As a new type of functional carbon material, expanded graphite (
Expanded Graphite, referred to as EG) is a loose, porous, worm-like substance obtained from natural graphite flakes through intercalation, washing, drying, and high-temperature expansion. In addition to the excellent properties of natural graphite, such as resistance to cold and heat, corrosion resistance, and self-lubrication, EG also has softness, compression resilience, adsorption, ecological environment coordination, biocompatibility, and radiation resistance than natural graphite does not have. And other characteristics. As early as the early 1860s, Brodie heated the natural graphite with chemical reagents such as sulfuric acid and nitric acid and discovered expanded graphite, but its application only began a hundred years later. Since then, many countries have successively launched the research and development of expanded graphite and made major scientific research breakthroughs.
Expanded graphite can instantly expand 150-300 times in volume when exposed to high temperature, changing from flake to worm-like, resulting in a loose structure, porous and curved, expanded surface area, increased surface energy, and enhanced adsorption of flake graphite. Vermicular graphite can be embedded in itself. Together, this increases its flexibility, resilience and plasticity.
Several development directions of expanded graphite are as follows:
1. Expanded graphite for special purposesExperiments show that graphite worms have the function of absorbing electromagnetic waves, and this property makes expanded graphite of high military application value. Both the U.S. military and our military have conducted experimental studies in this area.
This expanded graphite must meet the following requirements:
(1) Low initial expansion temperature and large expansion volume;
(2) Stable chemical properties, storage for 5 years, the expansion rate basically does not decrease;
(3) The surface of expanded graphite is neutral and does not corrode the shell.
2. Granular expanded graphiteSmall particles of expanded graphite mainly refer to 300 mesh expandable graphite with an expansion volume of 100ml/g. This product is mainly used for flame-retardant coatings and is in great demand.
3. Expanded graphite with high initial expansion temperatureThe initial expansion temperature of expanded graphite with high initial expansion temperature is 290-300ºC, and the expansion volume is ≥230ml/g. This kind of expanded graphite is mainly used for the flame retardant of engineering plastics and rubber.
4. Surface modified graphiteWhen expanded graphite is used as a flame retardant material, it involves the compatibility of graphite and other components. Because of the high degree of mineralization on the surface of graphite, it is neither a lipophilic substance nor a hydrophilic substance. Therefore, the graphite surface must be modified to solve the problem of compatibility between graphite and other components. Someone once proposed to whiten the graphite surface, that is, to cover the graphite surface with a strong white film. This is a difficult problem to solve. It may be done in laboratories for problems involving membrane chemistry or surface chemistry, but there are difficulties in industrialization. This type of white expandable graphite is mainly used for flame retardant coatings.
5. Low initial expansion temperature, low temperature expanded graphiteThe temperature at which this kind of expanded graphite starts to expand is 80-150°C, and the expansion volume reaches 250ml/g at 600°C. The difficulty in preparing expandable graphite that meets this condition lies in:
(1) Choose a suitable intercalation agent;
(2) Control and mastery of drying conditions;
(3) Determination of moisture;
(4) Resolution of environmental protection issues.
Currently, the preparation of low-temperature expandable graphite is still in the experimental stage.
PerformanceExtremely strong pressure resistance, flexibility, plasticity and self-lubricating;
Extremely strong resistance to high and low temperature, corrosion resistance and radiation resistance;
Extremely strong seismic characteristics;
Extremely strong conductivity;
Extremely strong anti-ageing and anti-distortion characteristics;
It can resist the melting and penetration of various metals;
Non-toxic does not contain any carcinogens, and does not harm the environment;
The expansion characteristics of expandable graphite flakes are different from other expansion agents. When heated to a certain temperature, due to the decomposition of the compound absorbed in the interlayer lattice, the expandable graphite starts to expand, which is called the initial expansion temperature, at 1000ºC The expansion is complete and the maximum volume is reached. The expansion volume can reach more than 200 times the initial volume. The expanded graphite is called expanded graphite or graphite worm, which changes from the original scale shape to a very low-density worm shape, forming a very good thermal insulation layer. Expanded graphite is not only the carbon source in the expansion system but also the heat insulation layer, which can effectively insulate the heat. It has the characteristics of a low heat release rate, low-quality loss, and less smoke in a fire.
However, it can be found from the existing literature that expanded graphite is an adsorbent with excellent performance, especially because it has a loose porous structure and a strong adsorption capacity for organic compounds. 1 g of expanded graphite can adsorb 80 g of petroleum, so Expanded graphite is designed as an adsorbent for various industrial greases and industrial oils.
Expanded graphite is very easy to adsorb oils, organic molecules and hydrophobic substances, and it has the irreplaceable effect of other substances when used in water environmental protection treatment. When it is used in granular form for water surface degreasing, its dosage is 1~lo%/rn, and the adsorption time can range from 15rain to several hours according to the size of the oil area on the water surface and the type of oil.
| Variety | Specification | Expansion rate (ml/g) | PH value | Volatile matter (%) | Moisture (%) | Carbon content (%) | Granularity (%)Sieve | Granularity
(%)Under sieve | Sulfur content (%) | Expansion temperature |
| Ordinary expandable graphite | 32mesh | 200-400 | 3-7 | ≤15.00 | ≤1.00 | 85.00-99.50 | ≥80.0 | / | ≤2.50 | ≥950ºC |
| 50mesh | 200-350 |
| 80mesh | 150-250 |
| 100mesh | 100-200 |
| 150mesh | 100-120 |
| Superfine expandable graphite | 100mesh | 30-60 | 3-7 | ≤10.00 | ≤1.00 | 85.00-99.50 | / | 85.00-99.50 | ≤2.50 | ≥950ºC |
| 200mesh | 20-50 |
| 325mesh | 10-20 |
| Ultra-low sulfur expandable graphite | 32mesh | 200-400 | 3-7 | ≤12.00 | ≤1.00 | 90.00-99.50 | ≥80.0 | / | ≤0.02 | ≥950ºC |
| 50mesh | 200-350 |
| 80mesh | 150-250 |
| 100mesh | 100-200 |
| High purity expandable graphite | 32mesh | 200-400 | 3-7 | ≤15.00 | ≤1.00 | ≥99.80 | ≥80.0 | / | ≤2.50 | ≥950ºC |
| 50mesh | 200-350 |
| 50mesh | 150-250 |
| 100mesh | 100-200 |
Flexible graphite sealing materials are mainly divided into two categories according to their uses: one is used for sealing packing on various pumps, valves, and reactors; the other is graphite gaskets used on various pipeline flanges.
Sealing packingSealing packing is a pre-formed packing that is cut into a suitable width and length of expanded graphite tape and wound in different specifications of metal moulds, and is directly formed on a press. It is suitable for various globe valves, gate valves, regulating valves, ball valves, adding valves, etc.
Sealing gasketUsually can be divided into two types, one is pure graphite gasket, which is formed by directly pressing expanded graphite pellets in a metal mould, or it can be directly punched or cut from expanded graphite sheet; the other is graphite wound gasket, It is made by overlapping metal tape and expanded graphite, which can be used under higher pressure.
Graphite packingGraphite packing is a sealing material woven with cotton fibre or graphite fibre and graphite roll foil. Graphite packing with cotton fibre core (SPM type) is suitable for sealing pipes, valves, pumps, etc. whose pressure is 12MPa and temperature is below 200ºC. The contact medium can be river water, tap water, groundwater, seawater, oil, etc. Graphite packing with graphite fibre as core (SPS type) is suitable for sealing pipes, valves, pumps, etc. whose pressure is 12MPa and temperature is below 350ºC. In addition to various water and oil, it can also contact acid and alkali substances.
Compared with other adsorbents, expanded graphite has many advantages. If activated carbon is used to remove oil from water, it will sink after adsorbing oil, the adsorption capacity is also small, and it is not easy to recycle; there are some adsorbents, such as cotton, straw ash, polypropylene fibre, perlite, vermiculite, etc. While absorbing oil, it also absorbs water, which brings difficulties to post-processing; expanded graphite has a large adsorption capacity for oil, and it floats on the water after absorbing oil. It is easy to catch and recover, and it is easy to recycle. It can use extrusion, centrifugal separation, vibration, and solvent. Methods such as cleaning, burning, heating and extraction, and will not form secondary pollution.
Because expanded graphite not only retains the excellent properties of natural graphite, such as high-temperature resistance, corrosion resistance, radiation resistance, and conductivity but also has many unique excellent properties, such as flexibility, resilience, self-adhesiveness, impermeability, adsorption and Low density and other characteristics, so it is particularly widely used in petroleum, chemical, atomic energy, electric power, pharmaceuticals, etc. With technological progress and high-tech development, expanded graphite, a new engineering material, has gradually replaced certain metal materials and organic synthetic materials in high-tech fields such as high speed, corrosion resistance, wear resistance, and energy saving.
Environmental protection field Expanded graphite has hydrophobic and lipophilic properties, and can selectively remove non-aqueous solutions in water, such as removing slick oil from the sea, rivers, and lakes. Expanded graphite can form a certain winding space when it absorbs oil, and can store oily substances much larger than its total pore volume. After absorbing a large amount of oil, it can be aggregated into blocks and float on the liquid surface, which is convenient for collection and can be regenerated and recycled. Since expanded graphite is basically composed of pure carbon, it is non-toxic and chemically inert, so it will not cause secondary pollution in water. In addition, expanded graphite can also be used to remove oil from industrial wastewater emulsions and to remove oil-soluble substances, such as pesticides, and has a good adsorption effect on many other organic or inorganic harmful components. In addition to selective adsorption in the liquid phase, expanded graphite also has a certain removal effect on the main components of air pollution generated by industrial exhaust gas and automobile exhaust gas, such as SOx and NOx. [2]
Sealing material Expanded graphite can be post-processed into flexible graphite for use as a sealing material. Compared with traditional sealing materials (such as asbestos, rubber, cellulose and their composite materials), flexible graphite can be used in a wider temperature range, in the air, the usable range is -200ºC-450ºC, and it can be reached in a vacuum or reducing atmosphere. 3000ºC and the coefficient of thermal expansion are small, does not become brittle, does not burst at low temperature, does not soften or creep at high temperature, so it is known as the "King of Sealing". It has been widely used in petrochemical, machinery, and metallurgy. , Atomic energy and other industries.
Biomedical Science Expanded graphite has the characteristics of good biocompatibility, non-toxicity, tasteless, and no side effects. It is a very important class of biomedical materials.
High-energy battery materials Adding expanded graphite to the zinc anode of a rechargeable zinc-manganese battery can reduce the polarization of the zinc anode during charging, enhance the conductivity of the electrode and electrolyte, inhibit the formation of dendrites, and provide good moulding characteristics, and inhibit anode dissolution and deformation, Extend battery life. In addition, lithium can form GICs with graphite through gas, liquid, solid and lithium salt electrolysis. Such GICs have low electrode potential and good intercalation and deintercalation reversibility.
Phase change heat storage material Phase change heat storage materials have poor thermal conductivity and poor heat transfer performance, which affect their energy storage and energy release efficiency. At the same time, the porous medium in the composite phase change material has a small porosity and contains less phase change material, resulting in low energy storage. These shortcomings limit the application and development of the material. The rich pore structure and high thermal conductivity of expanded graphite can make up for these defects.
Expanded graphite sheet has good electrical and thermal conductivity, with an electrothermal conversion rate of over 97%, and can generate far-infrared rays. It is a new type of heating material. Expanded graphite is pulverized into a fine powder, which has strong scattering and absorption characteristics for infrared waves, and is a good infrared shielding (stealth) material. Expandable graphite is made into pyrotechnic powder, which instantly explodes to form expanded graphite and is dispersed in predetermined airspace to form aerosol interference cloud smoke screen agent. In addition, expanded graphite can also be used as thermal insulation, sound insulation materials, electromagnetic shielding elements, and catalytic materials.
