Tetramethyl Tetrahydrogen Cyclotetrasiloxane — INCI reference

What it is

Tetramethyl Tetrahydrogen Cyclotetrasiloxane (CAS 2370-88-9, also known as D4H or TMCTS) is a four-membered cyclic siloxane ring in which each silicon atom bears one methyl group and one hydrogen atom. Unlike its fully methylated analog octamethylcyclotetrasiloxane (D4), D4H carries reactive Si-H bonds that enable hydrosilylation reactions. In cosmetic formulations, it serves primarily as a functional crosslinker and reactive intermediate for silicone network formation, contributing to smooth skin feel, light-texture spreadability, and film-forming properties. It is a colorless to pale yellow liquid with a boiling point of about 134 °C. The compound is moisture-sensitive and miscible with most organic solvents but immiscible with water. In personal care products it imparts the characteristic dry, silky after-feel associated with silicone-based systems.

Safety & warnings

The compound is classified as a flammable liquid (Category 3) with eye irritation potential (H319, H226). It is moisture-sensitive. The CIR Expert Panel noted insufficient data to assure safety of polymerized tetramethylcyclotetrasiloxanes in cosmetics as of 2014-2015. No evidence of skin sensitization has been reported at cosmetic use levels, but data gaps remain. Related cyclic siloxanes (D4, D5) face EU restrictions due to PBT/vPvB concerns; the regulatory status of D4H in the EU should be verified, as cyclic siloxane restrictions may extend to related compounds.

Origin & sourcing

Tetramethyl Tetrahydrogen Cyclotetrasiloxane is a synthetic organosilicon compound produced by controlled ring-forming hydrolysis and condensation of methyldichlorosilane intermediates. The manufacturing process yields a cyclic tetramer with alternating Si-O bonds in the ring backbone. It is commercially produced by silicone chemistry manufacturers.

History

Cyclic methylhydrosiloxanes such as D4H were developed alongside the broader silicone industry that grew from the mid-20th century. Their reactive Si-H functionality made them useful crosslinkers and intermediates in silicone polymer synthesis. Application in personal care formulations followed expanded use of silicone chemistry in cosmetics from the 1980s onward. The CIR Expert Panel reviewed polymerized tetramethylcyclotetrasiloxanes and issued a Notice to Precede requesting safety data in December 2014; as of that assessment, safety in cosmetics could not be assured due to insufficient data.

Questions everyone asks

What makes Tetramethyl Tetrahydrogen Cyclotetrasiloxane different from regular cyclotetrasiloxane (D4)?

Unlike octamethylcyclotetrasiloxane (D4), which has four methyl groups on each silicon, D4H has one hydrogen and one methyl group on each silicon. These Si-H bonds are chemically reactive, allowing D4H to act as a crosslinker in silicone networks via hydrosilylation reactions.

What does this ingredient do in sunscreen and skincare products?

It contributes to the smooth, lightweight, non-greasy feel of silicone-based formulas. As a reactive silicone, it can participate in crosslinking that forms flexible silicone films on skin, improving spreadability and sensory texture.

Is it safe for use on skin?

The CIR Expert Panel found insufficient published safety data to assure safety of polymerized tetramethylcyclotetrasiloxanes in cosmetics as of 2015. Standard cyclic siloxane data suggest low percutaneous absorption, but users concerned about silicone ingredients should check product-specific safety

Is it the same as D4 (the restricted silicone)?

No. D4H (Tetramethyl Tetrahydrogen Cyclotetrasiloxane) and D4 (Octamethylcyclotetrasiloxane) are distinct compounds with different structures and regulatory profiles. D4 is restricted in the EU as a PBT/vPvB substance; D4H has a different regulatory history, though related cyclic siloxanes remain un

Sources & references

Back to all ingredients