Modified silicone oil
” Modified silicone oil is a special silicone oil in which part of the methyl groups in the methyl silicone oil molecule is replaced by various organic groups, and has the interactive properties of organic groups and dimethyl silicone oil.”
In fact, depending on different application needs, there are more copolymer products composed of two or more groups on the same silicone backbone.
There is no clear boundary between “reactive silicone oil” and “non-reactive silicone oil”. If the active groups of “reactive silicone oil” are blocked, they will lose their reactivity. On the contrary, inactive silicone oil, such as polyether silicone oil, will be reactive if it contains unblocked hydroxyl groups.
Surfactant
1. The concept of surfactant
Any pure liquid under certain conditions has surface tension. At 20°C, the surface tension of water is 72.75ｍn·ｍ-1. When a solute is dissolved into a solvent, the surface tension of the solution changes due to the addition of the solute. The surface tension of the aqueous solution changes due to different solutes. For example, some inorganic salts can slightly increase the surface tension of water, and some lower alcohols can increase the surface tension of water. The surface tension of water decreases slightly, while soap and laundry detergent can cause the surface tension of water to decrease significantly.
The property that reduces the surface tension of a liquid is surface activity. Surfactants refer to substances that have strong surface activity and can significantly reduce the surface tension of liquids. In addition, as a surfactant, it should also have application properties such as solubilization, emulsification, wetting, decontamination, sterilization, defoaming and foaming. This is an important difference from general surface-active substances.
2. Structural characteristics of surfactants
Surfactant molecules generally consist of non-polar hydrocarbon chains and one or more polar chains. It is composed of sexual groups, and the length of the hydrocarbon chain is generally more than 8 carbon atoms.
Polar groups can be dissociated ions or undissociated hydrophilic groups. The polar group can be carboxylic acid and its salts, sulfonic acid and its salts, sulfate ester and its soluble salts, phosphate ester group, amino or amine group and their salts, or it can be hydroxyl group, amide group, ether bond, carboxyl group Acid ester group, etc.
For example, soap is a fatty acid (R-COO-) surfactant. The fatty acid carbon chain (R-) in its structure is a lipophilic group, and the dissociated fatty acid radicals (COO-) is a hydrophilic group.
3. Adsorption of surfactant
Positive adsorption of surfactant molecules in solution
When surfactant dissolves in water, when the concentration of surfactant in the water is very low, the surfactant molecules are oriented at the water-air interface, with the hydrophilic groups facing the water and the lipophilic groups Toward the air.
When the solution is dilute, the surfactant is almost completely concentrated on the surface to form a monomolecular layer. The concentration of surfactant in the surface layer of the solution is much higher than that in the solution, and will The surface tension of the solution is reduced below the surface tension of pure water. The phenomenon of surfactant accumulation on the surface layer of a solution is called positive adsorption.
Positive adsorption changes the properties of the solution surface. The outermost layer exhibits hydrocarbon chain properties, thus showing lower surface tension and resulting in better wettability. , emulsifying properties, foaming properties, etc. If the surfactant concentration is lower and the surface tension is reduced more significantly, the surface activity is stronger and positive adsorption is easier to form. Therefore, the surface activity of a surfactant is of great significance to its practical application.
Adsorption of surfactants on solid surfaces
When a surfactant solution comes into contact with a solid, the surfactant molecules may be adsorbed on the solid surface, causing the solid surface properties to change. The adsorption curve of ionic surfactants adsorbed by polar solid substances at low concentrations is S-shaped, forming a monomolecular layer, and the hydrophobic chains of surfactant molecules extend into the air.
When the concentration of the surfactant solution reaches the critical micelle concentration, the adsorption reaches saturation. At this time, the adsorption is double-layer adsorption, and the arrangement direction of the surfactant molecules is consistent with that of the first layer. In contrast, the hydrophilic groups point toward the air. As the solution temperature increases, the amount of adsorption will decrease. For non-polar solids, generally only monolayer adsorption occurs. The hydrophobic group is adsorbed on the solid surface and the hydrophilic group points to the air. When the surfactant concentration increases, the adsorption amount does not increase or even tends to decrease.
The adsorption of non-ionic surfactants on the solid surface is similar to the previous one, but the adsorption amount increases with the increase of temperature, and can be adsorbed from the monomolecular layer to the multimolecular layer. adsorption transition.
Organic silicone surfactant
“Reactivity” and “surface activity” are two different concepts. In layman’s terms, the so-called “reactivity” refers to the material’s ability to form molecular bonds with other material products. “Surface activity” means that the material has “hydrophilic” and “lipophilic” properties.
According to the surface activity characteristics of chemicals, part of the methyl groups in polydimethylsiloxane (PDMS) is replaced by carbon functional compounds, such as polyether, amino, Epoxy groups, carboxyl groups, quaternary ammonium groups, phosphate ester groups or copolymers composed of two groups on a silicone backbone can all be regarded as silicone surfactants.
“Silicon surfactant” is a new additive that has been active in the domestic materials industry market in the past decade. Silicone surfactant product development, production and application developmentThe reason for the rapid development is the rapidly developing market demand, as well as the excellent performance that imported products have achieved and the widespread use of application formula technology in China.
Among them, polyether-grafted polydimethylsiloxane (PDMSOPE), referred to as polyether silicone oil, is a typical one.
Characteristics of silicone surfactants
1. Surface activity and ease of spreading ( The important characteristics of silicone SAA (low surface tension) are its excellent surface activity and easy spreadability. This property comes from the low surface tension and weak intermolecular forces of polydimethylsiloxane. The silicon atom is at the center of the tetrahedron in the compound. According to the tetrahedral structure, the two methyl groups are perpendicular to the plane connecting silicon and two adjacent oxygen atoms; due to the large bond length of the SI-C bond, the three hydrogens on the two non-polar methyl groups are like braces. The open umbrella makes it very hydrophobic.
The three hydrogen atoms on the methyl group occupy a larger space due to the rotation of the methyl group, thereby increasing the distance between adjacent siloxane molecules; the intermolecular force is related to The distance between molecules is inversely proportional to the sixth power, so the intermolecular force of polydimethylsiloxane is much weaker than that of hydrocarbons. Therefore, its surface tension is smaller than that of hydrocarbons of similar molar mass, making it easy for siloxane to spread at the interface.
Another reason why polydimethylsiloxane chains easily spread on polar surfaces (such as water, metal, fiber, etc.) is because the oxygen in the silicon-oxygen chain can interact with Polar molecules or atomic groups form hydrogen bonds, which increases the molecular force between the silicon-oxygen chain and the polar surface, prompting it to spread into a monomolecular layer; thus, the hydrophobic siloxane lies on the polar surface, showing a unique The “stretch chain” configuration. The hydrophobic groups of ordinary SAA stand upright on the polar surface.
The above characteristics of silicone SAA determine that this type of material is easy to emulsify, has excellent permeability, is firmly combined with polar substrates, has good surface spreadability, and can form a thin film. Features. If it is not modified with a hydrophilic group, it still has hydrophobic characteristics.
2. The hydrophilic group in the temperature-resistant and hydrolysis-resistant silicone surfactant cannot be directly connected to the silicone surfactant. on the silicon atoms of the oxygen chain. The hydrophilic group directly connected to the silicon is easily hydrolyzed to form a silanol group, which can be further condensed or cross-linked, resulting in the complete destruction of the chemical structure of the silicone surfactant.
Therefore, when synthesizing silicone SAA, the hydrophobic group and the hydrophilic group must be connected through a isolating group. According to the difference between hydrophobic and hydrophilic connecting groups, polyether silicone oil can be divided into two types: silicon-oxygen-carbon chain-hydrophilic type and silicon-carbon-hydrophilic type. The former is usually used as a foaming agent in the polystyrene foaming process, and is also called “hydrophilic silicone oil”. Silicon-oxygen-carbon chain-hydrophilic based polyether silicone oil is usually not suitable for use in the presence of water, because the SI-O-C bond is prone to hydrolysis and condensation, causing the structure of the silicone surfactant to be destroyed and its performance to disappear, and it is not resistant to water. high temperature.
Silicon-carbon-hydrophilic based silicone surfactant, disperses stably in water and will not hydrolyze. It can be used for a long time below 150℃; it can also reach 200℃ in a short time.
3. The typical type of hydrophilic silicone SAA is polyether silicone oil. Due to the hanging or blocked polyether groups on its molecular chain (side chain or main chain), it has both lipophilic and hydrophilic properties. That is to say, this oil is not only miscible with alkane oily substances, but also easily dispersed in water. This is a very valuable feature.
Applications of silicone surfactants In addition to ordinary SAA, silicone SAA has the functions of wetting, dispersing, foaming, defoaming, antistatic, emulsifying, penetrating, softening, lubricating, and thickening. In addition to its solubility and other properties, it also has the characteristics of high surface activity, easy spreadability, physiological inertness, weather resistance, high temperature resistance, etc. These characteristics make it widely used in plastics and rubber. </p
