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SILICONES – Definition and Properties
A World of Unlimited Potential
We encounter silicones every day without noticing them. Under the hood, silicone rubber protects car electronics against moisture and dirt; in washing machines, silicone antifoam agents prevent the detergent from foaming over; in shampoo, silicones give hair its sheen; as silicone resin emulsion paints, they give masonry water repellency, while allowing water vapor and carbon dioxide to diffuse out of its interior. But also in medical applications requiring high resistance or cutting-edge products, silicones deliver genuine peak performance – serving as particularly pure material in medical tubing, wound dressings or orthopedic products, and as safe sealing and insulating materials in electrical equipment or insulators.
From Science to Success
The development of the Müller-Rochow process some 75 years ago paved the way for methylchlorosilane to be obtained from silicon metal and methyl chloride. This provided the first-ever access to the starting materials needed for the industrial manufacture of silicones and kick-started the global boom in the use of silicone products in many different applications. WACKER is a pioneer in this field, and was the first company in Europe to begin researching silicones. In the following years, WACKER processes laid the foundations for the modern and efficient manufacturing of organochlorosilanes and silicone products.
This was the beginning of the success story. These early scientific accomplishments underlie WACKER’s reputation as the European pioneer of silicone chemistry. The globally active WACKER Group now develops, produces and markets a versatile product portfolio in a range of complex chemical processes.
The backbone of silicones is the Si-O bond, which is much more stable than the C-C bond found in organic compounds. This has far-reaching consequences for the stability and resistance of silicones to a variety of influences. Thus, silicones are notable for their remarkable resistance to thermal and thermo-oxidative stress. They also have reduced sensitivity to electromagnetic or particle radiation (UV, alpha, beta and gamma radiation) compared to organic plastics.
A Versatile Formula
Silicones’ chemical structure allows them to be produced in countless variations. Not only can the functionality of the siloxane units be varied to yield products that possess oily, polymeric, resinous or rubbery properties – the scope for variation can also be further extended by altering the organic groups attached to the silicon. It is this variability which accounts for the impressive diversity found within silicone products: greases, release agents, antifoam agents, paint additives, paper coatings, hydrophobizing agents, high- or room-temperature-vulcanizing silicone rubber grades, and many, many more.
Silanes are the Starting Point
The starting point and chief building blocks for silicone production are silanes. These are produced from silicon and methyl chloride by direct synthesis (Müller-Rochow synthesis). Silanes are low-viscosity, water-white liquids that are soluble in organic solvents. They are also highly volatile due to their low molecular mass.
Structure of Silicones
Silicones, known to chemists as polydimethylsiloxanes, have a structure that resembles quartz modified with organic groups. They consist of an inorganic backbone composed of alternating silicon and oxygen atoms. The two remaining valences on the silicon atoms are occupied by organic groups (chiefly methyls), which are responsible for silicones’ exceptional chemical characteristics.
The Term “Silicone”
The term “silicone” was coined by F. S. Kipping (1863-1949), who recognized the formal similarity between these silicon-oxygen compounds and the equivalent oxygen compounds of carbon (silicones were initially called polysilicoketones). However, the Si-O-Si linkage is better described by the term “siloxane.” Strictly speaking, therefore, the correct term for all silicones is “polysiloxanes.” Nowadays, the term silicone is principally used in connection with the technical applications of polysiloxanes.