Silica sol is a colloidal dispersion formed by nano-SiO2 (silicon dioxide) particles dispersed in water (or organic solvents), also known as silicic acid solution. It is usually a colorless or milky white transparent liquid, often with a large specific surface area, high adsorption, and its special high dispersion, high fire resistance and thermal insulation and other excellent properties.
Usually, the particle size of silica sol commonly used in industry is 10~50 nm. Industrial silica sol is divided into acidic silica sol, neutral silica sol and alkaline silica sol according to pH; if divided according to the particle size of silica sol, it is mainly divided into: small-particle silica sol, with a particle size of less than 8 nm; ordinary silica sol, with a particle size of 8~20 nm; large-particle silica sol, with a particle size of more than 20 nm.
Among them, large-particle silica sol has excellent bonding properties, adsorption properties and stability, which makes it widely used in many fields. For example: using large-particle silica sol as a single crystal silicon polishing agent not only has good effect, but also fast polishing speed. Why can a good polishing effect be achieved after the particle size increases, and how to prepare large-particle silica sol? Let's take a look together.
The common silica sol particle size control processes are as follows:
① High temperature and high pressure process
Increasing the temperature and pressure of the reaction system is conducive to the increase of the average particle size of nano-silicon dioxide in silica sol. Within a certain temperature range, the higher the reaction temperature, the faster the polymerization rate of active silica, and the clustering phenomenon of colloidal monomers may occur in the reaction system, thereby increasing the particle size of the generated silica colloidal particles. However, the continued increase in temperature has a certain impact on the uniformity and dispersibility of the colloidal particles. When other conditions are constant, the lower the temperature, the better the uniformity and dispersibility of the product colloidal particles; the higher the temperature, the worse the uniformity and dispersibility of the product colloidal particles. By observing the homemade products, it is known that the higher the temperature and pressure, the color of the product gradually deteriorates and the fluorescence also decreases.
② Preparation first and then concentration process
The concentration process not only increases the concentration of the product, but also increases the particle size of the product to a certain extent. However, the higher the concentration of silica sol, the shorter the gelation time, which leads to poor stability of the product. Therefore, during the concentration process, a protective agent should be added to shorten the gel time of the product and improve the stability of the product.
The concentration methods of silica sol can generally be divided into two categories: one is ultrafiltration and the other is evaporation. Ultrafiltration is a physical process that uses an ultrafilter for concentration. The evaporation method controls the feeding state under normal pressure or pressurized conditions to concentrate, and it is a process in which small particles of silica sol continue to grow into large particles of silica sol. Since the silica sol concentrated by ultrafiltration has a small particle size, high viscosity, poor stability, and easy gelation, the concentration method is not suitable for the concentration requirements of large-particle silica sol. In industry, the method of constant liquid level concentration with the addition of a base material is commonly used. This method can produce silica sols with a mass fraction of more than 30%. If a suitable substrate is added, not only will the particle size grow larger, but the mass fraction can also reach more than 50%.
③Catalytic polymerization process
In the preparation process of silica sol, as the size of the colloidal particles gradually increases, the polymerization rate of active silica gradually slows down, thereby prolonging the reaction time; in addition, the slowdown of the polymerization rate of active silica causes the accumulation of silica monomers, which is very easy to form new nuclei, thereby affecting the uniformity of the silica sol product. In short, in the process of preparing large-particle silica sol, the polymerization rate of silica must be increased. At present, the main methods to increase the polymerization rate of active silica are: adding additives and increasing the reaction temperature and pressure of the system. Increasing the reaction temperature and pressure of the system can prepare large-particle silica sol products to a certain extent, but it will also affect the uniformity and stability of the product. Adding additives not only promotes the polymerization of active silica, but also does not affect the stability and uniformity of the product.
④Inhibition of secondary particle generation and growth process
Usually, when the active silica generated reaches supersaturation, the colloidal particles begin to grow and grow into target particles after a certain period of time. If the active silicate produced quickly exceeds the nucleation concentration, a large number of new nuclei will be continuously generated. In the subsequent reaction, the target particles and the new nuclei grow simultaneously, and the particles grown from the new nuclei become secondary particles. Due to the existence of secondary particles, the silica sol product is polydisperse and has a wide particle size distribution. Therefore, during the preparation process, the generation and growth of secondary particles should be suppressed to improve the uniformity of the product.