New Silicones, Through Necessity
Emerging technologies and new regulations are giving rise to novel uses for silicones.
The left half of this tube is coated with a low coefficient of friction coating; the right half is not. Photo: NuSil Technology LLC
As the demand for innovative answers to current and emergent challenges surface, developments in silicone technology increasingly provide the solutions. These advancements not only improve current uses of silicones, but also lead to new markets and applications. In fact, silicone applications span a range of industries, including medical devices and aerospace.
One market seeing a surge of interest in silicone—due to an increase in environmental regulations—is anti-fouling materials. Because of emerging laws from entities such as the Environmental Protection Agency (EPA), biocides previously used to deter marine growth—such as metals like tin or copper—are no longer acceptable. While effective, biocides typically disrupt marine life indiscriminately; the build up of these metals in harbors and bays results in unintentional harm. Users found that using biologically inert silicone without the aid of a biocide provides an effective, anti-fouling coating without the collateral damage previously experienced. Alterations in modulus or non-toxic leaving groups are being explored, which may result in silicone solutions that can achieve the desired goal.
Ice-phobic coatings are another example of a silicone solution developed as a result of strict environmental regulations. In the aerospace industry, glycols are used to de-ice planes while on the ground. Increased reclamation requirements, however, are pushing the industry into finding novel solutions to mitigate the cost of existing methods. One solution being investigated is the use of a silicone coating. Rather than trying to stop ice from forming, silicone coatings focus on limiting the adhesion of ice to the protected surface. In turn, mechanical means of removing the ice—such as forced air or even take-off speeds—will return the aerodynamic surface to its original form. Recent studies and tests have shown several silicone coatings work well, and adhesion of the ice has been minimal.
In the medical device industry, silicone is commonly used because of its biocompatibility and cured physical properties. The natural tacky finish of silicone is a challenge for medical device manufacturers. Historically, greases or oils have been utilized in some applications, but these solutions themselves present their own challenges. When coating or lubricating a molded part, it is important to consider the chemistry of the molded part versus the chemistry of the lubricant. If not addressed, fluid may diffuse into the elastomer, causing both swelling in the elastomer and a loss of fluid, reducing the lubricating characteristics.
In addition, fluids and greases can easily transfer to undesirable areas, causing a reduction in lubrication and contamination. A recent breakthrough in the evolving technology of silicone coatings addresses this concern: a silicone dispersion that covalently bonds to the silicone elastomer substrate and results in a dry, friction-minimizing finish. Once cured, the coating chemically bonds to the silicone elastomer substrate and mimics its mechanical properties, achieving a final product that, when coated, will bend, twist, elongate, etc., without cracking, flaking, or peeling.
Finally, for applications in which outgassing is a concern, processes have been developed to produce low outgassing silicones that meet American Society for Testing and Materials (ASTM) E595. These materials must meet the specifications outlined in National Aeronautic & Space Administration (NASA) SP-R-0022A and European Space Agency (ESA) PSS-014-702, with a maximum Total Mass Loss (TML) of 1% and Collected Volatile Condensable Material (CVCM) of less than 0.1%. Most notably, low outgassing pressure sensitive adhesives (PSAs) have been developed to meet these same standards and were created because existing PSAs are not as suitable for elevated temperatures or high-vacuum environments that are sensitive to contamination. These low outgassing PSAs are suitable for high-temperature applications over 100 C, and are easier to use.
Mixing, de-airing, and applying an adhesive at a specific bond line thickness is no longer necessary. Simply peel and apply, or even cut to desired shapes and apply—and the desired area is prepared at a standard thickness—ready for the next layer of the application.
Often, major innovations in industry standards occur when large changes are needed. Whether these evolutions stem from restrictive regulations or simply from a need for improvement, solutions will always be developed. As seen in the examples above, silicone has a place in a wide variety of applications as it provides a safe, eco-friendly solution to many previously accepted, damaging applications.