Frederic Stanley Kipping discovered silicones, which nearly fifty years later found widespread markets because of their unique properties.
Silicones are examples of what chemists call polymers
A fundamental scientific consideration with a silicone, as with any polymer, is how to obtain the desired physical and chemical properties in a product through close control of its chemical structure and molecular weight. Oily silicones with thousands of alternating silicon and oxygen atoms have been prepared. The average length of the molecular chain determines the flow characteristics (viscosity) of the oil. In samples with very long chains, rubberlike elasticity can be achieved through the cross-linking of the silicone chains in a controlled manner and the addition of a filler such as silica. High degrees of cross-linking could produce a hard, intractable material instead of a rubberlike one.
In the first four decades of the twentieth century, Frederic Stanley Kipping made an extensive study of the organic chemistry of silicon. He had a distinguished academic career, and he summarized his silicon work in a 1937 lecture before the Royal Society of London titled “Organic Derivatives of Silicon.” Because Kipping did not have available to him any naturally occurring compounds with chemical bonds between carbon and silicon atoms (organosilicon compounds), he needed to find methods of establishing such bonds. In this quest, Kipping drew on the discoveries of his predecessors.
Previous syntheses of compounds with silicon-carbon links involved the intermediacy of carbon derivatives of other elements. For example, Charles Friedel and James M. Crafts, who prepared the first organosilicon compound in 1863, used diethyl zinc as a reagent for transferring ethyl groups to silicon, replacing the chlorine atoms in silicon tetrachloride. Alfred Stock used dimethyl zinc to form silicon-to-carbon bonds and prepared a small sample of methylsilicone oil in 1919 but did not investigate the polymer further. Albert Ladenburg, who was a student of Friedel, used silicate esters as starting materials and employed sodium metal in some of his procedures. A. Polis also used sodium in a procedure to prepare tetraphenyl silicon. Kipping, in his earliest work, used sodium metal to activate organic halogen compounds, forming organosodium derivatives that could be used to replace the chlorine in silicon tetrachloride by organic groups. In 1901, the French organic chemist Victor Grignard reported on the synthetic uses of organomagnesium compounds—the famous “Grignard reagents”
Although Kipping was probably the first to prepare a silicone and was certainly the first to use the term “silicone,” he regarded silicones as a side issue and did not pursue their commercial possibilities. His careful experimental work was a valuable starting point for all subsequent workers in organosilicon chemistry, however, including those who later developed the silicone industry.
A few years after Kipping’s lecture before the Royal Society, important research was initiated at the General Electric (G.E.) Company’s corporate research laboratory in Schenectady, New York. On May 10, 1940, G.E. chemist Eugene G. Rochow discovered that when methyl chloride gas was passed over a heated mixture of elemental silicon and copper, it reacted to form organochlorosilane compounds with silicon-carbon bonds. Kipping had shown that these silicon compounds react with water to form silicones. The importance of Rochow’s discovery was that it opened the way to a continuous process that did not consume expensive metals such as magnesium or the flammable ether solvents required in the Grignard method. The copper acted as a catalyst, and the desired silicon compounds were formed with only minor quantities of by-products. This “direct synthesis,” as it has come to be called, is now realized commercially on a large scale and is the most important source of silicone precursors.
The action of water on the organochlorosilanes from Rochow’s direct synthesis is a rapid method of obtaining silicones, but it does not provide much control of the molecular weight. Further development work at G.E. and at Dow-Corning showed that the best procedure for controlled formation of silicone polymers involves treating the crude silicones with acid to produce a mixture from which high yields of an intermediate called D4 can be obtained by distillation. The intermediate D4 can be polymerized in a controlled manner through the use of acidic or basic catalysts. Wilton I. Patnode of G.E. and James F. Hyde of Dow-Corning made important advances in this area. Hyde’s discovery of the use of traces of potassium hydroxide as a polymerization catalyst for D4 made possible the manufacture of silicone rubber, which became one of the most commercially valuable of all the silicones.
Although Kipping’s discovery and naming of silicones occurred from 1901 to 1904, the practical use and impact of silicones started in 1940 with Rochow’s discovery of the direct synthesis. General Electric constructed a silicone plant at Waterford, New York, and Dow-Corning began production in Midland, Michigan. They were joined by Union Carbide in 1957 and Stauffer Chemical in 1965. The German companies Wacker-Chemie and Farbenfabriken Bayer, A.G., offered silicones beginning in the 1950’s; by 1968, silicones were being produced also in Belgium, Czechoslovakia, France, England, Japan, and the Soviet Union. Production of pure silicones rose from about 1,000 tons per year in 1950 to 14,000 tons in 1965 in the United States. World production in the late 1980’s was estimated at more than 500,000 tons. The major forms of silicone are oils, resins, and elastomers. Approximate percentages of the total production consumed in various industries are as follows: paper and textiles, 30 percent; electrical and electronics, 25 percent; construction, 15 percent; automobiles, 10 percent; office equipment, 10 percent; and medical and food processing, 10 percent.
In the United States, production of silicones was rapid enough to permit them to have some influence on military supplies for World War II. In aircraft communication equipment, use of silicones for extensive waterproofing of parts resulted in greater reliability of radios under tropical conditions of humidity, where condensing water could be destructive. Silicone rubber, because of its stability to heat, was used in gaskets for high-temperature use, such as in searchlights and in the superchargers on B-29 bomber engines. Silicone grease applied to aircraft engines also helped to protect spark plugs from moisture and promoted easier starting.
After World War II, the uses for silicones multiplied. Silicone rubber appeared in many products, from caulking compounds to wire insulation to breast implants for cosmetic and reconstructive surgery. Silicone is also an excellent material for joint implants and is a component of medical tubing. Silicone rubber boot soles walked on the Moon, where ordinary rubber would have failed. Ordinary materials failed in the space shuttle Challenger disaster in 1986; it is possible to speculate that if silicone rubber O-rings had been used, the accident might have been averted. Silicone oils continue to find a multitude of specialized uses, from lubricating recording tape to preventing tires from sticking in the molds in which they are made. Silicone oils are used in hydraulic fluids and transformer oils, for waterproofing paper and cloth, and for foam reduction in paper mills. They are also used as surface films for automobile finishes and razor blades. Silicone resins were originally developed to serve as binders for glass fibers to produce a composite material for electrical insulation. Their additional uses now include heat-resistant coatings for mufflers and catalytic converters on automobiles and electrically insulating varnish. Foamed silicone resins have been used for heat insulation and as components of sandwich-type composite materials.
The discovery of silicones cannot be isolated in time to a single day or even a single year. Moreover, silicones as we now know them owe much to years of patient developmental work in industrial laboratories. Basic research such as that conducted by Kipping, Stock, and others served to point the way and catalyzed the process of silicones’ commercialization.
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Challenger, F. “Frederic Stanley Kipping.” In Great Chemists, edited by Eduard Farber. New York: Interscience, 1961. This biography was written by one of Kipping’s collaborators. -
Hyde, James F. “Chemical Background of the Silicones.” Science 147 (1965): 829-836. This short article summarizes some of the chemistry involved in making silicones and focuses on the achievements of Dow-Corning researchers. -
Kipping, F. S. “Organic Derivatives of Silicon.” Proceedings of the Royal Society of London A159 (1937): 139-148. Kipping summarizes his work on silicon and places it in context with that of other workers. Many historical accounts draw on this lecture. Kipping ends on a note of pessimism, stating that “the prospect of any immediate and important advance in this section of organic chemistry does not seem to be very hopeful.” -
Leyson, Burr W. Marvels of Industrial Science. New York: E. P. Dutton, 1955. Leyson, an army officer, was impressed by the uses of silicones in the military. Discusses waterproofing of communication equipment and rubber for low-temperature applications. -
Liebhafsky, H. A. Silicones Under the Monogram. New York: John Wiley & Sons, 1978. This rambling and entertaining book is enriched with many photographs of individual scientists, laboratories, and samples of silicones. Includes extensive discussion of the commercial development of silicones and the patent litigation relating to it. The annotated bibliography is extensive and varied and could serve as an entry into the study of twentieth century industrial chemistry research. -
McGregor, Rob Roy. Silicones and Their Uses. New York: McGraw-Hill, 1954. McGregor prepared this monograph at the suggestion of Dow-Corning. Discusses physiological responses to silicones, applications of silicones in various industries, and the chemistry of silicone preparation in simple language. Many of the individual products mentioned are produced by Dow-Corning. -
Mueller, Richard. “One Hundred Years of Organosilicon Chemistry.” Translated from the German by E. G. Rochow. Journal of Chemical Education 42 (1965): 41-47. This article originally appeared in a German periodical and takes the form of a lecture presented March 27, 1963, in Dresden, Germany. Includes photographs of organosilicon pioneers and contains fifty-six references to their work. -
_______. Silicon and Silicones. New York: Springer-Verlag, 1987. Relates the story of silicon for the general reader. Tells the history of silicones along with several personal vignettes, including how Rochow treated his own house with silicone coatings to preserve the wooden siding. Discusses silicones within the wider context of silicon chemistry in general. -
Rochow, Eugene G. An Introduction to the Chemistry of the Silicones. 2d ed. New: York: John Wiley & Sons, 1951. This book appeared before any other on the subject and was translated into five languages. This edition contains a complete listing of Kipping’s publications on silicon chemistry as well as a wealth of information on the properties and uses of organosilicon compounds. -
Rochow, Eugene G., and Éduard Krahé. Perkin, Kipping, Lapworth: Success Through Sisterhood. New York: Springer, 2001. Biographical historical novel about the marriages of three sisters to the three chemists named in the title. Provides a factually accurate account of the influence these three women had on the work of their famous husbands.
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