What are the various methods for manufacturing nano-Silica? Their characteristics, applications, and how to Choose?
Nano-silica is one of the most widely used inorganic nanomaterials today. As a filler, it is extensively applied in polymer materials such as films, coatings, adhesives, rubber, and plastics. It can significantly improve the physical properties, wear resistance, expansion resistance, water resistance, and dielectric properties of polymers.
The production methods of nano-silica are mainly divided into physical and chemical methods:
Physical Method: This involves using a ball mill or crusher to pulverize silica in multiple stages, eventually obtaining the final product. The particle size is generally larger, approximately 1-5 µm. The advantages of this method include a simple production process and easy control of product fineness. However, it requires a lot of power, leading to high electricity consumption and low production efficiency. Additionally, it has disadvantages such as high impurities, low sphericity, and wide particle size distribution.
Chemical Methods: Chemical methods can produce nano-silica with high purity and uniform particle size distribution. These methods include vapor-phase method, precipitation method, and sol-gel method. The vapor-phase method uses silicon tetrachloride as a raw material, the precipitation method uses sodium silicate and inorganic acids, and the sol-gel method (Sol-gel) uses silicon ester (TEOS) as a raw material.
Vapor-Phase Method: This is currently the main method used by advanced countries for industrial production of nano-silica, also known as white smoke. The production principle is to turn the reactants into gases, allowing physical and chemical changes to occur in the gas phase, and then condensing during the cooling process to form nano-particles. The reaction equation is: using organic silicon halides as raw materials, hydrolyzed at high temperatures (generally as high as 1200~1600°C) in a hydrogen and oxygen flame to generate fine particulate smoke-like silica. The smoke-like silica aggregates into larger particles in the collector, and then after being collected into the deacidification furnace for deacidification treatment, the final nano-silica product is obtained. The particle size is generally below 50 nm, with high purity, good dispersibility, and small particle size. However, this method requires high equipment standards, has a complex process, high energy consumption, and high production costs. It is suitable for applications such as toner additives, composite materials, paint, and ink viscosity adjusters.
Sol-Gel Method (Sol-Gel): This uses highly active silicon compounds (such as TEOS) as precursors. The raw materials are uniformly mixed in the liquid phase, then acid is added to initiate the hydrolysis of silicate. The resulting silicate undergoes dehydration condensation reactions, forming a transparent and stable sol system in the solution. After standing for a while, the sol particles slowly polymerize to form a three-dimensional network structure gel. When the solvent in the gel loses fluidity, a gel is formed. The gel is dried, sintered, and solidified to produce nano-silica. The final particle size of the silica produced by this method is influenced by factors such as water and catalyst (acid or base) concentration, the type of silicon ester, different alcohols, and different temperatures. By controlling these factors, various structured nano-silica can be obtained. The silica produced by Sol-Gel technology has good sphericity, high purity, large specific surface area, and good dispersibility and suspension in solution. It is the most suitable process for high-end semiconductor material applications. Whether developing underfill, die attach adhesives, CCL, or ABF films, it is recommended to use Sol-Gel method silica. In recent years, its application in large particle size (100 nm) toner additives has also become increasingly prominent.
Precipitation Method: Nano-silica produced by the precipitation method is obtained through the acidification reaction of silicate. This method is simple, energy-efficient, has a wide range of raw materials, and is inexpensive. However, the particle size is influenced by factors such as the type and concentration of the acidifying agent and stirring speed, making it difficult to control the product's morphology. The pore size distribution is wide, and it tends to form agglomerates, often failing to exhibit the properties of nano-materials and usually serving as ordinary fillers for polymer toughening. Compared to the vapor-phase method, silica produced by the precipitation method has greater water absorption, weaker heat resistance, and electrical properties, and tends to foam during extrusion molding, making air vulcanization difficult. Therefore, its reinforcing effect is inferior to the vapor-phase method. The basic reaction mechanism is silicate polymerization and sol-gel formation of silica particles, mainly applied in the rubber, food, and pharmaceutical industries.
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