Silicone Contamination, Part III
Barbara Kanegsberg and Ed Kanegsberg, June 2004
Silicones have proven valuable in areas encompassing such diverse areas as
automotive, coating, biotechnology and nanotechnology. Because silicone compounds
are an important part of manufacturing, detection of low levels and techniques
for removal are of increasing importance.
Contamination Within the Analytical Laboratory
Some applications such as food and cosmetics processing require detection
of silicone levels of 10 ppm of silicone. Accurate determination of low levels
of silicone require scrupulous contamination control and careful experimental
design;1 concerns include avoiding contamination or losses during extraction
as well as understanding possible molecular modifications. Silicone contamination
can be characterized or speciated by gas chromatography. However, the sample
must be prepared using a lengthy (two to three hour), multi-step extraction
or digestion process. It is critical to avoid introduction of silicone during
extraction and analysis. For example, silicones can be transferred to the
sample
from stopcock grease or from the septa of the crimped vials used to hold
the extracts prior to analysis. If GC columns are subjected to acids, silicones
may be released from the liquid phase of the column. Conversely, silicone-containing
compounds may be lost through adsorption to glass extraction vessels or through
volatilization. Molecular structure may change after deposition or during
extraction.
In addition to negative controls and recovery studies, it is prudent to work
with the analytical laboratory to avoid contamination during sample collection.
For critical applications such as adhesion in biomedical devices, it
is tempting to assert that no silicone contamination is tolerable.
Analytical chemists
find the concept of absolute zero contamination to be unrealistic,
and probably unachievable. Tolerable silicone contamination is a
combination of function
and achievable quantification.
Silicone Removal
The method of choice for silicone removal depends on such factors as
the substrate, the specific silicone compound in question, other
soils, and the
required surface
quality. It is also important to assess such issues as flammability,
allowable inhalation or skin absorption, environmental requirements
(including status
as a volatile organic compound), as well as customer and FDA constraints.
Coatings removal may require abrasive or impingement techniques.
For thick films or mixed soils, organic solvents are generally
selected. The selection
process is often pragmatic. Some favor isopropyl alcohol, with
acetone for fluorinated silicones. However, these solvents tend not
to be
effective
for
removing silicones with viscosities greater than 50 centistokes.
Others, find more aggressive solvents such as trichloroethylene,
toluene, or
hexane to be
more successful.
Following the concept of “like dissolves like,” volatile
methyl siloxanes (VMS) have proven successful in some applications.
The linear
methyl siloxanes, while flammable, have the advantage of a lower
boiling point, more
rapid evaporation, and more favorable worker safety profile than
do the higher molecular weight cyclic siloxanes. VMS are relatively
costly.
However, because
the VMS are relatively mild solvents, they may be an option where
substrates would be damaged by aggressive solvents.
It may be necessary to remove silicones from processing equipment
on a regular basis, as in batch processes where some product
lines contain
silicones.
In other instances, it is considered valuable to introduce
a silicone removal process on preventive grounds. When any
cleaning
process
is introduced or
modified,
customer, FDA, and other regulatory agency validation requirements
must be considered.
One recent report illustrates a logical approach to comparing
efficacy of removal of silicones from spacecraft hardware.2
Several aqueous-based,
bio-based
(d-limonene),
and other organic compounds were investigated. Solubility
parameters were considered in initial selection; toxicity
and flammability
were also considered.
Turbidity
on mixing the proposed cleaning agent with specific silicone
compounds of interest was used as a qualitative discriminator
for comparing
solvents. For the silicones
tested, solubilization in isopropyl alcohol was not as
rapid as for some
other organic solvents, based on reduction of turbidity.
Toluene, hexane, and heptane
were identified as dissolving the silicone samples rapidly.
It was noted that a generic silicone contaminant probably
does not
exist;
specific solubility
depends on the specific silicone. This comparison study
is an example of a
good first step toward development of a rugged, well-monitored
process.
The Future
Given their unique, often desirable properties, eliminating
silicones from the manufacturing process is unrealistic
and counterproductive.
Appropriate
usage and controls allow these valuable materials to
be used productively.
Acknowledgment: The authors appreciate the comments of
Jennifer Stasser, Dow Corning Analytical Solutions
and of Thomas P.
Banigan, NuSil.
References
1 A. L . Smith, R.D. Parker. “Trace Analysis of Silicones,” The Analytical Chemistry of Silicones, Wiley-Interscience, A. Lee Smith (Editor), (1991). 2 K. Luey, D.J. Coleman. “Removal of Silicone Contaminants from Spacecraft Hardware,” Fourteenth Annual International Workshop On Alternatives To Toxic Materials In Industrial Processes, Scottsdale, AZ, (December 8 - 11, 2003).
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.
