Silicone Contamination, Part I
Barbara Kanegsberg and Ed Kanegsberg, April 2004
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.
References:
1 http://nai.arc.nasa.gov/astrobio/feat_questions/silicon_life.cfm
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.
4 R. Adler. http://www.adlersantiqueautos.com/articles/brake1.html
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
