The core principle of aluminum modification of acidic silica sol is to utilize the coordination or chemical bonding reactions between aluminum ions and the hydroxyl and siloxane chains in the silica sol system to optimize the stability, adhesion, and weather resistance of the silica sol.
Basic System and Modification Prerequisites: Acidic silica sol is a system of amorphous silica nanoparticles with a particle size of 10-100 nm dispersed in an acidic aqueous solution. The particle surface has a large number of silanol groups (Si-OH), and the system as a whole is positively charged. Its stability depends on the electrostatic repulsion and steric hindrance between particles. Aluminum modification generally uses aluminum salts (such as aluminum nitrate, aluminum sulfate, and aluminum isopropoxide) as modifiers. In an acidic environment, aluminum ions undergo hydrolysis to generate positively charged aluminum hydroxyl complexes (such as Al(OH)²⁺ and Al(OH)₂⁺).
Main Modification Reaction Mechanism
Surface Coordination Bonding Reaction
The Si-OH groups on the surface of silica sol particles undergo a dehydration reaction with aluminum hydroxyl complexes:
> Si-OH + Al(OH)ₙ³⁻ⁿ → Si-O-Al(OH)₂₋ₙ + H₂O
Through Si-O-Al chemical bonds, aluminum is anchored to the surface of silica particles, altering the charge distribution and functional group types on the particle surface.
Charge Regulation within the System
Incompletely coordinated free aluminum hydroxyl ions neutralize some of the positive charge on the surface of silica sol particles, while simultaneously forming weak bridging interactions between particles:
When the amount of aluminum added is low, it mainly increases the amount of positive charge on the particle surface, enhancing the electrostatic stability of the system;
When the amount of aluminum added is too high, excessive aluminum hydroxyl can induce particle aggregation, thus reducing stability.
Microstructure Optimization: After modification, aluminum forms a thin aluminum hydroxyl coating layer on the surface of silica particles. This fills defect sites on the particle surface, reducing the activity of hydroxyl groups; and improves the interfacial adhesion between the particles and organic or metal substrates, as Al-OH can further react with hydroxyl groups on the substrate surface to form chemical bonds.
Performance Changes After Modification
Improved Stability: Appropriate aluminum modification can broaden the pH range of silica sol from the original 2-4 to 3-6, while improving its anti-agglomeration ability under high temperature and electrolyte environments.
Enhanced Adhesion: The modified silica sol shows a 30%-50% increase in adhesion to glass, metal, and ceramic substrates (based on publicly available industry test data, 2024).
Optimized Weather Resistance: The coating layer can block the aging effects of ultraviolet rays on the silica sol film, extending the coating's service life.
Improved Film Formation Performance: The modified silica sol film exhibits increased hardness and enhanced wear resistance.
Modification Precautions
The amount of aluminum salt added must be strictly controlled, generally 1%-5% of the silica sol solid content. Excessive amounts will lead to gelation of the system.
Modification must be carried out in an acidic environment at room temperature to 60℃. Excessive temperature will accelerate the hydrolysis and agglomeration of aluminum ions.
Some aluminum salts can introduce anionic impurities. For high-purity modified systems, it is recommended to use organoaluminum modifiers such as aluminum isopropoxide.