The term "silica" denotes the compound silicon dioxide, SiO 2 . It is a ubiquitous chemical substance with rich chemical, geological, and commercial importance. Commercially, it has many uses, and it is the only source of elemental silicon.
The Evolution of Silicones
Silicon is a member of the Group IV elements in the Periodic Table. However, little of the chemistry of silicon can be inferred from carbon, one of its closest neighbors. Although silicon is the second most abundant element in Earth's crust (approximately 26%), it does not exist in nature as a free element. Silicon must be freed from its oxides through a chemical process known as carbothermic reduction. In this reaction, silica and a carbon source (generally wood) are heated together at extremely high temperatures to yield silicon in its elemental form. The Swedish chemist Jöns Jakob Berzelius (1824) was the first to isolate silicon from its natural matrix. Silicon is widely used in the electronics and chemical industries.
Silanes, siloxanes, and silicones are commercially produced from silicon and methyl chloride in a process known as the "direct reaction" or the "Rochow-Muller direct process."
Si + (excess) CH 3 Cl → (CH 3 ) x SiCl (4−x)
This reaction yields methylchlorosilanes (silicon compounds containing both methyl groups and chlorine). They are distilled (purified), and the isolated dimethyldichlorosilane is hydrolyzed to give siloxanes and silicones. This product can be formulated or finished into thousands of different products, which are sold to every major industrial segment. Although the direct reaction is used to commercially produce silicon-based materials today, the first example of an organosilicon compound (a material containing a silicon-carbon bond) was described in 1863 (as the U.S. Civil War raged and author Mark Twain was just beginning his literary career). French chemist Charles Friedel and U.S. chemist James Mason Crafts are credited for not only making the first compound that contained a silicon-carbon bond, but also determining the molecular weight of silicon during their years of research in this area of chemistry. By the turn of the twentieth century English chemist Frederic Stanley Kipping began to investigate this new area of science, publishing fifty-four papers on the subject between 1899 and 1937, but he failed to foresee the potential commercial value of his work. In one publication, he actually referred to the products from his work as "uninviting oils and glues."
In 1930 chemist J. Franklin Hyde, called the "father of silicone chemistry," met with Eugene Sullivan, vice president and director of research at Corning Glass Works. Sullivan was worried about the emerging plastics industry and its potential for competitive advantage over glass. Sullivan believed that it might be possible to combine the advantages of glass with some of the superior properties of organic plastics to produce a hybrid material. The idea was radical, but Hyde took Sullivan's idea for an inorganic-organic hybrid and used it as the foundation for what is now an $8 billion global industry, organosiloxanes. Hyde recognized the commercial importance of some of Kipping's observations and applied them to forge his hybrid technology. The Dow Corning Corporation was thus formed; at Dow the first commercial "silicones" were produced.
Silicones (or silicon-based materials) are an integral part of daily life. These materials make our denim clothing feel softer; they help our hair to shine and our skin to feel silky. Silicon-based materials are used in our food and drink to control foam. They lubricate and protect critical surfaces. They can be found in automobiles, buildings, and homes. These materials allow for the construction of skyscrapers as well as seal our aquariums and bathtubs. Have you ever wondered why you can peel a name tag from its paper backing and it still remains sticky enough to adhere to your clothing? The answer is silicones. Each day, we unknowingly come into contact with scores of products that contain silicones for the very special properties that they possess.
The German chemist Friedrich Wöhler first coined the term "silicone" in 1857. However, his strategy to name silicon-containing materials based on a nomenclature system derived from organic chemistry was lost when the silicon analog to a ketone could not be isolated. Today the term is often used as a generic for nearly all substances that contain a silicon atom. However, it is more properly described as an entirely synthetic polymer containing a Si-O backbone. To this backbone, organic groups are attached.
Bouncing Putty, later sold as Silly Putty ® , is an example of a silicon-based material with very unusual properties. It is a liquid that behaves as a solid or a solid that acts like a liquid. The material is a mixture of silicone polymers and boric acid. The polymers are terminated (end group) at both ends with an alcohol group (-OH) in contrast to the typical methyl group (-CH 3 ). The polymer ends (-OH) and the boric acid react with each other in a reversible fashion. This reversible reaction allows Silly Putty ® to be a very thick viscous liquid because bonds between the polymers and the boric acid are constantly being formed and broken. Stress (pulling the material sharply or striking it against the table) interferes with the reverse reaction, and the material behaves as a solid until the stress has been removed. The identity of the person who really invented bouncing putty is still debated today. R. R. McGregor and E. L. Warrick hold the U.S. Patent (2,431,878) that was issued in 1947 to Dow Corning Corporation. However, James Wright, an engineer for General Electric, has claimed that he conducted the initial experiments in 1943 that led to Silly Putty ® 's discovery. The truth is that neither company could find a good use for the interesting material. Silly Putty ® was brought to commercial success by Ruth Fallgatter (owner of the Block Shop toy store in New Haven, Connecticut) and her marketing consultant (Peter Hodgson of Marketing, Inc.) around 1950, with General Electric being the sole supplier of the material until 1959. Silly Putty ® remains a wonderful curiosity to all who handle the "liquid solid." It has been to the Moon (aboard Apollo 8) and was commemorated in a Smithsonian exhibit devoted to significant objects from the 1950s that shaped American culture.
This general description defines the broad class of polymers known as silicones or silicon-based materials. The most common example is poly (dimethylsiloxane) or PDMS. This polymer has a repeating (CH 3 ) 2 SiO unit. These materials are the basic building blocks of the silicone industry. Depending on the number of repeat units in the polymer chain and the degree of cross-linking (how the polymer chains are tied together), at least six classes of commercially important families of products can be produced: fluids, emulsions, compounds, lubricants, resins, and elastomers or rubbers.
How Are Silicones Used?
Silicones are highly valued materials because they have a combination of physical properties not found in other polymers. They have outstanding heat stability and can be used in applications where organic materials would melt or decompose. Many silicones seem to be impervious to the effects of aging, weather, sunlight, moisture, heat, cold, and some chemical assaults. Numerous silicones are used to stick, bond, or couple things together.
Unique surface properties further distinguish silicones from other materials. The low surface tension of silicone fluids makes them ideal for applications such as paper release agents, fiber lubricants, textile hand modifiers, mold release agents, antifouling materials, and water repellents. In fact, silicones have been used in foam control and as anticaking aids, corrosion inhibitors, emulsifiers, lubricants, conditioners, and gloss enhancers—all because of their special surface properties.
SEE ALSO Polymers, Synthetic .
Thomas H. Lane
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