Silica, also known as hydrated silica, is a white, non-toxic, amorphous micro powder. It has excellent properties such as high temperature resistance, dispersive porosity, electrical insulation, chemical stability and light weight. It is mainly used as filler for rubber, plastic, paper making, paint and other products . At present, about 70% of the world's white carbon black is used in the rubber industry. It is an excellent rubber reinforcing agent, which can improve the tear resistance of rubber compound, and its performance is better than that of common carbon black. In the paper industry, it can improve the paper's Baidu, strength and opacity. Silica can also be used in pesticide, cosmetics and other industries .
1. Overview of silica
1.1 introduction to silica
White carbon black is a general term for white powder X- ray amorphous silica and silicate products. It mainly refers to precipitated silica, fumed silica and ultrafine silica gel, and also includes powdered synthetic aluminum silicate and calcium silicate. Silica is a porous material, its composition can be expressed by SiO2 · nH2O, in which nH2O is in the form of surface hydroxyl. Soluble in caustic soda and hydrofluoric acid, insoluble in water, solvent and acid (except hydrofluoric acid). High temperature resistant, non combustible, tasteless, odorless, with good electrical insulation.
1.2 structure and properties of silica
The particle size of silica is very small, the primary particle size is about 0.01 ~ 1 μ m, and there are a large number of hydroxyl groups on the surface of silica particles. According to the research, there are three types of hydroxyl groups on the surface of silica: one is isolated and undisturbed isolated hydroxyl group; the other is the double hydroxyl group connected to a Si atom; the third is the adjacent hydroxyl group connected and forming hydrogen bond with each other. The presence of hydroxyl group makes it have strong hydrophilicity . At the same time, the active Si OH bond on the surface of the particles is easy to bond with other ions and play a reinforcing role. There is also a uniform layer of siloxane and silanol groups on the surface of silica, which have strong water absorption. Moreover, silanol is easy to be modified due to its reactivity .
2. Preparation of silica
According to its preparation method, silica can be divided into precipitated silica and gas phase silica. Among them, precipitation is the most important method to produce silica. The gas phase method is the main method to prepare nano silica . The traditional way to prepare silica is to use sodium silicate, silicon tetrachloride and tetraethyl orthosilicate as silicon source. Except sodium silicate, other costs are high. Nonmetallic ore method, which uses nonmetallic ore as silicon source, has the advantages of wide range of raw materials and low cost. It is a new method to prepare silica.
2.1 chemical vapor deposition
Chemical vapor deposition is also called dry or pyrolysis. Its primary particle size is between 7-40 nm. The specific surface area is generally greater than 100m2 / g, and the silica content is not less than 99.8%. In general, silicon tetrachloride gas is used to hydrolyze at high temperature in the mixture of hydrogen and air to synthesize silica aerosol, and then the processes of aggregation, cyclone separation and deacidification are carried out .
At present, the preparation of silica by gas-phase method is the main method to prepare nano silica. The silica product prepared by this method has high purity, large specific surface area, good dispersion, fine and spherical particles. The disadvantages are high equipment requirements, complex technology, expensive raw materials and large investment scale.
2.2 sedimentation method
The particle size distribution of silica products produced by precipitation method is uneven, and the quality of products with lower purity is different. The advantages are early start, mature technology, simple preparation process and low cost. The preparation methods of precipitated silica are carbonization, acidification, non-metallic mineral method, cellulose method, by-product recovery method, sol method, emulsion method and so on. Among them, the traditional preparation of precipitated silica is made by neutralization reaction of silicate and inorganic acid to generate hydrated silica, and then according to the requirements of the finished product, such as carbonization and acidification .
2.2.1 acidizing method
Acidification is the most traditional method to prepare silica. It is a method that the soluble silicate reacts with acid to remove sodium sulfate and water. The reaction equation is: Na2O · SiO2 + nH2O + 2H + → SiO2 · nH2O + 2Na + + H2O. At present, the key to the breakthrough of this technology is how to produce silica with excellent performance, such as gas phase method.
2.2.2 carbonation method
In recent years, as the greenhouse effect is more and more serious, people begin to pay attention to the emission and utilization of carbon dioxide. In order to reduce the consumption of inorganic acid and protect the environment, many scientists at home and abroad began to use carbon dioxide instead of inorganic acid to react with water glass to prepare silica. Compared with acidizing process, carbonation process is simple and easy to form scale. Compared with acidizing process, it can effectively recycle by-products, reduce emissions of three wastes, reduce production costs, improve economic benefits and protect the environment.
2.2.3 nonmetallic ore method
The common mineral silicon sources are mainly wollastonite, coal gangue, opal, hard kaolin, kaolin, halloysite, diatomite, bentonite, serpentine, quartz sand, fly ash, yellow phosphorus ore, sepiolite, etc. After crushing these ores, acid leaching or alkali leaching is used, and then pH adjustment, filtration, drying and other steps are carried out to prepare silica that meets the standard. Take kaolin as an example: after the kaolin is roasted in advance to enhance its acid dissolution reaction activity, it can be leached with acid to prepare silica .
On the one hand, the cost of silica produced by this method is lower than that of precipitation method, on the other hand, it can also produce nano silica with high efficiency next to gas phase method. At present, due to the rich non-metallic mineral resources, it is feasible technically, and has certain benefit prospects, which is the hot spot of research and application.
2.2.4 cellulose method
The cellulose raw material method is to prepare silica from rice husk, rice husk ash and other cellulose raw materials. It has the advantages of abundant sources, low cost, no harmful biological impurities such as heavy metals, and is suitable for people's livelihood industry; the content of radioactive elements is very low, and its application prospect in the electronic field is also very wide, so it is a preparation method of white carbon black with great market potential.
2.2.5 by product recovery method
The by-product recovery method mainly uses other by-products in production or reflection as raw materials to prepare silica. The common raw materials are sodium fluosilicate, the by-product of phosphate fertilizer, silicon tetrafluoride, etc. This method improves the utilization rate of resources and has good economic and environmental benefits, but the quality of silica is mostly low, which is not suitable for producing high-quality and high value-added silica products.
2.3 sol method
Silica prepared by sol method has large specific surface area and pore volume, which is suitable for the matrix of adsorbent and active metal catalyst. The disadvantage is that the silica prepared by sol method is similar to the sample prepared by traditional method, with high temperature instability and slightly smaller than the surface product.
2.4 emulsion method
The preparation of white carbon black by emulsion method is based on emulsifier and surfactant, which forms nanoscale micelles in the water system, and encapsulates the reaction materials, so that the total reaction is divided into various non interfering micro reactions, thus effectively avoiding the polymerization of primary particles of silica.
The white carbon black prepared by emulsion method is small and uniform, with narrow distribution and good dispersibility. At the same time, the disadvantages are also obvious, that is, all kinds of organic solvents and emulsifiers used in the reaction will remain in the product, which is difficult to completely remove, affecting the product performance.
3. Surface modification of silica
The polysiloxane inside the silica and the active silica alcohol group on the outer surface make it hydrophilic, so it is difficult to be wetted and dispersed in the organic phase, which affects the use effect of silica. There are hydroxyl groups on the surface of silica, the surface energy is large, and the aggregate tends to agglomerate, which affects the application performance of the product. In order to improve the wetting and dispersing of silica in organic phase and the reinforcing property of silica, it is necessary to modify the surface of silica. The surface modification of silica is to eliminate or reduce the amount of active silanol group on the surface of silica by using certain chemicals, so as to change its surface properties. The surface modification of silica can be divided into inorganic modification and organic modification. The inorganic modification is mainly titania coated silica. The modification of organics can be divided into dry modification, wet modification and hot pressing modification .
3.1 dry modification
At present, the modified fumed silica in developed countries all adopt the dry process. The main characteristics of this method are simple process, less post-treatment process, and the modified process is connected with the production process of fumed silica, which is easy to scale production. However, it has high requirements for equipment and large consumption of modifier, so the cost is high .
3.2 wet modification
There are two kinds of wet modification. One is to boil the dry silica and modifier in organic solvent, reflux and then separate them for drying. The main characteristics of this method are controllable product quality and less amount of modifier. The disadvantage is that the post-treatment process of the product is complex, easy to cause organic solvent pollution, and difficult to achieve large-scale production. The other is direct modification during the preparation of precipitated silica. The main features of this method are simple process, less auxiliary equipment, and cost saving .
3.3 modifier for silica modification
Generally, the substances which can react with the surface hydroxyl of silica can be used as modifiers, among which organic compounds have the best effect. The most commonly used modifiers include organohalosilanes, silane coupling agents, siloxanes, silazanes and alcohols.
The excellent structure and performance of white carbon black make its application scope expand continuously, which also puts forward higher requirements for the research and development of white carbon black. At present, the research of white carbon black mainly embodies in the following three aspects: (1) with the increasing attention to nano materials, scientists are also constantly exploring more economical and simple methods to prepare nano white carbon black. At present, most of the scientists focus on how to prepare nano-sized silica products with excellent properties by precipitation process. This may be an important breakthrough for the development of silica industry in the future. (2) Many scientists are studying how to modify white carbon black better and make it have better properties. At present, there is a big gap between the research of modified silica in China and the advanced level in the world. This is also a hot issue in the research and development of silica in China. (3) At present, more and more scientists at home and abroad use cheap non-metallic ore as silicon source to prepare silica. This method greatly reduces the production cost and saves energy. But its preparation technology is not mature, most of them are in the laboratory stage, but its huge economic benefits and practical significance attract more and more scientists to study this new method.
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