Within the silicone family, silicone oils and silicone emulsions are the most widely used members. Their unique structure and properties make them crucial in countless applications, from industrial production to everyday products.
I. Silicone Oil: From Molecular Structure to a Thousand Forms
Silicone oil is essentially a type of chain-like polyorganosiloxane with varying degrees of polymerization. Its creation involves several key steps: First, dimethyldichlorosilane is hydrolyzed with water to produce a preliminary condensation ring; this ring is then cracked and distilled to obtain a lower ring; finally, the ring, capping agent, and catalyst are combined to form a mixture with varying degrees of polymerization. After removing the low-boiling products, the silicone oil we commonly see is obtained.
The most commonly used silicone oil is methyl silicone oil—its organic groups are all methyl. However, to meet different needs, some methyl groups are replaced with other groups, such as hydrogen, ethyl, phenyl, and trifluoropropyl, resulting in a range of "modified silicone oils." These modifications give silicone oils distinct performance characteristics. For example, phenyl silicone oils offer greater resistance to high and low temperatures, while fluorinated silicone oils offer excellent chemical resistance.
Appearing to be colorless (or pale yellow), odorless, non-toxic liquids, silicone oils are generally non-volatile. Their properties are quite unique: they are insoluble in polar solvents like water and methanol, but are compatible with benzene, carbon tetrachloride, and kerosene. They also possess low vapor pressure, high flash and ignition points, and a low freezing point, making them exceptionally safe. Even more remarkable, depending on the molecular chain length (degree of polymerization), silicone oils can vary in viscosity from 0.65 centistokes to over a million centistokes. For lower viscosities, they can be prepared using acidic clay at 180°C or sulfuric acid at low temperatures. For higher viscosities, alkaline catalysts are required.
Based on chemical structure, silicone oil comprises a vast family of members: methyl silicone oil, ethyl silicone oil, phenyl silicone oil, methyl hydrogen silicone oil, methylchlorophenyl silicone oil... Each type has its own unique areas of expertise. When categorized by application, damping silicone oil, diffusion pump silicone oil, hydraulic fluid, insulating oil, heat transfer oil, brake fluid, and others shine in their respective roles.
What makes silicone oil so popular is its exceptional performance: strong heat resistance, making it resistant to deterioration at high temperatures; excellent electrical insulation, making it a "safeguard" for electronic equipment; excellent weather resistance, resisting aging in the face of wind and sun; strong hydrophobicity, allowing water to condense on its surface; high physiological inertness, making it less irritating to humans and organisms; low viscosity-temperature coefficient, high compression resistance, and some varieties are even radiation-resistant. These properties have made silicone oil a popular choice in aerospace, electronics, and machinery manufacturing.

2. Silicone Emulsions: A Popular Form of Silicone Oil
Simplely speaking, silicone emulsions are liquid dispersions of silicone oil. Among these, emulsions used as fabric softeners and defoamers are the most common. Especially in the textile industry, silicone emulsions are considered "fabric beauticians."
From "First Generation" to "Advanced": The Evolution of Fabric Softening Agents
Silicone fabric finishing agents have gone through two generations of development. The first generation consisted of mechanical mixtures of dimethyl silicone oil and hydrogenated silicone oil (and its derivatives). While these mixtures provided a softening effect, their performance was limited. In the second generation, hydroxyl-terminated polydimethylsiloxane emulsions became mainstream. These emulsions are formed by direct emulsion polymerization of octamethylcyclotetrasiloxane monomer, water, an emulsifier, and a catalyst under specific conditions. This "one-step" production method is not only time-saving and efficient, requiring minimal equipment, but also produces an exceptionally stable emulsion with uniform particle size. More importantly, the hydroxyl groups at the ends of the polymer can further react to form a film, further enhancing the fabric finishing effect, something mechanically mixed silicone oil emulsions cannot match.
Depending on the surfactant used, hydroxy silicone oil emulsions can be categorized as cationic, anionic, nonionic, and complex ionic, each with its own strengths.
Cationic hydroxy silicone oil emulsions often use quaternary ammonium salt emulsifiers (such as octadecyltrimethylammonium chloride) and ammonium hydroxide as a catalyst. They can make fabrics feel smoother, more elastic, and crisper. Even more impressively, they are excellent waterproofing agents—when used in combination with methyl hydrogenated silicone oil emulsions, they achieve high levels of waterproofing and durability. They are commonly used for waterproofing applications such as vinylon tarpaulin and polyester fabrics.
The greatest advantage of anionic hydroxy silicone oil emulsions is their compatibility. In textile printing and dyeing, many auxiliaries are anionic. Using cationic emulsions can easily lead to demulsification and oil bleaching, while anionic emulsions avoid this problem, resulting in a wider range of applications.
Complex ionic hydroxy silicone oil emulsions were developed to address the shortcomings of cationic emulsions. While cationic emulsions offer advantages, they are not resistant to hard water and cannot be used in the same bath as 2D resins, magnesium chloride catalysts, or anionic brighteners, resulting in numerous limitations. This led to the idea of combining cationic and nonionic emulsifiers. This resulting emulsion is not only resistant to hard water and can coexist peacefully with various additives, but is also heat-resistant and freeze-resistant, significantly improving stability.
Nonionic hydroxyl silicone oil emulsions are the most adaptable all-rounders. They are more stable than ionic emulsions and can handle complex operating environments, leading to significant research and development efforts in many countries. For example, Ciba-Geigy's UltrateX FSA is a nonionic emulsion of hydroxyl-capped polydimethylsiloxane with a molecular weight exceeding 200,000, which offers superior performance compared to earlier anionic products.
Customized Finishes with Special Functions
To give fabrics additional "superpowers" such as oil and stain resistance, antistatic properties, and hydrophilicity, or to imbue synthetic fabrics with the texture of natural fabrics, researchers have begun introducing specialized reactive groups into silicone molecules—amino, amide, ester, cyano, carboxyl, and epoxy groups, among others.
These groups act like "add-ons" to silicone oils for specialized functions: amino groups are suitable for shrink-proofing and softening wool fabrics; amide groups enhance stain resistance and softness; cyano groups enhance oil resistance; copolymers of polyoxyethylene ether and silicones offer significant antistatic properties; and organofluorine-modified silicones, combining oil, stain, static, and water repellency, have become the new favorite for high-end fabric finishing.
From the diverse forms of silicone oils to the refined classification of silicone emulsions, these silicone products, with their unique appeal, are constantly reshaping our understanding of material properties and enabling more industries to achieve greater efficiency and higher quality.