Tracing the History of Polymeric Materials: Silicones

Almost all polymers are based on carbon. The backbone chemistry of our most common polymers — polyethylene, polypropylene, PVC, and polystyrene — is exclusively carbon with different pendant groups defining the properties of each material. The same is true of many other polymers like acrylic and ABS as well as elastomers like EPDM and nitrile rubber. Other polymers incorporate elements such as oxygen and nitrogen into the backbone to achieve higher levels of mechanical and thermal performance.

But there is a class of materials where no carbon is present in the polymer backbone. Instead, silicon is the essential backbone ingredient, coupled with oxygen while the carbon that is present is relegated to the side groups attached to the backbone. This class of materials is referred to generically as silicones and more properly as siloxanes.

If you look at a periodic table, you can see that carbon and silicon appear in the same column. It has been known for some time that elements in the same column enter into similar types of chemical reactions. For example, the elements in the far right column of the table are referred to as the noble gases. Except under the most extreme lab conditions, none of these elements combine with other elements to form compounds and they are found in nature in pure form as gases. The next column to the left contains the halogens, elements that all combine with hydrogen to form acids and are present in compounds that are useful as flame retardants.

Given the knowledge of this pattern in the periodic table, it makes sense that silicon is capable of participating in reactions similar to those that involve carbon, which would mean that polymers could be produced with a backbone based on silicon. Historically, carbon has been readily available from the environment in various forms, including soot, charcoal, and even diamonds. In addition, hydrocarbons in the form of bio-materials and petroleum products provide a readily obtainable class of raw materials from which polymers can be produced.

Silicon is the most abundant element in the earth’s crust. But it was not isolated as a chemical element until 1823 when the great Swedish chemist Jons Jackob Berzelius processed a silicate-based mineral, potassium fluorosilicate, in the presence of excess potassium. He then heated this elemental silicon in the presence of chlorine gas to form silicon tetrachloride (SiCl₄). This became the starting point for producing the compounds that would eventually form the chemistry for silicone polymers.

Silicon is the most abundant element in the earth’s crust, but it was not isolated as a chemical element until 1823. 

But as is often the case, it was a long road. It would be more than a century between the work of Berzelius and the development of the commercial materials that we know today as silicone polymers. The process of developing silicon chemistry towards the production of organic materials was advanced by German chemist Friedrich Wohler, who made significant contributions in both organic and inorganic chemistry. In organic chemistry, he was the first to produce urea from ammonium cyanate, and in the process dealt a significant blow to the belief of the time that organic compounds could only be made by living organisms. In inorganic chemistry he was the first person to isolate beryllium and yttrium in pure form. It is fitting, therefore, that he played a role in the early development of silicones, producing silane (SiH₄) and trichlorosilane (SiHCl₃) in 1857. Wohler was also the first person to use the term “silicone.”

In 1863, Charles Friedel and James Crafts, two chemists well known to anyone who has studied organic chemistry, prepared tetraethysilane by reacting the silicon tetrachloride that Berzelius had discovered with diethyl zinc. Thus was created the first true organosilicon compound, bridging the gap between organic and inorganic chemistry. In 1872, Albert Ladenburg, who worked with Friedel, extended this work by reacting triethoxychlorosilane with diethyl zinc in the presence of sodium to produce an organosilane with multiple functional groups, opening the door to the possibility of polymerization. He continued to experiment with other combinations, introducing aromatic rings into the chemistry by reacting diphenyl mercury with silicon tetrachloride to produce trichlorophenylsilane in 1874.

A period of active experimentation in organosilane chemistry continued into the 20^th century. A prominent figure in this story was Professor Frederic Stanley Kipping. Kipping and his colleagues at University College in Nottingham, England, devoted nearly half a century to researching the synthesis and properties of the materials that have become popularly known as silicones.

In 1901 he produced a material that today we would identify as poly(diphenyl siloxane) and revived the term “silicone” to describe this class of compounds. In 1904, he developed an important mechanism for forming the carbon-silicon bonds that were the key component for creating organosilane chemistry using the Grignard route, a well-known mechanism in organic chemistry that employs organometallic chemicals based on magnesium. This is a crucial part of the story of commercial development that would come later. Kipping and his teams published 57 papers on organosilicon materials between 1899 and 1944.

Despite Kipping’s foundational role in the development of these materials, he was not optimistic that they would ever become commercially useful. He referred to them as “sticky messes” and in 1937 he gave an address to the Royal Society where he stated, “The prospect of any immediate and important advances in this section of organic chemistry does not seem to be hopeful.”

But at about the same time that these words were being spoken, someone else had a very different viewpoint. Dr. Eugene Sullivan worked at Corning Glass Works in Corning, N.Y. Sullivan saw the potential for developing materials that would provide a property profile between the rigid and brittle glass and ceramics and the softer, more flexible, but less thermally stable carbon-based polymers that were being rapidly developed in the early part of the 20^th century. English chemist J. Franklin Hyde began research into the feasibility of producing silicones at a commercial level. Sullivan hired Hyde in 1930 and in a relatively short period of time Hyde was using Kipping’s Grignard process to prepare laboratory quantities of silicone polymers.

It had been recognized for some time that in motor windings, the incumbent material, phenolic, was limited in thermal performance and a solution was needed. By 1938, Hyde had produced samples of glass cloth impregnated with silicone resin. These were introduced at a meeting between Corning Glass Works and General Electric in this same year. This started the fast track to commercialization that would happen five years later. But another giant of the chemical industry would become involved in the process. And that story will be the subject of our next installment.

ABOUT THE AUTHOR: Michael Sepe is an independent materials and processing consultant based in Sedona, Ariz., with clients throughout North America, Europe, and Asia. He has more than 45 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting and failure analysis. Contact: (928) 203-0408 • mike@thematerialanalyst.com