1 Polymer-based flame retardant composite materials
Expandable graphite is a new type of inorganic flame retardant. It has no disadvantages of organic flame retardant and is easy to bloom. It can be used for the flame retardancy of various polymers, which has caused widespread concern.
When the expandable graphite is used for the flame retardancy of the rigid polyurethane foam, it is found that the expandable graphite has better flame retardancy than the halogen-free flame retardant such as ammonium polyphosphate, triethyl phosphate, melamine, melamine cyanurate, and the like. There is synergistic or anti-synergy between the halogen-free flame retardant and the expandable graphite, and the synergistic effect of ammonium polyphosphate and triethyl phosphate with expandable graphite is the best. Red phosphorus and expandable graphite also have good flame retardant synergistic effects, such as composite flame retardant of red phosphorus and expandable graphite. When the mass fraction is 10%, the flame retardant polypropylene has an oxygen index of 23.4, and The mechanical properties are good, the tensile strength is 35.9 MPa, and the notched impact strength is 0.71 kJ·m 2 .
The addition of expandable graphite to the fireproof coating can improve the looseness of the expanded carbonaceous layer of the steel structure fireproof coating and easy to crack, and the fire resistance limit time of the steel structure is prolonged. Compared with fire retardant coatings with ammonium polyphosphate/pentaerythritol/melamine as flame retardants, fire retardant coatings with expandable graphite as flame retardant have more excellent flame retardant properties, better thermal insulation properties and longer fire resistance time.
In addition, the flame retardant which is mainly composed of expandable graphite is used for flame retardant of felt, exhibits excellent flame retardant effect, and the flame retardant of felt is UL 94-V-0 grade.
2.2 Polymer-based conductive composites
The expandable graphite is expanded to form expanded graphite, and then the expanded graphite is dispersed in a polymer matrix in a nano-graphite sheet to obtain a polymer-based nano-conductive composite material. Since the graphite is nano-dispersed in the matrix, the mechanical properties of the matrix are not greatly reduced, and the graphite is lamellar, so it is easy to form a conductive network in the matrix, and a small amount of graphite can achieve good electrical conductivity. This is not possible with other conductive fillers. Therefore, research on the preparation of polymer-based conductive composites using expanded graphite as a conductive filler has become a hot topic in recent years.
Expanded graphite as a conductive filler for the preparation of polymer-based conductive composites can be used in situ polymerization intercalation method. Chen Guohua et al. disperse the expanded graphite in a 70% aqueous solution of ethanol, ultrasonically disperse for 8 hours, filter and dry to obtain nanographite flakes, then mix the nanographite flakes with methyl methacrylate monomer and polymerize to obtain polymethyl. Methyl acrylate/nano-graphite composite, the conductive percolation threshold of the composite is only 1.0%, which is much lower than the conductive percolation threshold of polymethyl methacrylate/common graphite composite 5.5%, when nano-sheet graphite When the mass fraction is 4.0%, the electrical conductivity of the composite material is 103 S·cm. In addition, some researchers have used in-situ polymerization intercalation to prepare nano-conductive composites of polystyrene and polyamide and expanded graphite.
Expanded graphite as a conductive filler and polymer composite can also be used to expand the intercalation method of expanded graphite and polymer. The mixed intercalation method has two methods: solution intercalation method and melt intercalation method. The conductive composite material obtained by solution intercalation method The conductive performance is good, the conductive percolation threshold is about 2% (volume fraction), but the preparation process is complicated and consumes a large amount of organic solution, and the process cost is high, and the practical application is not significant. The melt intercalation method is the most promising method, but the conductive composite material obtained by directly intercalating the expanded graphite with the polymer has poor conductivity and high conductivity percolation threshold. The expanded graphite is first treated with a surfactant and then melt intercalated with the polymer. The process is simple, and the prepared polymer-based conductive composite material has excellent electrical conductivity and a low conductive percolation threshold.
Expanded graphite worms can be used as conductive fillers in the preparation of conductive coatings, which have very different electrical resistance properties than conventional powder-mixed coatings. Its absolute volume resistivity is an order of magnitude lower than that of the latter, and it hardly changes with the amount of filler added. It has a very stable volume resistivity, and its coating has an electromagnetic shielding efficiency of more than 40 dB and a water heating power density of 7 W. About cm2.
3. Applied research in the field of the environment
The surface of expanded graphite has developed pores, mostly mesopores or macropores, and the surface is non-polar, so it has excellent adsorption capacity for non-polar macromolecules. The study found that expanded graphite has strong adsorption capacity for oil, diesel, lubricating oil, kerosene, etc., and the oil absorption rate can be as high as 70-80 times or even higher.
Expanded graphite is an excellent oil adsorbent in water. After adsorbing a large amount of oil, it can be aggregated into pieces, floating on the water surface, easy to collect, and can be recycled and recycled. At the same time, the expanded graphite is basically composed of pure carbon, non-toxic and chemically inert, and does not cause secondary pollution during use. Therefore, expanded graphite can be used to absorb oil in the sea, rivers, and lakes, to remove oil from industrial wastewater emulsions, and to remove oil-soluble substances such as pesticides, and to have good properties for many other organic or inorganic harmful components. The adsorption effect.
The expanded graphite is used to treat the printing and dyeing wastewater of the woolen mill. The average removal rate of COD in the wastewater can reach 40% and the average chroma can be reduced by 40%. The average removal rate of COD in the wastewater can reach 20%, color. The average can be reduced by 20%. The high-temperature coal tar produced by the expanded graphite coking industry also has a good adsorption effect and can be an effective means for treating coking wastewater.
In addition to being used for selective adsorption in the liquid phase, expanded graphite can also utilize the selective adsorption characteristics of expanded graphite to purify the gas. Expanded graphite has good adsorption and removal ability for formaldehyde exhaust gas, and also has certain removal effects on industrial exhaust gas and main components of automobile exhaust gas such as SO x and NOx.
4. Applied research in the military field
Expanded graphite is pulverized into fine powder and has strong scattering absorption characteristics for infrared waves. It is a good infrared shielding (stealth) material. The expandable graphite is made into a pyrotechnic composition, which instantaneously explodes to form expanded graphite and is dispersed in a predetermined air space, and can form an aerosol interference cloud smoke agent. For example, a strong oxidant potassium chlorate and potassium perchlorate are used to make a pyrotechnic composition, and a suitable ignition sequence is used to realize the combustion and explosion of the pyrotechnic composition, and the expansive graphite is rapidly expanded and dispersed by the explosion process to realize the interference function.
The study found that the expanded graphite smoke screen can attenuate 3 mm and 8 mm wave radiation well, and the attenuation of 3 mm and 8 mm waves is 17.0 dB and 16.6 dB, respectively, while the aluminum foil strips are only 7.4 dB and 4.1 dB respectively, so they can be utilized. A smoke screen produced by an expandable graphite smoke agent interferes with millimeter wave radar detection.
Peng Junfang et al. uniformly mixes the iron hydroxide powder and the expandable graphite powder, and then rapidly expands at a high temperature, so that the fine iron oxide particles having magnetic properties are relatively uniformly dispersed in the graphite matrix, thereby obtaining an excellent electromagnetic wave having both conductivity and ferrimagnetism. Shielding material.
5 Applied research in other fields
Expandable graphite catalyzes certain chemical reactions. It has been found that expandable graphite has high catalytic ability for the synthesis of n-butyl acetate, benzyl acetate, dimethyl fumarate, pentaerythritol bis-benzaldehyde and methyl acrylate. For example, the synthesis of n-butyl acetate catalyzed by expandable graphite, the reaction time is only 55 min, and the ester yield is over 98%.
Due to the excellent adsorption properties of expanded graphite for organic and biological macromolecules, it has broad application prospects in biomedical materials. Tsinghua University and other developed expansive graphite to make medical dressings instead of medical gauze. After more than 300 experiments with mice, rats, guinea pigs and rabbits, it proved that it is non-toxic, has no side effects, no irritation to the wound surface, no blacking, and promotes Heal. In the 4 hospitals of the First Military Medical University, the Department of Burns, 114 clinical trials were carried out. The effect is better than that of traditional gauze drainage. It has obvious anti-infection, antibacterial and anti-inflammatory effects, and can replace 50%-80% of gauze. This achievement has obtained the invention patent and is being industrialized.
The addition of expanded graphite to the composite adsorbent block and the composite phase change heat storage material can significantly improve the thermal conductivity and the mass transfer performance. By utilizing the characteristics of strong adsorption of expanded graphite and its own good conductor, the expanded graphite is pressed into a certain shape and an electrolyte fixed body with a certain strength by using a crosslinking agent, and a lightweight storage battery with excellent performance and convenient use can be obtained. Further, expanded graphite can also be used as an electrode material or the like.
In the future, in addition to further research on high-quality flexible graphite and its composite materials, the application of expandable graphite in polymer-based composite materials, environmental fields and electric heating elements should be further studied to make it widely available in these fields. Promote the application.