Ingredient Profile—Dimethicone

By: Michael J. Fevola, PhD

Posted: March 30, 2012, from the April 2012 issue of Cosmetics & Toiletries.

Polydimethylsiloxane (PDMS), more commonly known as dimethicone, is the most widely utilized silicone material in cosmetics and personal care.^{1, 2} It is found in a wide variety of rinse-off and leave-on products such as shampoos, hair conditioners, skin moisturizers and color cosmetics.^{3, 4} Despite its relatively simple chemical structure, dimethicone exhibits unique physical properties that render it effective for conditioning and protecting the hair and skin, and improving the sensorial attributes of formulations. Dimethicone is also a key component of hundreds of modified silicones, which represent some of the most sophisticated and high performance ingredients available today.

Chemistry

Dimethicone is a hybrid inorganic-organic homopolymer comprised of dimethylsiloxane repeat units, i.e., the polymer consists of an inorganic siloxane backbone (–Si–O–) that bears two methyl (–CH3) groups on each silicon (Si) atom. According to the International Cosmetic Ingredient Dictionary & Handbook, the name dimethicone refers specifically to fully methylated PDMS homopolymers that are end-capped with trimethylsiloxy (TMS) groups.³ Other International Nomenclature of Cosmetic Ingredients (INCI) names are used to refer to PDMS homopolymers with different end-groups. For example, hydroxyl (–OH) end-capped PDMS is called dimethiconol, and hydrogen end-capped PDMS is called bis-hydrogen dimethicone.

The degree of polymerization (DP) can span several orders of magnitude for dimethicone. Thus, n can be as low as 0–3 for the volatile silicones, to as high as several thousand for high viscosity silicone gums.¹ Note that when n = 0 and n = 1, the INCI names for the compounds are disiloxane and trisiloxane, respectively.

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