Dimethicone

Dimethicone is the plainest member of the silicone family and the one most widely used in cosmetics. It is a chain of alternating silicon and oxygen atoms, with two methyl groups on every silicon.^[1] It is supplied in viscosities ranging from thinner than water to a gum that must be cut with a solvent before it can be poured.

Its usefulness is almost entirely physical. It spreads further than a vegetable or mineral oil of the same thickness, it does not oxidise into rancidity, and it leaves a surface that feels dry rather than greasy. Most modern foundations, primers and hair conditioners contain some.

The backbone is a siloxane: silicon and oxygen repeating in alternation, with a methyl group on either side of each silicon. The silicon to oxygen bond is longer and more freely rotating than a carbon to carbon bond, which is why silicone fluids stay pourable across a wide temperature range and why a very long chain can still behave as a liquid.

Dimethicone is made by hydrolysing dimethyldichlorosilane. That reaction became industrially practical after Eugene Rochow passed methyl chloride over a heated copper and silicon mixture at General Electric in May 1940, a route since known as the direct process.^[2] Hydrolysis yields both rings and chains; Winton Patnode and Donald Wilcock separated and described the two families in 1946.^[3] The chains, capped at each end and sorted by length, are dimethicone. The rings are the cyclic siloxanes.

Silicones reached the market as war materials. Corning Glass Works and Dow Chemical formed the Dow Corning joint venture in 1943 to manufacture them.^[4] The early demand was industrial - greases, insulators, hydraulic fluids - rather than for anything applied to a face. The move into skin and hair products came later, as part of the wider post-war turn of the beauty industry towards synthetic raw materials.^[5]

Hair care did most to spread it. A conditioner that deposits a thin silicone layer alters how hair feels immediately, and the change can be judged in the hand rather than argued for. That immediacy mattered as much to dimethicone's adoption as any technical case made for it.

In a formulation dimethicone does three separable jobs. It thins the feel of heavy oils and waxes, so a cream can carry pigment without dragging. It slows water loss from the surface of the skin by sitting on it as a continuous layer. And in hair products it coats the cuticle, reducing friction between strands and making combing easier.

Low-viscosity grades act mainly as spreading agents. High-viscosity gums act as film formers and are usually supplied already dispersed in a volatile carrier, because they cannot easily be handled otherwise. In the United States dimethicone is also a listed over-the-counter skin protectant active ingredient, permitted at one to thirty per cent under 21 CFR 347.10, as in force on 22 September 2026.^[6]

Dimethicone is in ordinary cosmetic use. It is a chain rather than a ring, and it is not itself named in the European restrictions on cyclic siloxanes: Commission Regulation (EU) 2018/35 of 10 January 2018 restricted the rings D4 and D5 in wash-off cosmetic products,^[7] and Commission Regulation (EU) 2024/1328 of 16 May 2024 extended entry 70 of Annex XVII to the REACH Regulation to D6 and to leave-on products.^[8] All three of those substances are rings.

Entry 70 nevertheless reaches dimethicone indirectly. As amended, the restriction bites on D4, D5 and D6 at or above one tenth of one per cent by weight whether they are present as substances on their own, as constituents of other substances, or in mixtures.^[8] Commercial dimethicone can carry a residue of cyclic siloxane left over from manufacture, so a grade carrying residual cyclics above that threshold is within the scope of the instrument - which is why cosmetic grades are bought against a specification rather than as a generic fluid. European regulatory attention to silicones over the past decade has been directed at their persistence in the environment rather than at their behaviour on skin.