Physical and chemical properties

Dimethyl silicone is also known as methyl silicone oil and polydimethylsiloxane liquid. It appears as colorless transparent viscous liquid, being tasteless, odorless and non-toxic. The molecular formula is CH3 [Si (cH3) 2] nSi (CH3) 3. The average molecular weight is 5000 ~ 100000. Based on the differences on the molecular weight, kinematic viscosity varies from 1.0 × 10-6 ~ 100000 × 10-6 square meters / second. Long-term use temperature range is -50 ~ 180 ℃. In isolated air or inert gas, it can be subject to long-term use at 200 ℃. Viscosity coefficient: 0.31 ~ 0.61. Surface tension: 1.59 ~ 2.15 × 10-4N / cm. Dielectric constant (23 ℃, 100Hz): 2.18 ~ 2.17. Volume resistivity (23 °C): 1.0 × 1014 to 1.0 × 1016Ωcm3. Dielectric loss tangent (23 ℃, 100Hz):0.00002~0.00004. Breakdown voltage strength: 13.7 ~ 17.7kV / mm. It is soluble in benzene, toluene, xylene and ethyl ether, partially soluble in ethanol, butanol and acetone, insoluble in cyclohexanol, methanol, paraffin oil and vegetable oil. It has excellent hydrophobic moisture resistance, good light transmission, and chemical stability. Widely used in insulation, heat, moisture-proof filler, defoamer, mold release agent, lubricant and surface treatment agent. It is produced by the reaction between octamethylcyclotetrasiloxane and hexamethyldisiloxane in the presence of concentrated sulfuric acid or tetramethylammonium hydroxide catalyst.

Toxicity

It has no obvious acute and chronic poisoning reaction to human beings and mammals, nor be mutagenic and carcinogenic. Oral, inhalation and skin contact have no obvious irritation or allergic reaction to the eyes, skin, and can’t be absorbed by the gastrointestinal tract and skin.

Chemical Properties

Dimethicones are clear, colorless liquids available in various viscosities. Dimethicone is a fluid mixture of fully methylated linear siloxane polymers end-blocked with trimethylsiloxy units. Methicone is a linear monomethyl polysiloxane. Dimethicone are safe as used in cosmetic formulations, vulcanization.

Uses

A silicone oil consisting of a mixture of fully methylated linear siloxane polymers end-blocking with trimethylsiloxy units. Commonly used in skin and hair products.

Indications

Silicone (dimethicone) ointments are excellent water-protective agents because they have an extremely low surface tension and penetrate crevices in the skin to form a plastic-like barrier. Further, they are nontoxic, inert, stable, and water repellent. As such, they are useful as barrier creams in industry or wherever constant or frequent exposure to aqueous compounds is a problem. They will not, however, protect well against solvents, oils, or dusts.

Definition

ChEBI: A polydimethylsiloxane macromolecule, composed of dimethylsiloxane repeating units.

Production Methods

Dimethicone isapoly (dimethylsiloxane) obtained by hydrolysis and polycondensation of dichlorodimethylsilane and chlorotrimethylsilane. The hydrolysis products contain active silanol groups through which condensation polymerization proceeds. By varying the proportions of chlorotrimethylsilane, which acts as a chain terminator, silicones of varying molecular weight may be prepared. Different grades of dimethicone are produced that may be distinguished by a number placed after the name indicating the nominal viscosity. For example, ABIL 20 (Evonik Goldschmidt UK Ltd) has a nominal kinematic viscosity of 18–22mm2/s (18–22cSt).

Brand name

Sentry Dimethicone (Witco); Sentry Dimethicone Dispension (Witco).

Pharmaceutical Applications

Dimethicones of various viscosities are widely used in cosmetic and pharmaceutical formulations. In topical oil-in-water emulsions dimethicone is added to the oil phase as an antifoaming agent. Dimethicone is hydrophobic and is also widely used in topical barrier preparations. Therapeutically, dimethicone may be used with simethicone in oral pharmaceutical formulations used in the treatment of flatulence. Dimethicone is also used to form a waterrepellent film on glass containers.

Safety

Dimethicone is generally regarded as a relatively nontoxic and nonirritant material although it can cause temporary irritation to the eyes. In pharmaceutical formulations it may be used in oral and topical preparations. Dimethicones are also used extensively in cosmetic formulations and in certain food applications. The WHO has set a tentative estimated acceptable daily intake of dimethicone with a relative molecular mass in the range of 200–300 at up to 1.5mg/kg body-weight. Injection of silicones into tissues may cause granulomatous reactions. Accidental intravascular injection has been associated with fatalities.

storage

Dimethicones should be stored in an airtight container in a cool, dry, place; they are stable to heat and are resistant to most chemical substances although they are affected by strong acids. Thin films of dimethicone may be sterilized by dry heat for at least 2 hours at 160°C. Sterilization of large quantities of dimethicone by steam autoclaving is notrecommended since excesswater diffuses intothe fluid causing it to become hazy. However, thin films may be sterilized by this method. Gamma irradiation may also be used to sterilize dimethicone. Gamma irradiation can, however, cause crosslinking with a consequent increase in the viscosity of fluids.

Regulatory Status

Accepted for use as a food additive in Europe. Included in the FDA Inactive Ingredients Database (oral capsules and tablets, topical creams,emulsions,lotions,and transdermal preparations).Included in nonparenteral medicines licensed in the UK. Included in the Canadian List of Acceptable Non-medicinal Ingredients.