Trimethylsiloxysilicate

Trimethylsiloxysilicate is the part of a long-wear cosmetic that does the wearing. It is a silicone resin rather than a silicone oil: a compact, three-dimensional, silica-like core whose surface is capped with trimethylsilyl groups. The result is a solid that dissolves readily in silicone and hydrocarbon solvents and turns brittle again when they go.

Formulators call it an MQ resin, after the two structural units it is built from. The Cosmetic Ingredient Review assessed it alongside three related silylates and surface-modified siloxysilicates and lists the functions of that group as including binder, emollient, occlusive and viscosity increasing agent; the functions the report records for trimethylsiloxysilicate itself are antifoaming agent and skin-conditioning agent - occlusive.^[1]

Siloxane units are named by how many oxygens each silicon shares with its neighbours. M is a silicon sharing one, the trimethylsilyl cap. Q is a silicon sharing four, the fully condensed unit that builds silica. A resin made almost entirely of M and Q units is therefore an MQ resin: a cluster of silica-like Q units wrapped in a shell of M caps, which is what keeps it soluble instead of sandy.

The preparation described by William Daudt and Leslie Tyler at Dow Corning begins with an aqueous silica sol, acidified, then treated with hexamethyldisiloxane and ethanol. The mixture separates into two phases and the resin is recovered from the organic one.^[2] The ratio of M units to Q units decides whether the product is a viscous oil or a hard solid.

The chemistry is older than any cosmetic use documented here. Daudt and Tyler filed on 13 September 1950 and the patent issued on 20 April 1954, at a point when the copolymers were wanted for water-proofing, for anti-flotation of paint pigments and for thickening dimethylsiloxane fluids rather than for anything worn on a face.^[3]

The earliest cosmetic record traced here is a Shiseido application published on 18 July 1986, which discloses a liquid lip composition containing forty per cent MQ resin and twenty per cent volatile hydrocarbon oil and reports that it does not transfer to objects such as drinking glasses; it survives as prior art cited inside Procter and Gamble's later patent rather than as a document consulted directly.^[4] That patent, with a priority date of 7 November 1995, names MQ resins explicitly, specifies a preferred ratio of M to Q units of about 0.7, and identifies particular commercial grades by supplier code.^[4] The arrangement set out in it is the one still in use: resin, plus volatile carrier, plus pigment, applied wet and left dry.

On its own the resin is a hard, glassy material and cannot be spread. Dissolved in isododecane or in a volatile silicone it becomes a pourable liquid carrying a substantial load of solids.^[5] Applied to the skin or the lip, the solvent leaves and the resin knits into a continuous film with the pigment locked inside it.

That film is why the colour does not come off on a cup, and while it is in place it also slows water loss from the surface beneath it - the sense in which the Cosmetic Ingredient Review lists the material as a skin-conditioning agent, occlusive.^[6] It is likewise why long-wear products feel tight and resist removal with water alone. The same property produces both effects, and no formulation choice separates them cleanly; what formulators adjust is the balance between the resin and the softer, tackier materials set against it.

The Cosmetic Ingredient Review Expert Panel assessed trimethylsiloxysilicate alongside the silylates and surface-modified siloxysilicates and published its conclusion in 2013: safe as used, provided the finished product is formulated and delivered so as not to be irritating or sensitising to the respiratory tract.^[7] The qualification is about aerosols, not about lipstick.

The panel's reasoning rests on the material being a macromolecule, insoluble in water and chemically inert under physiological conditions, used at up to around ten per cent in aerosolised products, with the great majority of particles in cosmetic aerosols too large to reach the deep lung.^[7] That is a conclusion about a defined pattern of use at a stated date rather than a general statement about the substance.