Frederick Kipping Develops Silicones
Frederick Kipping Develops Silicones
Silicones are a class of mixed inorganic/organic polymers developed in the early twentieth century. Since their initial discovery, they have been investigated and their uses expanded greatly because of their relative chemical inertness and their tolerance to a wide range of temperatures and environmental conditions. This versatility has made them useful in lubricants, as synthetic rubber, as water repellents, and a number of other uses.
Polymers are large molecules that consist of a large number of connected smaller molecules called monomers. Monomers link together in long chains of tens or hundreds of thousands of units, forming the polymers. Polymers are common in nature. Cellulose, lignin, proteins, and other important biological molecules are all polymers. Perhaps the best known and most important biological polymer is DNA, which consists of a very long sequence of bases (the monomers) that, chemically, are very similar to one another. Adolf Spittler created the first artificial polymer in 1897, probably by accident. The first commercially successful polymer, Bakelite, was developed in 1909 by Leo Baekeland (1863-1944).
Most polymers contain carbon as their chemical "backbone," making them organic molecules. Some, however, contain silicon as the primary atom, linked to oxygen with carbon groups attached to this backbone. Such organic/inorganic polymers are called silicones. In the early twentieth century, English chemist Frederic Kipping (1863-1949) developed and investigated silicones. Kipping, under the misapprehension that he had developed ketones (a type of organic, biological molecule) with silicone substituting for one of the primary carbon atoms, called his discovery "silicone" (as opposed to silicon, the element).
Over the next 45 years, Kipping continued his researches into the properties of what became an entire class of polymers, publishing over 50 papers on the subject. During World War II, silicones became important to the war effort, used as lubricants, synthetic substitutes for rubber (much of which was produced on plantations now controlled by the Japanese), and as a water-proofing agent.
In many ways, silicones were nearly ideal as polymers. Like plastics, they could be molded into nearly any shape, making them ideal for a wide variety of uses. Unlike most plastics, they were very resistant to the effects of heat and cold, giving them a far greater versatility and value to industry. In addition, being largely inert chemically, silicones were ideal for many applications in both the chemical industry and for use in surgical procedures. Unfortunately, the chemical bond between the silicon and oxygen atoms that comprises the polymer's backbone is susceptible to some chemical attacks, including hydrolysis, acids, and bases, keeping these compounds from being universally used. However, even with these few minor weaknesses, silicones remain supremely useful in many facets of industry and modern society.
It is difficult to overstate the impact that the family of silicones has had on our society. Silicone rubber, for example, provides a soft cushion for one's eyeglasses on the bridge of the nose and, in the insole of many athletic shoes, provides an equally soft cushion between a runner's heel and the pavement. Silicone lubricants, including the ubiquitous WD-40, keep bicycles running smoothly, keep doors opening quietly, and keep engines and gears operating efficiently for years at a time. As a hydraulic fluid, silicone oils provide reliable service under the incredible variety of conditions found in military and commercial aircraft, nuclear submarines, and the family automobile. Still other compounds are used as caulking or other sealants to make showers, windows, piping systems, and boats watertight. Other uses include electrical insulation, flexible molds, heat-resistant seals, and as surgical implants (most notoriously as silicone breast implants that resulted in numerous legal suits in the 1990s, but also for plastic surgery and other surgical uses).
All of these uses take advantage of one or more of the silicones' chief chemical advantages: resiliency over a wide range of temperatures, physical and chemical versatility, and chemical inertness. The impact of these compounds, then, will be discussed in terms of each of these characteristics and the fields in which they occur.
As mentioned above, silicone compounds retain their physical properties over a wide range of temperatures. This makes them ideal for lowand high-temperature applications because, unlike conventional polymers, they remain flexible at low temperatures and they refuse to soften or melt in the heat. Take, for example, a high-performance aircraft. High-altitude air temperatures are typically far below freezing. Unlike conventional compounds, silicone grease doesn't congeal at these temperatures and silicone hydraulic fluids become only marginally more viscous. So, at relatively low speeds at high altitudes, the aircraft's control surfaces work properly. At high, supersonic speeds, the airplane will heat up considerably because of friction against the air. While most aircraft do not become red-hot, skin temperatures can rise to a few hundred degrees. As temperatures rise, the silicone grease stays semi-solid, remaining on the components it is lubricating, instead of melting and dripping off as would be the case with conventional greases. The hydraulic fluid, too, retains its viscosity, giving a uniform feel and performance to the hydraulic system (which operates the control surfaces and landing gear) under a wide range of operating conditions. Silicone compounds are also used as grease to pack pump bearings on ships that might have to operate in the sweltering equatorial waters or the below-freezing waters of polar seas.
Silicone rubbers share this relative insensitivity to temperature. Obviously, at sufficiently high temperatures any compound will melt, just as any liquid will eventually solidify. However, silicone rubber will remain useable far longer than will conventional plastics, not melting, softening, or losing its strength at temperatures of a few hundred degrees. Interestingly, this same resiliency has helped make silicones ideal for some commercial applications. For example, silicone rubber shoe inserts not only cushion the impact of one's heel against the ground, but it does so reliably, for years, without needing replacement, under conditions ranging from a midsummer's casual walk to a runner's January run. These same properties, plus their resistance to the chemicals found in perspiration and skin oils, make silicone rubber one of the favorite compounds for the nosepieces on eyeglasses.
Silicones are a chemically and physically diverse group of materials, giving them even more versatility. This family of compounds can be formulated into liquids, solids, or gels. Within each of these families, compositions can vary as well, allowing a specific product to be fine-tuned for its proposed use. Part of the reason for this versatility lies with the chemical properties of silicon, which shares many chemical properties with carbon because it lies directly beneath carbon in the Periodic Table. Like carbon, which can also assume many chemical guises, silicon can be persuaded to take on a variety of chemical bonds of different strengths, depending on which atoms it is bonded with. This means that, in some circumstances, it can be strongly linked to oxygen, making a solid material with great strength. In other circumstances, it is more loosely bonded with oxygen or even more loosely bonded with both oxygen and carbon groups, yielding a silicone grease or oil that is liquid at room temperatures. By controlling the nature of these silicon bonds and the number of silicon-based monomers that link together (or polymerize), a chemist can have a great deal of control over the final physical and chemical properties of the resulting compound.
Finally, silicone is relatively chemically inert, allowing it to be used in a wide variety of situations in which this is desirable. For example, regardless of lubricating abilities, one would not pack ball bearings in an acid because the acid would etch or corrode the bearings. The ideal lubricants are not only slippery, but are chemically unlikely to attack the surfaces they lubricate. Similarly, bodily fluids are a hostile environment for many materials since they are typically salty and replete with chemicals. Ordinary steels corrode quickly, and even many stainless steels corrode over time. Chemically reactive polymers or other compounds used in the body are also likely to either corrode or to initiate undesirable chemical reactions, leading to a breakdown of the implant, generation of unwanted chemical reaction products, or both. Many silicones manage to avoid these perils because they are not attacked by the body's chemistry and they are remarkably resistant to breakdown. This gives them widespread use in some surgical procedures in which such properties, combined with the physical properties noted above, are wanted. The use of silicone gel breast implants gained a high degree of notoriety in the 1990s because they were apparently linked with breakage and infection. However, scientific studies performed after most of these suits were settled indicated with a high degree of confidence that the silicone gel did not, in fact, cause the problems noted, even when the implants did rupture. It must also be noted that not all plastic surgeries are for the sake of vanity or aesthetics; in many cases, plastic surgery is performed to correct physical injuries from accidents or to repair birth defects.
It seems, then, that Kipping's discovery of silicones in 1904 was the impetus for a great many innovations that have had an impact on technology and society. Some of these impacts are directly noticed, such as silicone rubber shoe inserts, caulking for a leaking shower, and silicone water repellents for shoes and camping gear. Others, such as silicone-based greases and hydraulic fluids, materials for surgical implants, and the like, are not as obvious, but are equally important to society because of their role in our transportation systems, our self-image, and our commerce.
P. ANDREW KARAM
Rochow, Eugene G. Silicon and Silicones: About Stone-Age Tools, Antique Pottery, Modern Ceramics, Computers, Space Materials, and How They All Got That Way. New York: Springer-Verlag, 1987.