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.
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.
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.
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).