In recent years, with the development of nanotechnology, people have successfully prepared a variety of nano powders, such as nano-SiO2, nano-TiO2, nano-CaCO3, and so on.
Among them, nano SiO2 is currently one of the most widely used inorganic nanomaterials, due to its non-toxic, odorless, pollution-free, high strength, high stiffness, and UV absorption characteristics.
Nano SiO2 has been widely used as an additive in polymer materials, such as coatings, adhesives, rubber, plastics, and other fields.
The application of nano silica in polymers can significantly improve the mechanical properties, wear resistance, elasticity, water resistance, and photostability of polymers. The broad commercial prospects and economic value have made the preparation and modification of nano SiO2 a hot topic in scientific research.
At present, there are two main methods for preparing nano SiO2 at home and abroad: physical and chemical methods. The physical method generally uses high-energy ball mills or ultrasonic airflow crushers to perform multi-stage crushing on SiO2 aggregates, ultimately obtaining products with a particle size of generally 1-5 μ m.
The advantages of using physical methods to prepare nano SiO2 are simple production process and easy control of product particle size. However, this process generally requires a lot of power, resulting in high energy consumption, low preparation efficiency, and easy mixing of impurities, which affects the performance of the product.
In addition, the nano SiO2 prepared by physical methods also has disadvantages such as poor particle spheroidization, wide particle size distribution range, and uneven distribution.
Compared with physical methods, using chemical methods can produce pure and uniformly distributed nano SiO2 particles. At present, chemical methods mainly include gas phase method, precipitation method, sol gel method, ionic exchange method and micro lotion method.
However, in terms of industrial production, the main preparation methods include gas phase method with silicon tetrachloride as raw material, precipitation method with sodium silicate and inorganic ACID as raw materials, sol-gel method with silicate ester as raw material and microemulsion method.
Gas-phase method
Gas phase method is currently the main method used in developed countries for industrial production of nano SiO2, and the nano SiO2 produced by this method has been widely used in various fields.
The production principle of gas-phase method is to directly use gas or through various means to convert reactants into gas, causing physical and chemical changes in the gaseous state, and then condensing and growing to form nanoparticles during the cooling process.
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Gas phase reaction equation
The specific process is to use organic silane halides as raw materials to undergo high-temperature hydrolysis (usually up to 1200-1600 ℃) in hydrogen and oxygen flames, generating extremely fine particulate smoke like SiO2.
The smoke like SiO2 is integrated into larger particles in the aggregator, and then collected by a cyclone separator into a deacidification furnace for deacidification treatment to obtain finished nano SiO2.
The nano SiO2 particle size obtained by this method is generally between 7-40nm, resulting in high purity, good dispersibility, and small particle size. However, it requires high equipment requirements, complex processes, high energy consumption, and high production costs.
Precipitation method
The precipitation method of nano SiO2 is obtained through the acidification reaction process of silicates. This method has a simple preparation process, low energy consumption, wide raw material sources, and low cost. However, the particle size of the product is affected by factors such as the type, concentration, and stirring speed of the acidifier.
In addition, the morphology of the product is difficult to control, the pore size distribution is wide, and it is easy to form aggregate particles. The quality of the product is not as good as that of the products produced by the vapor phase method and the sol gel method, and it usually cannot show the characteristics of nano materials. Instead, it is often used as a common filler for polymer reinforcement and toughening.
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Schematic diagram of the reaction principle of precipitation method
The precipitation method for preparing SiO2 not only has high water absorption, but also weaker heat resistance and electrical properties compared to the gas-phase method. Moreover, foaming phenomenon is prone to occur during its extrusion molding, making it difficult for hot air to undergo vulcanization. Compared to the gas-phase method, the reinforcement effect is poor.
The actual reaction mechanism of preparing nano-SiO2 by precipitation method is complex, which can be simplified into silicic acid polymerization and sol gel formation of SiO2 particles. At present, the precipitation method for preparing nano silica technology includes the following categories:
(1) Preparation of highly dispersed nano silica in organic solvents;
(2) Acidifiers react with silicate aqueous solutions, and the precipitates are separated and dried to prepare nano silica;
(3) The alkali metal silicate and inorganic acid are mixed to form nano silica hydrosol, which is transformed into gel particles, and then dried, washed with hot water, dried and calcined to obtain nano silica;
(4) Preparation of nano silica by carbonation of water glass;
(5) Non spherical silica nanoparticles with smooth edges were prepared by spray granulation.
By using precipitation method to prepare high-performance nano silica for reinforcing silicone rubber, the reinforcing performance is equivalent to that of gas-phase nano silica;
The particle has a comprehensive physical performance balance, and can be mixed with silicone rubber under low shear conditions to obtain a reinforcing structure. By determining the appropriate formula and achieving the best mechanical properties of the compound at a certain hardness level.
Sol gel method
Sol gel method uses silicon compounds with high chemical activity (such as silicates and silicates) as precursors, mixes these materials evenly in the liquid phase, and then adds acid to induce hydrolysis of silicate;
Dehydration condensation reaction occurs between the generated orthosilicic acid, forming a transparent, uniform and stable sol system in the solution. After sol aging, the colloidal particles slowly polymerize to form a three-dimensional network structure of gel;
When the middle of the gel network is filled with solvent that has lost its fluidity, gel is formed. Nano-SiO2 can be prepared by drying and sintering the gel.
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Reaction equation of sol gel method
The final particle size of SiO2 prepared by this method is influenced by the concentration of water and catalyst (acid or base), the type of silicate ester, different alcohols, and different temperatures.
By regulating these factors, various types of nano SiO2 structures can be obtained. The SiO2 prepared by Sol Gel technology has good morphology, high purity, large specific surface area, and is easy to achieve good dispersion and suspension in solution.
However, compared with the precipitation method, the raw materials used in the sol gel method are expensive, the preparation time is longer, and the preparation process is more complex.
