news

Gas phase silicon dioxide (commonly known as gas silicon) is a nano inorganic powder prepared by high-temperature hydrolysis of silicon tetrachloride in a hydrogen oxygen flame. The primary particle size is usually 7-40 nm, and the particles agglomerate to form three-dimensional flocculent aggregates. According to whether the surface has been organically modified, it can be divided into two categories: hydrophilic and hydrophobic fumed silica. The most essential difference between the two is the content of silicon hydroxyl groups on the particle surface, which further determines the surface polarity, dispersion behavior, rheological control ability, and application boundary of the powder in the coating system. In formula design, it is necessary to select the appropriate type based on the resin system and coating performance requirements.

Hydrophilic gas-phase silica has not undergone organic surface modification, and a large amount of active silicon hydroxyl groups (Si OH) remain on the particle surface. Silicon hydroxyl groups have strong basicity, and particles can form a three-dimensional network structure through hydrogen bonding interactions. The abundant silicon hydroxyl groups make it highly wettable by polar media such as water and alcohol, and have good dispersibility in water-based coatings, water-based inks, and polar solvent systems. In terms of rheological control, the hydrophilic water silicon hydrogen bond network has a strong effect, with outstanding thickening and thixotropic efficiency. A small amount of addition can achieve anti settling and anti sagging effects in the system. However, this hydrogen bond network is highly sensitive to polar small molecules. When water, polyols, and surfactants are present in the formula, polar substances compete to consume silicon hydroxyl groups, disrupting the network structure constructed by gas and silicon, resulting in a decrease in system viscosity and thixotropic failure. At the same time, a large amount of polar silanol groups have strong hygroscopicity. When the powder is dispersed into the coating and cured, the residual silanol groups will become channels for water molecules to penetrate, enhancing the coating’s water absorption tendency and reducing its water resistance, moisture resistance, and salt spray resistance. Therefore, it is mostly used in water-based systems that do not require high water resistance.

Hydrophobic silica is a surface organic modification of hydrophilic silica using silane reagents such as dimethyldichlorosilane and hexamethyldisilazane. During the modification process, organic groups react with the surface silanol groups of particles, consuming the majority of active silanol groups. The particle surface is covered by non-polar organic groups, and the powder surface changes from polarity to hydrophobicity. The significant reduction of surface silicon hydroxyl groups weakens the hydrogen bonding forces between particles, and the initial efficiency of thickening and thixotropy in pure systems is slightly lower than that of hydrophilic gas silicon; But its rheological network is not easily disturbed by polar components such as water, alcohol, and surfactants, and its long-term rheological stability is better in formulations containing polar additives. After hydrophobic modification, the moisture absorption of the gas silicon is significantly reduced. Adding it to the interior of the paint film does not introduce a large number of polar hydrophilic groups, nor does it provide a permeation path for moisture. It has little negative impact on the coating’s water resistance, moisture resistance, and salt spray resistance. It is widely used in solvent based coatings, epoxy, polyurethane, acrylic paint, waterproof coatings, and sealant systems. The disadvantage is that hydrophobic silica is difficult to spontaneously wet in the aqueous phase and requires high-speed dispersion process. If the dispersion process is not properly controlled, the powder is prone to agglomeration, forming particle defects, which affect the appearance and transparency of the coating.

In the process of coating formula design, there is a clear trade-off between two types of silicon gas. Hydrophilic fumed silica has a lower cost, is easy to disperse in aqueous systems, and has a fast thickening effect. It is suitable for formulations that require anti settling and anti sagging properties, but have lower water resistance requirements for the coating; Although hydrophobic fumed silica has higher dispersion process requirements, it has strong rheological anti-interference ability and can significantly reduce the negative impact on the water resistance of the paint film. It is suitable for high-end coating systems with high weather resistance, anti-corrosion, and high temperature and humidity resistance. In some complex systems, the two can also be compounded to balance rheological efficiency and coating water resistance, but the dispersion matching needs to be fully evaluated during compounding to avoid coating defects such as agglomeration and gloss fluctuations.