Room-temperature vulcanization rubber

RTV. (room-temperature-vulcanizing). Rubbers that have good physical properties and electrical properties similar to silicone rubber. 

Because of its imique properties and somewhat higher price eompared to the other common elastomers, silicone rubber is usually classed as a specialty elastomer, although it is increasingly used as a eost-effective alternative in a variety of applications. Two types of silieone elastomers are available, each providing the same fundamental properties. These are the thermosetting rubbers that are vulcanized with heat, and RTV (room temperature vulcanizing) rubbers. 

PDMS has the distinction of having the lowest glass transition temperature (Tg a -135°C) of any polymer. While silicone materials are relatively weak when compared with hydrocarbon-based polymers, their low-temperature flexibility allows them to be used in many applications where strength is not a primary concern. Silicones are often referred to as room-temperature vulcanizing rubbers. Typical uses include gaskets and seals. 

These materials are reviewed elsewhere in this book except RTV. The RTV (room temperature vulcanization) silicone plastic is a very popular type. It solidifies by vulcanization or curing at room temperature by chemical reaction, made up of two-part components of silicones and other elas-tomers/rubbers. RTV are used to withstand temperatures as high as 290° C (550°F) and as low as —160° C (—250°F) without losing their strength. Their rapid curing makes them... 

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. 

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. 

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

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. 

Dibutyltin and dioctyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as catalysts for the curing of room-temperature-vulcanized (RTV) silicone elastomers to produce flexible silicone rubbers used as sealing compounds, insulators, and in a wide variety of other applications. Dioiganotin carboxylates also catalyze the curing of thermosetting silicone resins, which are widely used in paper-release coatings. 

Indirect tooling methods are many. Examples include cast aluminum, investment metal cast, cast plastics, cast kirksite, sprayed steel, spin-castings, plaster casting, electroforming, room temperature vulcanizing (RTV) silicone elastomer (Chapter 2 Silicone Elastomer), elastomer/ rubber, reaction injection, stereolithography,338 344 (Table 17.4), direct metal laser sintering, and laminate construction. 

The spectral dispersion for organosilicones may be considerable for certain families of compounds. This is reflected in the Si chemical shifts of siloxanes, -(SiRR 0) -, an important class of compounds which includes resins, fluids, room-temperature vulcanized and heat-cured rubber consumer products. The first, Si NMR results (5,78) reported on polydimethylsiloxanes showed that individual resonance... 

Applications Disulfur dichloride is utilized in the manufacture of sulfur dichloride, thionyl chloride and sulfur tetrafluoride and with polyols for the production of additives for high pressure lubricating oils and cutting oils. In addition it is used as a catalyst in the chlorination of acetic acid. Solutions of sulfur in disulfur dichloride is utilized in the room temperature vulcanization of rubbers. 

When silicone adhesives react with air moisture they are called RTV-1 silicone adhesives (RTV room temperature vulcanization). The RTV adhesives have a rubber-Uke character. The term vulcanization is not technically correct, but is rather an historical reference. Silicones are elastomers and were at first crosslinked like other elastomers, albeit not with sulfur. In the integrated state, silicones contain blocked crosslinking agents that lose their blocking function upon contact with moisture. There are various different types of crosslinking agents with different... 

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 ... 

FIGURE 8.6 Room temperature vulcanized (RTV) silicone rubber. 

Silicone elastomers are either room-temperature vulcanization (RTV) or heat-cured silicone rubbers, depending on whether cross-linking is accomplished at ambient or elevated temperature. [The term vulcanization (see Chapter 1 and Chapter 2) is a synonym for cross-linking. While curing is also a synonym for cross-linking, it often refers to a combination of additional polymerization plus cross-linking.] RTV and heat-cured silicone rubbers typically involve polysiloxanes with degrees of polymerizations of 200-1500 and 2500-11,000, respectively. 

Room-temperature vulcanizing silicone rubbers (RTV rubbers) are low-molecular-weight liquid silicones with reactive end groups and loaded with reinforcing fillers. Several types are available on the market. 

Poly(Dimethyl Siloxane) Sihcone Rubber, Usually Copolymer with Vinyl groups (VMQ) Poly(Dimethyl Siloxane) Copolymer with Phenyl-Bearing Siloxane and Vinyl Groups (PVMQ) Room Temperature Vulcanizing Silicone Polysulfide (ET and EOT) Polyurethane (AU and EU)... 

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