The one-component, RTV, silicone adhesive-sealants covered by this specification have outstanding resistance to ultraviolet radiation and ozone, chemical inertness, thermal stability, low-temperature flexibility and general long-term aging characteristics. Since the silicone adhesive-sealants are available as thixotropic pastes or as self-leveling liquids, they lend themselves to a variety of application techniques which are easily adapted to specialty uses as well as to production line methods. The materials are currently used in a wide variety of applications in the automotive, marine-appliance, metal working, aerospace, aircraft, building construction, communication, computer, and other industries. The silicone adhesive-sealants covered by this specification are divided into groups according to the intended use and the outstanding property which makes the RTV silicone adhesive-sealants suitable for a specific application (see 1.2).

Group I Silicone adhesive-sealants are intended for general-purpose adhesive-sealant applications. Group II Silicone adhesive-sealants are intended for use in applications requiring very high mechanical bond strength and tear strength, such as mechanical bonding and sealing, aerospace, automotive and industrial assembly sealing and gasketing, and other high stress applications. Group III Silicone compounds are intended for use in applications requiring very high temperature resistance such as encapsulating and sealing of heating elements in appliances, aerospace gasketing and sealing, and other critical bonding, sealing, potting, insulating, encapsulating and protective coating applications where parts must perform at high temperatures 401°-500°F (205-260°C).

Each group of silicone adhesive-sealant is subdivided into two Types according to physical state (see 1.2).

Type I Pastelike RTVs are intended for use on horizontal, vertical or overhead surfaces for applications such as bonding, sealing, and formed-in-place gaskets. Type II Self-leveling liquid (pourable) RTVs are intended for use in applications requiring flow into small surface crevices or hard-to-reach places.

Silicone adhesive-sealant selection should be made by individuals having expertise in silicone chemistry after careful consideration of the design and the performance required in the application, and the environment to which it will be exposed. The RTV adhesive-sealants covered by this specification give off acetic acid and are highly corrosive to wiring, connectors, solder joints, printed wiring boards and other components of avionic, electronic and electrical equipment. These RTVs are also prohibited from use on spacecraft because of stringent outgassing requirements in space environments. For avionic, electronic and electrical equipment and spacecraft applications, consideration should be given to silicone RTV adhesive sealants covered by MIL-A-46146.

These adhesive-sealants are not resistant to many types of fluid such as fuel, and hydraulic fluid. Materials meeting this specification utilize atmospheric moisture and liberate acetic acid during cure. When cured in contact with certain metals such as copper and other sensitive metals, a slight corrosion may occur. It has also been shown that this type material can cause fracture of stressed high strength steel when applied under high relative humidity conditions. These factors should be thoroughly investigated prior to the use of these materials.

The use of a primer is recommended for various substrates to achieve consistent results and obtain optimum adhesion when the silicone adhesive-sealant is exposed to water, high humidity and elevated temperature conditions. Primers should be used only as directed by the manufacturer, and should be used only with the adhesive-sealant for which the primer was supplied. The use of silicone adhesive-sealant and primer combinations other than those recommended by the manufacturer may lead to loss of adhesion or bond failure.

With these one component silicone adhesive-sealants, which require moisture from the air to cure, the thicknesses should be limited to ¼ inch (6 mm), and the glue lines which is 1 inch (25 mm) maximum between non-porous substrates. (The glue line or bond line is the layer which attaches two adherends.)