Two words, “silicone contamination,” are dreaded throughout the industrial world. Silicones are polymers made of silicon, carbon, and oxygen. The term has been liberally applied to describe all organosilicon polymers and even some monomers. Silicones are widely used throughout the industry. Applications include lubricants, adhesives, films and barriers, as well as process additives like surfactants or plasticizers. Some of the same physical and chemical properties that make silicones attractive, namely a high degree of chemical inertness, thermal stability and resistance to oxidation, make silicone contamination a ubiquitous problem. Because silicones are so widely used and because they can be difficult to remove, care in the choices of materials and an understanding of possible sources of contamination is crucial.
It is not surprising that silicones are so popular. Silicon bears certain similarities to carbon. Silicon is just below carbon in the periodic table. The atomic structure of the two are similar. Like carbon, silicon can combine with four other elements, allowing a wide array of compounds. Analogies have been drawn between carbon and silicon-based molecules. The term silicone was first used in the 19th century, the “one” was originally derived from the oxygenated carbon based ketones. NASA, in fact, provides an informative discussion of silicon-based chemistry, including similarities to and differences from carbon-based chemistry, and a debate on the likelihood of silicon-based life forms, a perennial premise in Science Fiction.1
While complex Si based polymers can be constructed, silicon is NOT carbon. The differences in chemistries contribute to making Si contamination such a difficult problem. For instance, organic contaminants can be oxidized by atomic oxygen and easily removed. With silicones, however, only the organic functional groups will be oxidized, leaving a glassy non-volatile silicate surface. This can lead to cracking or crazing degrading the optical properties of the surface or exposing subsurface layers to oxidation. This silicate surface is very hard to remove. This is a problem for NASA satellites in low-earth orbit where there is an abundance of atomic oxygen.2
Silicones make excellent lubricants and mold-release agents. The same properties cause them to be enemies of adhesion, therefore a serious contaminant in bonding applications. Since silicones are relatively chemically inert, and unaffected by most organic or aqueous solvents, they are difficult to remove. Silicones are often used as vacuum pump oils. If metallization is attempted on a critical surface and silicones are present, there can be unacceptable adhesion. Silicones are also a potential problem in compressed medical gases. Silicone particle contamination of pharmaceuticals can be difficult to remove by filtration.
Some silicone compounds with high vapor pressure can off-gas from their matrix and pose problems for certain devices.3 Magnetic and chemical sensors are affected by deposition of silicone compounds on their surfaces. For example, parts-per-million (ppm) emission from components inside a portable hydrocarbon detector can polymerize on the surface of the sensor and severely impede its performance in a matter of hours. Minimal exposure of hard disk read-write heads to silicone compounds can result in errors or drive failure.
Silicone contamination problems are not restricted to the critical components manufactured in cleanrooms. In wood or metal coating operations, silicone contamination is a nightmare. Even traces of silicone cause primers and paints or other coatings to “fisheye,” separate, and lose adhesion. This is a particular problem in automotive refinishing, where silicone-based cleaning and polishing products have been used. In another automotive application, if silicone brake fluid gets through a leaking vacuum booster into an engine, it burns to form silica sand and quickly wears down an engine’s internal parts.4 Silicone can also affect rubber brake components.
Wood finishers, manufacturers of connectors, designers of biomedical devices, Q.C. departments in pharmaceutical facilities, those in the space program, and many others are all concerned with thin film or particulate silicone contamination. An awareness of the vast number of sources of silicone-based materials as well as the pathways by which contamination may occur is an important step in prevention of contamination. In the next column, we continue with a discussion of detecting and avoiding silicone contamination.
2 B. A. Banks, K. K. de Groh, S. K., Rutledge, C. A. Haytas. “Consequences of Atomic Oxygen Interaction with Silicone and Silicone Contamination on Surfaces in Low Earth Orbit,” International Society for Optical Engineering, Denver, CO (1999). Abstract at http://www.grc.nasa.gov/WWW/epbranch/other/silctitles.htm#5.
3 E. Butrym. “Analysis of Silicone Contaminants on Electronic Components by Thermal Desorption GC-MS,” http://www.sisweb.com/referenc/applnote/app-88.htm.