RTV silicone

RTV Silicone Chemistry. There are two basic cure chemistries used by RTV sihcones the acetoxy-based and the alkoxy-based cure systems. Acetoxy-based RTV sihcones were first commercialized in the early 1960s (422,423). The general chemical reactions of these first-generation products are shown in Figure 8. 

Room temperature vulcanising silicone rubbers (r. t. v. rubbers) have proved of considerable value where elaborate processing equipment is not available. These rubbers are low molecular weight silicones with reactive end-groups and loaded with reinforcing fillers. The RTV silicone rubbers may be classified into two types ... 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Conventional machining operations are used preferably from the same plastic to be used in the product (Chapter 8, SECONDARY EQUIPMENT). Different casting techniques are used that provide low cost even though they are usually labor intensive. The casting of unfilled or filled/reinforced plastic used include TS polyurethane, epoxy, structural foam, and RTV silicone. Also used are die cast metals. 

Development work by the US Navy has included attempts to use expls other than RDX and HMX viz, HNS and TACOT (Tetranitro-1,2,5,6-tetraazadibenzocyclooctatetrene) together with binders such as RTV silicon rubbers. As reported by Stott (Ref 43) such work was not fruitful because of the low deton vel and d of the developed compns... 

Dibutyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as homogeneous catalysts for room-temperature-vulcanizing (RTV) silicones. The dialkyltin compounds bring about the cross-linking of the oligomeric siloxanes, to produce flexible, silicone rubbers having a host of different uses, such as electrical insulators and dental-impression molds. Recent work has also shown (560) that various dibutyltin dicar-boxylates catalyze both the hydrolysis and gelation of ethyl silicate under neutral conditions. 

RTV silicon, polyurethanefoams Bu2Sn(OCOC12H25)2, Bu2Sn(OCOC H23)2... 

One-part clear acrylic latex sealant formulation, 22 42t One-part manganese dioxide-cured polysulfide formulation, 22 4 It One-part pigmented siliconized acrylic latex sealant, 22 42t One-part RTV silicones, 22 596 One-part silicone cross-linkers, 22 33t One-part silicone sealant formulation, 22 34t... 

Room temperature-vulcanizing (RTV), silicon rubbers make use of the room temperature reaction of certain groups that can be placed on polydimethylsiloxanes, which react with water. When exposed to water, such as that normally present in the atmosphere, cross-links are formed creating an elastomeric product. 

Improved Room-Temperature Vulcanized (RTV) Silicone Elastomers as Integrated Circuit (IC) Encapsulants... 

Of all the commercially available organic and inorganic polymeric materials, RTV silicone elastomer has proved to he one of the most effective encapsulants used for mechanical and moisture protection of the Integrated Circuitry (1C) devices. A general overview of the RTV silicone elastomer and its commercial preparation and cure mechanism are described. Improved electrical performance of the RTV silicone encapsulant, by immobilizing the contaminant ions, such as Na, K" , Cl , with the addition of the heterocyclic poly-ethers as the contaminant ion scavengers seems to have a potential application as the contaminant ionic migration preventor in the electronic applications. 

For incorporation of crown ethers and cryptates into the RTV encapsulant system as sodium and potassium ion scavengers, the total ionic contaminants must first precisely be determined. Atomic absorption is used to measure these ions in commercial silicone RTVs and silicone fluids. Values of "10 ppm for sodium and potassium were obtained in the best samples. Chloride level was determined by potentiometric titration of the silicone with AgN03. A quantity of ion trap (either crown ethers or cryptates) was then added to the RTV silicone encapsulant, and its molar concentration was equal to the combined sodium and potassium contaminant levels. 

The formulated RTV silicone is usually cured at room temperature for 16 hours and then at 120°C for 4 hours to ensure the complete removal of organic solvent. A rubbery and non-tacky elastomer is usually obtained after the curing cycle. 

Figure 5. Triple-track resistor electrical testing performance of crown ethers in commercial RTV silicon encapsulants. Conditions bias, 180 V relative humidity, 96% temperature, 100°C.

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