An article explaining silicones: What are the classifications and differences of silicones? What are the specific applications of various silicone oils?

Silicone is a type of material between inorganic and organic, containing both an inorganic skeleton mainly composed of silicon-oxygen bonds and organic components connected by silicon-carbon bonds. Due to its excellent hydrophobicity, resistance to UV radiation, chemical and physiological inertness, compatibility, anti-smudge performance and skin feel improvement effect, it is now widely used in many segments of the cosmetics industry, especially in makeup, hair and daily care products.

Silicone products have been used in the daily chemical industry since the 1950s, making them a longer-standing product than many others. Let's discuss: What exactly is silicone? How many types are there? And what are some of the everyday products you use?

Organosilicon compounds contain silicon-carbon (Si-C) bonds, with at least one organic group directly attached to a silicon atom. They typically exist as polysiloxanes and possess a unique chemical structure and excellent physical properties. The basic structural unit of organosilicon is composed of silicon-oxygen chains, with side chains connected to other organic groups via silicon atoms. This structure allows it to combine the properties of organic compounds with the functions of inorganic compounds.

Silicone Oil

Silicone oils possess excellent hydrophobicity, physiological inertness, antioxidant properties, and low surface tension. By introducing functional groups into silicone oil molecules, new functional properties can be imparted to silicone oils, thereby expanding their applications in cosmetics. The basic synthesis method for silicone oils involves the polymerization of various monomers in the presence of a catalyst, primarily through hydrosilylation and siloxane hydrolysis-condensation reactions.

Silicone Wax
Long-chain alkyl silicone oil is a long-chain alkyl (C8+) modified polysiloxane, also known as silicone wax (when the carbon chain reaches a certain length, the product becomes a white wax). Silicone wax is a hydrophobic silicone polymer, formed by a silylation reaction between hydrogenated silicone oil and long-chain olefins. It is solid at room temperature and moisturizes the skin, reduces stickiness, and imparts excellent skin-protecting properties, hydrophobicity, breathability, UV protection, and softness to cosmetics. In sunscreen cosmetics, it can be miscible with mineral oil before being added to the oil phase, providing excellent radiation protection and synergistic effects with UV absorbers and sunscreens. In hair care products, it leaves hair soft and shiny.

Silicone Emulsifiers
Silicone emulsifiers have low surface tension, enhancing emulsion stability while providing a soft, smooth feel. The mainstream silicone emulsifier is polyether-modified polysiloxane, which combines hydrophilic, hygroscopic polyether segments with hydrophobic, compliant polysiloxane segments. This gives the product excellent emulsification and surface spreading properties. However, polyether raw materials may contain banned substances such as dioxane, posing certain application risks. Therefore, extensive research is underway to develop silicone emulsifiers without polyether segments.

Silicone film-forming agents
Silicone film-forming agents possess excellent adhesion and flexibility, forming continuous, soft, and strongly adherent films on hair, skin, and nails. They offer excellent durability and outstanding smudge resistance. MQ silicone resin, a common silicone film-forming agent, is derived through the polycondensation of M and Q monomers. Polyacrylates offer excellent weather resistance and are cost-effective. The high bond energy of the silicon-oxygen bonds in the silicone backbone contributes to excellent film-forming properties and migration resistance, while also improving the SPF of sunscreen products.

Silicone elastomers
Silicone elastomers form a flexible, three-dimensional network structure through crosslinking between molecular chains. The cavities within this network can absorb low-viscosity silicone oil to form a silicone elastomer gel. Silicone polymer powders can also be produced without silicone oil. Silicone elastomers can provide a smooth, velvety texture, reduce the stickiness of product formulas, form a breathable film on the skin, enhance moisturizing properties, and prolong the wear of cosmetics. To meet the demand for functional cosmetics, silicone elastomers are often modified. Depending on the structure after modification and crosslinking, silicone elastomers can be categorized as polyurethane-modified, polyether-modified, and long-chain alkyl-modified.

Among silicone products, silicone oil is the most widely used. Silicone oil boasts numerous unique properties, including low temperature-viscosity ratio, high and low temperature resistance, oxidation resistance, high flash point, low volatility, good insulation, low surface tension, non-corrosiveness to metals, and non-toxicity. So, what is silicone oil? What types are there? And how are they used in cosmetics?

What is silicone oil?

Are silicone oil and dimethyl silicone oil the same thing? Silicone oil generally refers to an organic polysiloxane that is liquid at room temperature. Silicone oil and dimethyl silicone oil are not identical. Silicone oil can be divided into several types based on the different organic groups in the silicone oil. Among them, polydimethylsiloxane, also known as dimethyl silicone oil, is a specific type of silicone oil that shares most of the properties of silicone oil. Among all silicone oils, methyl silicone oil is the most widely used and the most important type of silicone oil.

The basic structure of silicone oil:

Silicone oil is a polysiloxane with a chain structure. It is divided into linear silicone oil and branched silicone oil (Figure 1). R represents an organic group, and R' represents an organic group or hydrogen. When both R and R' are methyl, it is a methyl silicone oil. When R' is an ethyl, propyl, long-chain alkyl, phenyl, or other modifying group, it can impart specific properties to the silicone oil. For example, long-chain alkyl groups can enhance affinity for organic matter and lubricity, while polyether segments can increase hydrophilicity.

From a molecular perspective, silicone oils possess three distinct structural characteristics: They are composed of four basic units with varying functionalities-M, D, T, and Q (Figure 2)-linked together in various covalent bond patterns. The molecular chain contains a variety of chemical bonds, which impart distinct properties. For example, Si-O-Si bonds impart heat resistance, flame retardancy, and weather resistance, while Si-C bonds confer surface activity, hydrophobicity, and defoaming properties. Silicone methyl groups are essential; all silicone oils are derivatives of methyl silicone oils. The silicon-carbon bond within the silicon methyl group is the most stable, endowing silicone oils with unique properties.

Generally speaking, silicone oils are primarily synthesized by polymerizing monomers or coupling agents of varying functionalities through specific methods, or by grafting onto polysiloxane backbones or side chains. These reactions occur through siloxane equilibration reactions, condensation reactions between siloxane functional groups, and hydrosilylation reactions.

① Methyl silicone oil: Methyl silicone oil's molecular structure consists entirely of methyl groups. It exhibits high surface activity, low surface tension, easy spreading, and hydrophobic properties. It forms a hydrophobic, breathable protective film on the skin and helps hair maintain its natural sheen. Therefore, methyl silicone oil can be used as a spreading agent, lubricant, and emollient, often replacing traditional oily ingredients such as paraffin and petrolatum.

② Phenyl silicone oil: When some of the methyl groups in the methyl silicone oil molecular chain are replaced by phenyl groups, methylphenyl silicone oil is produced. Methylphenyl silicone oil is a colorless or yellow, transparent liquid. The presence of the phenyl group improves compatibility with cosmetic bases and enhances gloss, making it useful as a radiation-resistant agent in sunscreens.

③ Methyl long-chain alkyl silicone oil. When part of the methyl groups in methyl silicone oil are replaced by long-chain alkyl groups, it is called methyl long-chain alkyl silicone oil. The introduction of long-chain alkyl groups improves the lubricity, defoaming and anti-sticking properties of the product. The longer the alkyl chain in the molecular chain, the higher the surface tension, refractive index and viscosity of the product, and the lower the density. When methyl long-chain alkyl silicone oil is used in hair oil, it can improve the lubricity and spreadability of the product and improve hair quality; if used in lipstick, it can enhance the moisturizing and breathability of the lipstick; when used in sunscreen products, it has excellent anti-radiation effects and has a good synergistic effect with UV absorbers and sunscreens.

④ Polyether-modified silicone oil Polyether-modified silicone oil refers to silicone oil containing polyether segments in its molecules. The siloxane segments in the molecular chain of polyether-modified silicone oil give it low surface tension, while the hydrophilic polyether segments give it hydrophilicity. The two segments connected together make it an excellent surfactant. When used in cosmetics, it can not only improve the lubricity and moisturizing properties of the product, but also reduce the stickiness. According to the different chemical structures, polyether-modified silicone oils can be divided into Si-O-C and Si-C types. The former has two segments connected by Si-O-C bonds, which are hydrolyzable polyether-modified silicone oils and are unstable; the latter has two segments connected by Si-C bonds, which are non-hydrolyzable polyether-modified silicone oils, which are relatively stable and easy to store. Polyether-modified silicone oil has the characteristics of water solubility, emulsification, surface activity and physiological inertness. When added to the formula of daily chemical products, it can produce products that are non-irritating to the skin, have good moisturizing properties and antibacterial properties. It is an excellent cosmetic auxiliary agent.

⑤ Aminoalkyl-modified silicone oil: Aminoalkyl-modified silicone oil refers to silicone oils in which some of the methyl groups in the dimethyl silicone oil molecular chain have been replaced by aminoalkyl groups, including primary, secondary, and tertiary amines. The properties and functions of the silicone oil vary depending on the aminoalkyl group. Depending on the position of the aminoalkyl group, amino-modified silicone oils can be divided into side-chain aminosilicone oils and terminal aminosilicone oils. The most commonly used aminosilicone oil is aminopropyl silicone oil. Due to the polarity and reactivity of the amino group, it is easier to emulsify and disperse than dimethyl silicone oil. Amino groups can also physically or chemically react with groups on the hair surface, adsorbing onto the hair surface in a targeted manner, making it smoother and more lustrous. They are widely used in shampoos and in textile finishing to improve fabric washability, shrinkage resistance, and softness. However, amino silicone oil can easily cause yellowing of fabrics, has poor stability and poor hydrophilicity, and usually requires the use of a large amount of emulsifiers for emulsion preparation. Therefore, polyether-modified amino silicone oil not only has superior anti-yellowing properties and stronger stability, but also can improve the smoothness, softness and hydrophilicity of fabrics, significantly improving the comfort and durability of fabrics.

⑥ Quaternary ammonium-modified silicone oils: Silicone oils containing quaternary ammonium groups in their molecules are called quaternary ammonium-based silicone oils. The bactericidal cations in quaternary ammonium-modified silicone oils attract and bind negatively charged bacteria and fungi, inhibiting their respiration, penetrating bacterial cells, and disrupting enzyme metabolism, thereby achieving an antibacterial effect. Quaternary ammonium-based silicones are also resistant to yellowing and can be used in fabric treatments. When added to shampoo, quaternary ammonium-based silicone oils can also improve hair combing, leaving it soft and shiny.

⑦ Carboxyl-modified silicone oil Silicone oil with carboxyl groups introduced at both ends or side chains of methyl silicone oil is called carboxyl-modified silicone oil. The main carboxyl hydrocarbon groups are: -C2H4COOH, -CH2CHMeCOOH, -C10H20COOH, etc. Carboxyl groups are reactive and polar and are used in polishes, lubricants, release agents and fabric products.

⑧ Mercapto-modified silicone oils contain mercapto groups in their molecules and are used to modify various polymers. Mercapto groups also interact with hair, so mercapto-modified silicone oils are used in hair care products and dyes to increase hair wettability and maintain hair color.

⑨ Epoxy-modified silicone oils: Epoxy-modified silicone oils are silicone oils that have epoxy groups introduced into methyl silicone oil. These epoxy groups impart reactivity, adsorption, and crosslinking properties. When used as a surface treatment agent, they can improve the wrinkle resistance and sewability of fabrics. When used as a wig treatment agent, they can enhance the smoothness of hair and prevent yellowing.

These four categories are commonly used silicone products in cosmetics. Many people cannot tell the difference between them. In fact, in the cosmetic silicone system, the core differences between silicone oil, silicone wax, silicone elastomer, and silicone powder come from their chemical structure (linear/cross-linked/powdered) and physical form (liquid/semi-solid/elastomer/powder), which leads to significant differences in their skin feel, function, and application scenarios.

1. Silicone oil is the most basic organosilicon in cosmetics. It is essentially a linear polysiloxane fluid.
2. Silicone wax is a modified silicone oil. By introducing "wax-like groups" (such as stearyl and cetyl groups), it has the "semi-solid/solid" form and setting properties of traditional waxes (such as paraffin and beeswax).
3. Silicone elastomers are based on polydimethylsiloxane. Cross-linking groups such as vinyl groups form a three-dimensional network. The intermolecular spaces can encapsulate silicone oil or solvents, giving them elasticity.
4. Silicone powders are based on "silicone cross-linked polymers." The molecules form a three-dimensional network structure. The powder particles have hydrophobic groups on the surface, making them less susceptible to moisture absorption.