Polysiloxane

OVERVIEW

The term polysiloxane (pol-ee-sill-OK-sane) refers to a class of compounds whose molecules consist of a siliconoxygen backbone (-Si-O-Si-O-Si-O-Si)_n arranged either in a linear or cyclic (ring) pattern. Each silicon in the chain has two additional oxygen atoms attached to it. In many cases, the polysiloxanes also have one or more alkyl groups attached to the main chain replacing one or more of the oxygens. An alkyl group is an alkane, a saturated hydrocarbon, lacking one hydrogen atom. Examples of alkyl groups are the methyl (-CH₃) and ethyl (-CH₂CH₃) groups. In one common type of siloxane, all of the oxygens that are not a part of the backbone of the chain are replaced by methyl groups. Polysiloxanes that contain alkyl groups are known as organosiloxanes or, more commonly, silicones.

KEY FACTS

HER NAMES:

Siloxane; organosiloxane; silicone

FORMULA:

Varies

ELEMENTS:

Varies: Silicon (always), oxygen (always), carbon (almost always), hydrogen (almost always)

COMPOUND TYPE:

Inorganic polymer

STATE:

Solid or liquid

MOLECULAR WEIGHT:

Varies

MELTING POINT:

Varies

BOILING POINT:

Not applicable

SOLUBILITY:

Varies

Researchers have now learned how to modify organosiloxane polymers by using various alkyl groups in chains of various lengths and conformations to produce a very wide array of products. All are organosiloxanes, but with very different physical and chemical properties and, hence, with very different applications. They range from liquid to gel to a semi-solid, rubber-like material. In general, the organosiloxanes tend to be chemically inert and resistant to heat. Although not identical, the terms polysiloxane, siloxane, and organosiloxane, and silicone are often used interchangeably.

The chemistry of silicon has long fascinated chemists. The element lies just below carbon in the periodic table, meaning that its physical and chemical properties are similar to carbon from which millions of organic compounds are made. The hope has long been to discover if silicon also can make a diverse number of compounds, as carbon can. One researcher who pursued this question in the early twentieth century was the English chemist Frederic Stanley Kipping (1863–1949). Kipping made use of a new kind of chemical reaction, called the Grignard reaction, after the French chemist Victor Grignard (1871–1935) who invented it. With the Grignard reaction, Kipping was able to make a number of silicon compounds to which were bonded alkyl groups in a variety of conformations. These compounds were the first organosiloxanes produced and studied in any detail, earning Kipping the title of "Father of Silicone Chemistry." During his lifetime, Kipping wrote more than 50 scholarly papers on his work. He did not, however, see the possibility of commercial applications for his discoveries.

The commercialization of silicones did not become possible until the early 1940s when a research chemist at the General Electric Company, E. G. Rochow (1909–2002) found an efficient way of making organosiloxanes in large quantities. Rochow's discovery came at an opportune time, the beginning of World War II. A number of military applications were found for the new product almost immediately. Within five years, a number of major chemical companies, including General Electric, Dow-Corning, Union Carbide, Stauffer Chemical, Wacher-Chemie, and Farbenfabriken Bayer A. G., had begun making organosiloxanes in large quantities.

HOW IT IS MADE

A variety of production methods is available for making different types of organosiloxanes. Probably the most popular approach begins with the reduction of silicon dioxide (sand; SiO₂) in an electric furnace. In that reaction, the silicon dioxide breaks down into silicon and oxygen: SiO₂ → Si + O₂. The silicon is then treated with methyl chloride (CH₃Cl), which produces a mixture of methylchlorosilanes. Methylchlorosilanes are compounds that contain a single silicon atom to which are attached one or more methyl groups and one or more chlorine atoms. The methylchlorosilane produced in largest amount is dimethyldichlorosilane, (CH₃)₂SiCl₂. When this compound is treated with water, all of the chlorines are replaced by oxygens and the resulting compound polymerizes spontaneously. That is, individual molecules of the newly-formed silicon-carbon-oxygen compound begin to react with each other to form long organosiloxane chains. The size and character of the chain can be controlled by adding compounds that stop the polymerization reaction at some given point, resulting in the formation of a liquid, a gel, a semi-solid, or some other form of the product.

COMMON USES AND POTENTIAL HAZARDS

Organosiloxanes are very heat resistant, so they do not easily melt, like most other organic compounds. They are also water-repellant and can withstand extremes of sunlight, moisture, cold, and attack by most chemicals. These properties make them useful for protective coatings, electrical insulation, adhesives, lubricants, paints, and rubber-like materials. Some silicones are also used to make nonstick surfaces, such as pans and spatulas for the kitchen. They are also used in making other kitchen items, such as oven mitts, because of their heat resistance.

Methyl silicones are also a major ingredient in personal care products. They are added to shaving lotions to provide lubrication and to give these products a non-greasy, yet silky, feeling. They help hair-styling products to spread more easily, and they increase the skin protection factor (SPF) in sunscreens. Silicones are also used in deodorants, perfumes, and nail polishes.

Interesting Facts

• One of the most famous footprints in the world—that made by Neil Armstrong during his landing on the Moon in 1969—was made with a boot with a silicone rubber sole.
• Silicone was first used for breast implants in the 1960s for women who had undergone mastectomies, surgical removal of their breasts. Silicone implants later became popular with women who had no medical problems, but wanted larger breasts. By the 1980s, many women with breast implants began complaining of pain, chronic fatigue, inflammation of breast tissue, and other medical problems thought to be associated with their silicone breast implants. Some of these women sued Bristol-Meyers Squibb, Dow Chemical, 3M, and other companies who made silicone for breast implants. By 1995, more than 20,000 legal claims had been brought against Dow alone. Faced with this staggering number of claims, the company went bankrupt.

Some industrial and commercial applications of silicones include:

• As adhesives and sealants in many aircraft parts, including doors, windows, wings, fuel tanks, hydraulic switches, overhead bins, wing edges, leading gear electrical devices, vent ducts, engine gaskets, electrical wires and black boxes;

• In the construction business as sealants for all kinds of building materials, including concrete, glass, granite, steel and plastic;
• In the manufacture of many automobile parts, including air bags, gaskets, headlamps, hydraulic bearings, ignition cables, radiator seals and hoses, shock absorbers, spark plug boots and ventilation flaps;
• As heat transfer agents;
• For the weatherproofing of concrete and other surfaces;
• In the production of surgical membranes and implants;
• In a great variety of electrical and electronic appliances, devices, and parts, including computer cases, keyboards, copy-machine components, and telephones; and
• For providing attractive and sturdy finishes for fabric and clothing.

Words to Know

ALKANE

a hydrocarbon where all the bonds between atoms are single bonds, the carbons are linked in a chain (except for methane, CH₄), and every carbon atom is saturated, or bonded to the maximum number of hydrogen atoms.

ALKYL GROUP

an alkane that is lacking one hydrogen atom.

HYDROCARBON

a chemical that consists of carbon and hydrogen.

FOR FURTHER INFORMATION

"The Basics of Silicon Chemistry." Dow Chemical Company. http://www.dowcorning.com/content/sitech/sitechbasics/silicones.asp (accessed on October 26, 2005).

"Heat and Chemical Resistant Silicone Rubber. Silicones 2. Organic Silicon Chemistry." ChemCases.com Kennesaw State University. http://www.chemcases.com/silicon/sil2cone.htm (accessed on October 26, 2005).

"Silicones." Macrogalleria, University of Southern Mississippi. http://www.pslc.ws/macrogcss/silicone.html (accessed on October 26, 2005).

"Silicones Science On-Line." Centre Européen des Silicones. http://www.silicones-science.com/ (accessed on October 26, 2005).

"What Are Silicones?" Silicones Environmental, Health and Safety Council of North America. http://www.sehsc.com/index.asp (accessed on October 26, 2005).