Rubber, silicone

Silicone rubbers play a huge role in automotive applications due to their low temperature characteristics, thermal and chemical resistance, and general purpose use. From Table 8.1, we can see that the useful temperature range goes from -50 to 250°C. For automotive applications, this range covers all the operating system temperatures that a vehicle will see. Table 8.2 shows service life at continuous use for silicone rubbers. The time durations shown here are extensive for an elastomer and represent a great improvement over unsaturated carbon-based elastomers. 

Service Life of Silicone Rubbers at Different Temperatures 

FIGURE 8.4 Silicone rubber from addition polymerization. 

High Temperature Vulcanized Silicone (Peroxide Cured) 

FIGURE 8.5 Silicone rubber from peroxide cure. 

Among the silicone rubbers there are different product groups which differ in their crosslinking mechanism and application areas. 

In spite of their high cost, silicone rubbers have over the last 40 years established themselves in a variety of applications where heat resistance and retention of properties over a wide range of temperatures are required. 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

The main chemical routes for vulcanisation of silicone elastomers are 1) Elevated temperatures cures and 2) Room temperature vulcanisation mechanisms. Organic peroxide cures are used in elevated temperature cures. Since the organic peroxides are inhibited by most carbon blacks, non black reinforcing fillers such as precipitated silicas, titanium dioxide and zinc oxide are used. Room temperature vulcanisation is normally used with low consistency silicone elastomers. 

This high polymer gum is mixed both with a cross-linking agent and a 

Silicone rubbers have various properties which cannot be matched by other rubbers, and which justify their relatively high price. Their flexibility at low temperature (even to —80°) and their useful life at -I-250° of about 50-100 times that of hydrocarbon rubbers make them very suitable for numerous engineering applications, particularly in the aircraft industry. Their chemical inertness has resulted in many biological uses. 

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The design and operation of a flow visualization system for highly viscous fluids, such as silicon rubber, has been reported by Ghafouri and Freakley (1994). This system consists mainly of a rotating roll and fixed-blade assembly, as is shown in Figure 5.7, and can be used to generate and maintain, essentially. 

Simulation of the Couette flow of silicon rubber - generalized Newtonian model... 

Keeping all of the flow regime conditions identical to the previous example, we now consider a finite element model based on treating silicon rubber as a viscoelastic fluid whose constitutive behaviour is defined by the following upper-convected Maxwell equation... 

Silicone rubbers have excellent ozone and weathering resistance, good electrical properties, and good adhesion to metal. 

Foamed polymers Foamed sheet Foamed silicone rubber Foam fractionation... 

Self-Adhering Silicone Rubber Tape" in GeneralElectric Company bulletin. Insulating Materials Data PD 1302, General Electric Co., Schenectady,... 

Table 6. Properties of Different Classes of Silicone Rubbers ...

Table 9. Electrical Properties of Typical Silicone Rubber ...

Polymer-Fluid Equilibria and the Glass Transition Most polymer systems fall in the Class HI or Class V phase diagrams, and the same system can often change from one class into the other as the polymer s molecular weight changes. Most polymers are insoluble in CO9 below 100°C, yet CO9 can be quite sohible in the polymer. For example, the sorption of CO9 into silicone rubber is highly dependent upon temperature and pressure, since these properties have a large influence on the density and activity of CO9. 

These are silicon rubber, glass tape or glass mica tape sheathed, w ith an elastomer, having fire retardant and low smoke properties... 

PLUNGER STEEL/SILICONE RUBBER M/C (MOULDED SILICONE RUBBER END SEAL)... 

Whilst exhibiting the excellent low-temperature flexibility (with a Tg of about -80°C) and very good heat resistance (up to 200°C) typical of a silicone rubber, the fluorosilicones also exhibit good aliphatic oil resistance and excellent aging resistance. However, for some applications they have recently encountered a challenge from the polyphosphazenes (see Section 13.10). 

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

The ISO and ASTM DI418 use the following classification for silicone rubbers ... 

MQ Silicone rubbers having only methyl substituent groups on tbe polymer chain (polydimethyl siloxanes). 

VMQ Silicone rubbers having both methyl and vinyl substituent groups on the polymer chain. 

Substantial improvements in the heat-resisting capability of silicone rubbers were achieved with the appearance of the poly(carborane siloxanes). First described in 1966, they were introduced commercially by the Olin Corporation in 1971 as Dexsil. The polymers have the essential structure... 

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 ... 

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