Published: Jan 2012
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SILICONE-BASED COATINGS ARE AMONG THE MOST useful materials for a wide variety of applications. Because the basic bond energies of Si-C and Si-O bonds are high, the chemical processes usually associated with aging of coated surfaces are often much slower and in many situations virtually eliminated for silicone coatings. Also, because the Si-C and Si-O bonds are not present in the natural organic world, biocompatibility and resistance to degradation via biochemical and biophysical processes are significantly reduced. In a similar manner, some silicone polymeric coatings and fluorosilicone-based coatings, in particular, have excellent solvent resistance. Silicone coatings based on trifluoropropylmethyl polysiloxanes have resistance to swelling from such agents as gasoline, jet fuel, solvents, and various other reagents. Chemically, highly branched polymeric silicone coatings begin to approach the properties of silica surfaces as the organic pendant content is reduced. As the organic pendant groups are reduced, the SiO4/2 content increases and the chemical resistance increases. Such polymeric coatings can provide physical scratch resistance as well as chemical resistance. Elastomeric silicone coatings, however, do not provide good resistance to strong acids and/or bases. Strong acids or bases, in particular at elevated temperatures, can cause depolymerization of the siloxane backbone, resulting in failure or, in the case of silicone elastomeric coatings, dissolution of the coating. In a similar manner, silicone coatings are resistant to virtually all frequencies of the electromagnetic spectrum. For compliant coatings, silicones are unsurpassed in resistance to hard radiation, such as that from a Cobalt-60 source for doses in excess of 20 Mrd, as well as from the ultraviolet, visible, and infrared frequencies. When combined with their hydrophobicity, oxygen, and ozone resistance properties, silicones provide excellent weatherability characteristics, and when these properties are combined with the resistance to atomic oxygen encountered in low earth orbit conditions, silicone coatings provide protection for organic substrates in various spacecraft applications. Coating various medical devices is another applications area that utilizes the high quality chemical and biochemical performance characteristics associated with silicone coatings. Such coatings are used to encapsulate and seal permanent implants such as heart pacemakers. They have also been used to coat temporary implants such as catheters and surgical drains. Thin elastomeric silicone coatings are used to provide soft tissue replacements by forming an envelope to encapsulate gels and/or normal saline solutions. Recent applications for biocompatible silicone coatings include drug delivery devices for both transdermal and long-term implantable, controlled-release drug delivery. A final characteristic that makes silicone coatings useful is their inherently low or nonflammability. Typically, silicone elastomeric coatings have been rated SE-l when tested via Underwriters' Laboratories Flame Test (UL-94). This property makes silicone coatings ideal for conformal coating of various electrical circuits and devices. In the event of catastrophic thermal degradation, the silicone coatings can and do provide a SiO2 “ash” coating that may permit the emergency operation of the electrical device on a shortterm, temporary basis. Lynch et al.  have investigated silicone and other coatings as thermal barrier coatings. They found that the only system that met their requirements of protecting a thin steel plate during a direct flame impingement test and withstanding low temperature flexure tests was a fiberglass-polysilicone composite. Other investigators have studied the effect of silicone fabric coatings on mechanical properties when used in glass fabric/polyester composites  and on water absorption of such fabrics .
Petraitis, D. J.
Nusil Technology, Carpinteria, CA