The science of silicones

This page answers some basic questions around the chemistry behind silicones, and how they are made. 

What are Silicones?

Silicones are high-performance oligomers or polymers that can take a variety of physical forms, ranging from solids to water-thin liquids and semi-viscous pastes, greases and oils. They are noted for their ability to function in conditions that most conventional materials would not withstand 

Silicones are produced by reacting silicon—one of the earth’s most common elements- with methyl chloride and further reaction with water which removes the chlorine atom. This reaction mainly produces linear and cyclic siloxanes characterized by a chain of alternating silicon and oxygen atoms. Siloxanes are then further reacted into longer chains to form silicone polymers. 

Where do silicones come from?

Silicones are essentially organically modified quartz. Silicon quartz consists of silicon and oxygen atoms. In the making of silicones, two oxygen atoms attached to every silicon atom have been replaced by methyl groups.

Silicon is a chemical element widely distributed on earth in various combinations with oxygen only (silica) or oxygen and other elements (silicates). Silicon is the second most abundant element on the earth’s crust after oxygen (approximately 28% by mass) and it naturally forms long-lived, stable compounds. In many biological systems silica is an essential element of mechanical structures.

Silicon is always found in chemically and thermally stable mineral combinations but never in its pure form. Silicon is the key to all silicone chemistry as its atomic structure dictates the properties of silicones.

How is silicon obtained?

Today elemental silicon is obtained through the electro-thermic reduction of SiO2 with carbon at 1,400 degrees Celsius.

Elemental silicon is dark grey, metallic shiny, hard, and brittle. It has a melting point of 1,423°C and a boiling point of 2,630°C. Like carbon, silicon has a crystalline structure, similar to that of diamonds. Read more about the similarities and differences between silicon and carbon in our factsheet.

 

SiO– the structure of silicon quartz

What are D4, D5, D6?

Cyclic siloxanes (cyclosiloxanes) are basic members of the broad family of silicone materials. All silicone materials share a common chemistry but each substance is different when it comes to its properties and use.

A common denominator for cyclosiloxanes is that they contain repeating units of silicone (Si) and oxygen (O) atoms in a closed loop, giving it a “cyclic” structure. The three most common cyclosiloxanes in commercial production are D4, D5 and D6.

Octamethylcyclotetrasiloxane (D4), contains four repeating units of silicone (Si) and oxygen (O) atoms in a closed (cyclic) loop.

Where is it used?

D4 is an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production of silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D4 is consumed with only a tiny amount remaining in final products.

Decamethylcyclopentasiloxane (D5), contains five repeating units of silicon (Si) and oxygen (O) atoms in a closed (cyclic) loop.

Where is it used?

D5 is an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production of silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D5 is consumed with only a tiny amount remaining in final products.

D5 can be added as an ingredient in silicone mixtures used in cosmetic applications such as skin creams and sunscreen, where it may be labelled "cyclomethicone" or "cyclopentasiloxane". D5 can also be used as a dry-cleaning solvent in closed systems, which significantly limits exposure to workers, consumers and the environment.

Dodecamethylcyclohexasiloxane (D6), contains six repeating units of silicon (Si) and oxygen (O) atoms in a closed loop, giving it a "cyclic" structure.        

Where is it used?

D6 is also an odourless, colourless liquid mostly used as an intermediate or basic raw material in the production silicone rubbers, gels and resins. When used as an intermediate during the manufacturing process, virtually all D6 is consumed with only a tiny amount remaining in final products.

D6 can also be used as an ingredient in silicone mixtures used in cosmetic applications such as skin creams and sunscreen, where it may be labelled "cyclomethicone" or "cyclohexasiloxane".

How are silicones made?

Obtain & GRIND silicon

1400 degree Celcius reaction with carbon

REACT

React silicon metal to make chlorosilanes

SEPARATE

Separate (distill) chlorosilanes

React with water and strip

Hydrolysis and stripping creates siloxanes, such as PDMS and cyclic volatile methyl siloxanes (cVMS) D4, D5 and D6

SEPARATE

Separate (distill) siloxanes

Polymerise

Mix to sealants, adhesives, elastomers Polymerise into silicones rubber, fluids, resins and myriad other forms

Silicones have countless performance qualities

They play an increasingly important role supporting improvement, innovation and progress in a wide range of industry sectors, from automobiles to electronics to textiles. In essence, silicones make things work better. Their main characteristics can be divided according to the following concepts.

Adhesion

Adhesion

Silicones' excellent adhesion properties are a product of the chemical bond between organic polymer matrices and substrates that are traditionally difficult to adhere to, like glass, metal and stone.

Stability

Stability

Silicones resist high temperatures and their physical characteristics vary very little, even under extreme temperature conditions, which is why many industrial fluids are formulated with silicone as a base.

Insulation

Insulation

Silicones play a major role in the protection and insulation of electro-technical equipment.

Lubrication

Lubrication

Silicone lubricants, based on fluids, pastes or greases, are designed to reduce friction between different body parts that are in contact such as plastics, rubbers, metals and glass.

Foam control

Foam control

Silicone foam controllers are used in detergents as well as in the separation of crude oil from water and the production of pulp for papermaking in the medical and construction industries.

Surface protecting

Surface protecting

Over the years, buildings suffer considerable physical and chemical damage caused by water penetrating into building materials. However, thanks to silicones, water repellent products protect these materials from such damage.

Spreading

Spreading

Paints, varnishes and printing inks are applied as thin layers on a variety of substrates. The applied films have relatively large surface areas compared to their volume, and a correspondingly large contact area with respect to the substrate.

Biological compatibility

Biological compatibility

Silicones provide qualities that people value and seek in cosmetics. For example, silicones give the skin a smooth and soft feeling and help it contain its moisture.

Forms of silicones

LIQUID/OIL

Used as lubricants and in hydraulic fluids

GEL

Used in contact lenses, and for scar reduction

RUBBER

Used in food storage containers and with products such as silicone sealants

FOAM

Used as surfactants and as fire protection

SOLIDS

Used as lightweight automotive parts

What are the major uses of Silicones? 

The distinctive aspects of Silicones make them one of the world’s most important and adaptable raw materials, used in literally thousands of products and applications – from healthcare, aerospace and personal care, to electronics, transportation, construction and energy.

To read more about uses and benefits of silicones, click here.