Rubber, moisture-curing silicone

RTV silicone adhesive is a rubberlike polymer called polydimethsiloxane. RTV stands for room temperature vulcanizing, or simply a rubber that cures at room temperature. Silicone rubber adhesives are made using a complicated process that turns elemental silicon metal made from sand (silica) into a rubbery polymer. When cured, silicone rubber adhesives/sealants have excellent resistance to heat (500—600 °F) and moisture that makes them exceptionally suited for outdoor weathering applications such as sealant and caulking compounds in the construction industry. Because of its exceptional properties, silicone adhesive has been used in some exotic applications such as the soles of the boots worn by the first astronauts to walk on the moon. Silicone adhesive/sealants are used to seal windows, doors, and portholes on the space shuttle and many satellite missiles. A special silicone adhesive is used to bond the heat shield tiles on space shuttles. 

Room-temperature-vulcanizing silicone rubber (General Electric and Dow Corning) is available at hardware stores and is very useful as an adhesive and sealant. Atmospheric moisture is necessary to effect the cure, so broad areas of impermeable materials should not be cemented with these preparations. The uncured material evolves acetic acid, and the cured material appears to lose some weight in high vacuum but if used with moderation, it can be considered a satisfactory vacuum sealant for most chemical vacuum systems. 

In the rubber industry, moisture absorbed on the surface of silicate, impacts the rate and extent of cure and results in sponge-like textures. In moisture cured systems such as polyurethanes, polysulfides and silicones, moisture causes a premature reduction in shelf-life. In extrusion and injection molding the moisture absorbed on fillers contributes to various defects and a strict regime must be followed regarding the drying time and the conditions prior to processing. Lacing, a less well known phenomenon, is caused by the absorption of moisture on the surface of titanium dioxide. ... 

Their main field of application are sealing compounds (silicone rubber) for which they are sold in the form of reactive one-component systems (mainly in cartridges) (RTV-l-systems, room-temperature vulcanization). Just like the one-component polyurethanes described in Section 4.2.2, they cure under the influence of moisture from the ambient air. In certain formulations, this reaction leads to the separation of acetic acid perceptible by its characteristic odor. Adhesive and sealing layers on a silicone basis show the following characteristics ... 

Storage Moisture and light-sensitive Uses Electroplating platinizing pumice and the like for catalysts etching zinc for printing platinum mirrors indelible ink ceramics microscopy RTV addition-cured, 2-component silicone rubber catalyst photographic chemicals... 

To prevent the mold from adhering to the model, it is necessary to seal porous or, in the case of condensation-curing rubber, moisture-absorbing surfaces as well as surfaces consisting of silicate materials such as glass or porcelain prior to applying the silicone rubber. Suitable coating materials are polyvinyl alcohol, methyl cellulose (wallpaper adhesive), soft soap or Vaseline. 

Silicone adhesives cure without the application of heat or pressure to form permanently flexible silicone rubber. The rubber remains flexible despite the exposure to high or low temperatures, weather, moisture, oxygen, ozone, or UV radiation. This makes them useful for joining and sealing joints in which considerable movement can be expected, such as intermediate layers between plastics and other materials of construction (e.g., acryUc glazing). Several types of silicone adhesives/sealants are available, including one-part and two-part systems. 

Liquid silicone rubbers (LSRs) are two-part grades which can be coinjection-molded with thermoplastics to make door locks and flaps for vents.1 LSRs can be biocompatible and have low compression set, low durometer hardness, and excellent adhesion. One-part silicones are cured by ambient moisture. They are used for adhesives and sealants with plastic, metal, glass, ceramic, and silicone mbber substrates. A solventless, clear silicone/PC has been developed which requires no mixing, and can be applied without a primer. ... 

On this basis, a number of active sites have been developed for the production of RTV-1 type silicon rubbers (Table 5). The key for this curing behavior is that atmospheric moisture is sufficiently active to start and accelerate the crosslinking reactions. 

Examples in industry include attaching a silicone rubber seal to an aluminium parachute container. A primer is brushed on to the silicone rubber and the cyanoacrylate is applied to the aluminium. Once fully cured the bonded seal assembly prevents dust or moisture entering the safety critical box. 

Room-temperature-vulcanizing (RTV) silicone-rubber adhesives and sealants form flexible bonds with high peel strength to many substrates. These resins are one-component pastes that cure by reacting with moisture in the air. Because of this unique curing mechanism, nonporous substrates should not overlap by more than 1 in. 

One-part type For one-part-type siHcones, both the catalyst and the cross-linking agent are incorporated in the base compound at the time of manufacture, which involves compounding a silanol-stopped siHcone polymer with an excess of cross-linker. The one-component silicones typically cure with moisture. Exposure to atmospheric moisture causes further curing of the silicone rubber to take place. Diffusion of moisture into the rubber progresses vulcanization from the surface inward. One-part component type silicones are excellent adhesive sealants and can be formed in place as gaskets. They can also be used to form films by solvent dispersion. 

As already mentioned, one-component silicone sealants cure by moisture pick-up from the atmosphere to form permanently elastic rubber. There are different types of curing systems for one-part silicone sealants (Table 6.5). However, the most commonly available systems are the acetoxy and neutral curing systems. 

Storage Moisture-sensitive handle under nitrogen Uses Rubber accelerator, component of a catalyst system for polymerization of vinyl monomers, curing agent for water-repellent silicone materials, agent for plating with zirconium... 

Like the one-component polysulfides or polyurethanes, the one-component silicones must be protected from moisture during manufacture and storage. However, package stability of the silicones is superior. If a cartridge is only partially used, the plug of cured rubber which forms in the tip retards further moisture penetration. 

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