First, it is important to understand that silicon is element 14 in the periodic table and the second most abundant element in the Earth's crust, accounting for approximately 25% of its total composition. Silicon does not exist in a free state in nature; it primarily occurs in the form of silicates. Polysiloxanes and their derivatives are polymers composed of silicon, carbon, hydrogen, and oxygen, and may sometimes also contain nitrogen and sulfur. Over the past thirty years, polysiloxanes and their derivatives have been widely used in cosmetics and personal care products, including shampoos, conditioners, various hair care products, and makeup products. Their unique physical properties are highly effective in conditioning and protecting hair and skin, while also improving the sensory properties of products. Polydimethylsiloxane (PDMS) is a key component of hundreds of modified polysiloxanes and represents one of the high-end, high-performance cosmetic ingredients currently available on the market.
I. Basic Properties
2. Physicochemical Properties
(1)
Polydimethylsiloxane is the most widely used linear, non-reactive, and unmodified dimethylsiloxane polymer, commonly known as silicone oil. Depending on its relative molecular mass, its appearance ranges from a colorless and transparent volatile liquid (such as 1 cSt fluid) to an extremely high-viscosity liquid (<1,000,000 cSt) or silica gel (>40,000,000 cSt).
(2) Viscosity
Commercially available polydimethylsiloxanes are colorless, odorless, oily liquids with viscosities ranging from 1 to 1,000,000 cSt. The most commonly used polydimethylsiloxanes have viscosities ranging from 100 to 300,000 cSt. The viscosity of polydimethylsiloxane is related to both the relative molecular mass (Mr) and the molecular mass distribution of the polymer. The desired viscosity can be achieved by controlling the Mr during polymerization or by blending polydimethylsiloxanes with different viscosities.
(3)
The viscosity of polydimethylsiloxane changes very little with temperature and remains liquid over a wide range of relative molecular masses. Low-Mr polydimethylsiloxane behaves as a Newtonian fluid, whereas when the Mr exceeds 35,000, it exhibits the characteristics of a non-Newtonian fluid.
3. Stability
Polydimethylsiloxanes (PDMS) are generally resistant to certain substances and environmental factors, such as UV radiation, heat, and oxidation, whereas hydrocarbons are more sensitive to these effects. Compared with hydrocarbons, PDMS polymers exhibit greater thermal and oxidative stability. This high stability results from the strength of the Si–O–Si bonds. Furthermore, PDMS polymers also demonstrate excellent low-temperature resistance, with a pour point of -60°C and a glass transition temperature of -120°C.
Due to the weak intermolecular forces of polydimethylsiloxane, it exhibits low surface tension, a high gas diffusion coefficient, and excellent spreadability. Therefore, unmodified polydimethylsiloxane polymers can spread into a uniform film on the skin and other substrates. This film has high permeability, allowing oxygen, carbon dioxide, and water vapor to pass through. This property is one reason why it is widely used in both skincare and makeup products.
Specific Applications in Different Cosmetics
1. Skincare Products
(1) Face Cream/Lotion: Adjusts product consistency, improves application feel, and enhances long-lasting moisturizing ability.
(2) Sunscreen: Improves film-forming speed, reduces sunscreen loss, and enhances waterproof and sweat-resistant performance.
(3) Serum: Acts as a carrier ingredient, helping active ingredients penetrate deeply while reducing product stickiness.
(4) Eye Cream: Reduces friction on the delicate skin around the eyes and helps minimize the deepening of fine lines caused by rubbing.
2. Makeup Industry
(1) Foundation/Concealer: Improves the spreadability of the formula, making makeup more adherent and less prone to creasing.
(2) Lipstick/Lip Gloss: Increases the smoothness of the formula, reduces dryness, and improves application comfort.
(3) Mascara: Enhances adhesion between the formula and eyelashes while utilizing hydrophobic properties to achieve waterproof and sweat-resistant effects.
(4) Eyeshadow/Blush: Reduces powder fallout and enhances color payoff and long-lasting wear.
3. Hair Care Products
(1) Shampoo/Conditioner: Adheres to the surface of hair cuticles, fills damaged areas, and reduces friction-related hair damage.
(2) Hair Mask/Essential Oil: Forms a waterproof protective film that resists the effects of moisture on hair and increases shine.
(3) Styling Products: Provides a lightweight styling effect while reducing damage caused by high-temperature styling tools.
4. Cleaning and Body Care Products
(1) Shower Gel/Hand Soap: Reduces irritation to the skin and forms a protective film after washing to prevent dryness and tightness.
(2) Shaving Cream: Acts as a lubricant to reduce friction between the razor and the skin, thereby lowering the risk of cuts.
Precautions for Use
(1) Long-term and excessive use may clog pores. It is recommended to use these products together with cleansers that have strong cleansing abilities. Due to their hydrophobic nature, silicone-based substances are not easily rinsed away with water alone, so using a makeup remover for thorough cleansing is recommended.
(2) Organosilicon substances degrade slowly in the natural environment. Therefore, it is advisable to choose products with biodegradable formulas whenever possible.
(3) Before first-time use, it is recommended to perform a patch test on a small area of skin, such as behind the ear, to check for possible allergic reactions.
