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Silicone rubber

Properties

Silicone rubber offers good resistance to extreme temperatures,

being able to operate normally from 55 C to +300 C. At theextreme temperatures, the tensile strength, elongation, tear

strength and compression set can be far superior to conventional rubbers although still low

relative to other materials. Organic rubberhas a carbon to carbon backbone which can leavethem susceptible to ozone,UV, heat and other ageing factors that silicone rubber can withstand

well. This makes it one of the elastomers of choice in many extreme environments.

Compared to organic rubbers, however, silicone rubber has a very low tensile strength.[1] For this

reason, care is needed in designing products to withstand even low imposed loads. The materialis also very sensitive to fatiguefrom cyclic loading. Silicone rubber is a highly inert material and

does not react with most chemicals. Due to its inertness, it is used in many medical applications

and in medical implants.

Special grades

There are also many special grades and forms of silicone rubber, including: steam resistant,

metal detectable, high tear strength, extreme high temperature, extreme low temperature,

electrically conductive, chemical/oil/acid/gas resistant, low smoke emitting, and flame-retardant.

A variety of fillers can be used in silicone rubber, although most are non-reinforcing and

lower the tensile strength.

Silicone rubber is available in a range or hardness levels, expressed as Shore A or IRHD

between 10 and 100, the higher number being the harder compound. It is also available invirtually any colour and can be colour matched.

Synthesis

Silicones are synthesized from chlorosilanes,tetraethoxysilane, and related compounds. In thecase ofPDMS, the starting material is dimethyldichlorosilane , which reacts with wateras

follows:

n Si(CH3)2Cl2 + n H2O [Si(CH3)2O]n + 2n HCl

Duringpolymerization, this reaction evolves hazardous hydrogen chloride gas. For medical uses,

a process was developed where the chlorine atoms in the silane precursor were replaced with

acetate groups. The reaction product of the final curing process is the less dangerous acetic acid

(the acid found in vinegar). As a side effect, the curing process is much slower in this case. Thisis the chemistry used in many consumer applications, such as silicone caulkand adhesives.

http://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Ozonehttp://en.wikipedia.org/wiki/Ozonehttp://en.wikipedia.org/wiki/UVhttp://en.wikipedia.org/wiki/Elastomerhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Silicone_rubber#cite_note-0http://en.wikipedia.org/wiki/Fatiguehttp://en.wikipedia.org/wiki/Fatiguehttp://en.wikipedia.org/wiki/Medical_implanthttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Conductivehttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Chlorosilanehttp://en.wikipedia.org/wiki/Chlorosilanehttp://en.wikipedia.org/wiki/Tetraethoxysilanehttp://en.wikipedia.org/wiki/Tetraethoxysilanehttp://en.wikipedia.org/wiki/Polydimethylsiloxanehttp://en.wikipedia.org/wiki/Dimethyldichlorosilanehttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Polymerizationhttp://en.wikipedia.org/wiki/Hydrogen_chloridehttp://en.wikipedia.org/wiki/Acetic_acidhttp://en.wikipedia.org/wiki/Caulkhttp://en.wikipedia.org/wiki/Adhesivehttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Ozonehttp://en.wikipedia.org/wiki/UVhttp://en.wikipedia.org/wiki/Elastomerhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Silicone_rubber#cite_note-0http://en.wikipedia.org/wiki/Fatiguehttp://en.wikipedia.org/wiki/Medical_implanthttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Conductivehttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Chlorosilanehttp://en.wikipedia.org/wiki/Tetraethoxysilanehttp://en.wikipedia.org/wiki/Polydimethylsiloxanehttp://en.wikipedia.org/wiki/Dimethyldichlorosilanehttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Polymerizationhttp://en.wikipedia.org/wiki/Hydrogen_chloridehttp://en.wikipedia.org/wiki/Acetic_acidhttp://en.wikipedia.org/wiki/Caulkhttp://en.wikipedia.org/wiki/Adhesive

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Silane precursors with more acid-forming groups and fewer methyl groups, such as

methyltrichlorosilane, can be used to introducebranches orcross-links in the polymer chain.

Ideally, each molecule of such a compound becomes a branch point. This can be used to producehard silicone resins. Similarly, precursors with three methyl groups can be used to limit

molecular weight, since each such molecule has only one reactive site and so forms the end of a

siloxane chain.

Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild andcontrollable manner than chlorosilanes.

Vulcanization of silicones

An example of an RTV silicone rubber keypad

"Room-temperature vulcanizing" (RTV)silicone is constructed of reactive oil base polymers

combined with strengthening mineral fillers. There are two types of room-temperaturevulcanizing silicone:

RTV-1 (One-component systems)

RTV-1 hardens due to the action of atmospheric humidity, a catalyst and acetoxysilane.Acetoxysilane, when exposed to humid conditions will form acetic acid.[3] The curing process

begins on the outer surface and progresses through to its core. The product is packed in airtight

cartridges and is either in a fluid or paste form. RTV-1 silicone has good adhesion, elasticity

and durability characteristics. The Shore A hardness can be varied between 18 and 60.

Elongation at break can range from 150% up to 700%. They have excellent aging resistance

due to superior resistance to UV radiation and weathering. Industrial RTV-1 products arereferred to as CAFs.

RTV-2 (Two-component systems)

RTV-2 elastomer are two-component products that, when mixed, cure at room-temperature to a

solid elastomer, a gel, or a flexible foam. RTV-2 remains flexible from -80 C to +250 C.

Break down occurs at temperatures above 350 C leaving an inert silica deposit that is non-

flammable and non-combustible. They can be used for electrical insulation due to their

dielectric properties. Mechanical properties are satisfactory. RTV-2 is used to make flexible

moulds, as well as many technical parts for industry and paramedical applications.

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Epichlorohydrin

Two special types of epi is co and copolymer with ethylene eco.special property aliphatic andaromatic fluid resistance.good ozone electrical thermal about150*.applicatio making wire cable

jackets,hoses,belting packaging

Production

Epichlorohydrin is manufactured from allyl chloride in two steps, beginning with the

hydrochlorination using hypochlorous acid, which affords a mixture of two alcohols:[3]

CH2=CHCH2Cl + HOCl HOCH2CHClCH2Cl and, or ClCH2CH(OH)CH2Cl

In the second step, this mixture is treated with base to give the epoxide:

HOCH2CHClCH2Cl and, or ClCH2CH(OH)CH2Cl + NaOH CH2CHOCH2Cl + NaCl +H2O

http://en.wikipedia.org/wiki/Allyl_chloridehttp://en.wikipedia.org/wiki/Hypochlorous_acidhttp://en.wikipedia.org/wiki/Hypochlorous_acidhttp://en.wikipedia.org/wiki/Epichlorohydrin#cite_note-Ullmann-2http://en.wikipedia.org/wiki/Epoxidehttp://en.wikipedia.org/wiki/File:Epichlorohydrin-manufacture-step2-2D-skeletal.pnghttp://en.wikipedia.org/wiki/File:Epichlorohydrin-manufacture-step1-2D-skeletal.pnghttp://en.wikipedia.org/wiki/Allyl_chloridehttp://en.wikipedia.org/wiki/Hypochlorous_acidhttp://en.wikipedia.org/wiki/Epichlorohydrin#cite_note-Ullmann-2http://en.wikipedia.org/wiki/Epoxide

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Poly sulphide

Polysulfides, as sulfides, can inducestress corrosion cracking in carbon steel and

stainless steel.

Polysulfides are a class ofchemical compoundscontaining chains ofsulfuratoms. There are twomain classes of polysulfides: anions and organic polysulfides.Anionshave the general formula

Sn2. These anions are the conjugate bases of the hydrogen polysulfides H2Sn. Organic

polysulfides generally have the formulae RSnR, where R = alkyloraryl

http://en.wikipedia.org/wiki/Stress_corrosion_crackinghttp://en.wikipedia.org/wiki/Stress_corrosion_crackinghttp://en.wikipedia.org/wiki/Carbon_steelhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Alkylhttp://en.wikipedia.org/wiki/Alkylhttp://en.wikipedia.org/wiki/Arylhttp://en.wikipedia.org/wiki/Stress_corrosion_crackinghttp://en.wikipedia.org/wiki/Carbon_steelhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Alkylhttp://en.wikipedia.org/wiki/Aryl