Advantages of surfactants in defoamers
1. First of all, it has temperature resistance and hydrolysis resistance.
The hydrophilic group in the silicone surfactant cannot be directly connected to the silicon atom of the siloxane chain. The hydrophilic group directly connected to the silicon is easily hydrolyzed to form a silanol group, which can further condense and cross-link, thereby affecting the complete elimination of the chemical structure of the silicone surfactant.
Therefore, the synthetic silicone SAA must connect the hydrophobic group and the hydrophilic group through an isolating group. According to the difference in the connecting groups of the hydrophobic group and the hydrophilic group, polyether silicone oil can be: silicon-oxygen-carbon chain-hydrophilic group type, defoamer silicon-carbon-hydrophilic group type. The former is generally used as a foaming agent in the polystyrene foaming process, also called “hydrophilic silicone oil”. Silicon-oxygen-carbon chain-hydrophilic group type polyether silicone oil is generally not suitable for use in the presence of water, because the SI-O-C bond is easily hydrolyzed and condensed, resulting in the destruction of the structure and disappearance of the performance of the silicone surfactant, and it is not resistant to high temperatures. Silicon-carbon-hydrophilic group type silicone surfactants are stable in dispersion in water and will not hydrolyze. It can be used for a long time at less than 150°C; in a short time, it can reach 200°C. The above characteristics of silicone SAA determine that this type of substance has the characteristics of easy emulsification, excellent permeability, strong bonding with polar substrates, good surface spreadability, and thin film formation. If the hydrophilic group is not modified, it still has the characteristics of hydrophobicity.
2. Surface activity and easy spreadability (low surface tension)
The important characteristics of silicone SAA are its excellent surface activity and easy spreadability. This characteristic comes from the low surface tension and weak intermolecular force of polydimethylsiloxane. The silicon atom is at the center of the tetrahedron in the compound. According to the tetrahedral structure, the two methyl groups of tributyl phosphate are perpendicular to the plane connecting silicon and two adjacent oxygen atoms; because the SI-C bond length is large, the three hydrogens on the two non-polar methyl groups are like an open umbrella, which makes it very hydrophobic. The three hydrogen atoms on the methyl group occupy a large space due to the rotation of the methyl group, thereby increasing the distance between adjacent siloxane molecules; the intermolecular force is inversely proportional to the sixth power of the intermolecular distance, so the intermolecular force of polydimethylsiloxane is much weaker than that of hydrocarbons. Therefore, its surface tension is smaller than that of hydrocarbons with similar molar mass, making it easy for siloxane to spread on the interface. Another reason why polydimethylsiloxane chains are easy to spread on polar surfaces (such as water, metal, fiber, etc.) is that the oxygen in the silicon oxygen chain can form hydrogen bonds with polar molecules or atomic groups, increasing the molecular force between the silicon oxygen chain and the polar surface, prompting it to spread into a monolayer; thus, the hydrophobic siloxane lies horizontally on the polar surface, presenting a unique “extended chain” configuration. The hydrophobic group of ordinary SAA is upright on the polar surface.
3. Hydrophilicity
The typical type of silicone SAA is polyether silicone oil. The reason for tributyl ester is that polyether groups are suspended or blocked on its molecular chain (side chain or main chain), making it not only lipophilic but also hydrophilic. That is, the oil is not only miscible with alkane oily substances, but also easily dispersed in water. This is an advantage with great application value.
