Gas phase method (also known as "pyrolysis method") uses silicon sources such as silane (SiH ₄) or silicon tetrachloride (SiCl ₄) to undergo hydrolysis/oxidation reactions at high temperatures, generating gaseous silica, which is then cooled and aggregated to obtain ultrafine powders. This process produces products with higher purity and smaller particle size (10-50nm), but at a higher cost. It is mainly used for high-end transparent toothpaste or functional toothpaste that requires strong adsorption properties (such as odor adsorption and drug loaded toothpaste). The process is as follows:
Raw material vaporization: Heat high-purity silicon tetrachloride (SiCl ₄, purity ≥ 99.99%) to 100-150 ℃ for vaporization, and mix it with dry air (or oxygen) and hydrogen in proportion (hydrogen as fuel, air as oxidant).
High temperature reaction: The mixed gas is introduced into a high-temperature hydrolysis furnace (temperature 1000-1200 ℃), and the reaction occurs: SiCl ₄+2H ₂+O ₂ → SiO ₂+4HCl, and the generated silicon dioxide exists in the form of extremely fine gaseous particles.
Cooling and collection: The reaction product enters the cooler (temperature drops to 200-300 ℃), and the gaseous silica particles gradually aggregate into nanoscale powder, which is then collected through a bag filter.
Deacidification and modification: The collected powder needs to undergo deacidification treatment (to remove residual HCl and avoid abnormal pH values in toothpaste), and some products also need to undergo surface modification (such as using silane coupling agents to improve compatibility with the oil components in toothpaste).
Quality inspection and packaging: Using the same precipitation method, the focus is on detecting particle size distribution (10-50nm), specific surface area (300-600m ²/g), and purity (SiO ₂ ≥ 99.8%) to ensure compliance with high-end toothpaste requirements.
