Elastomers fluorosilicone

The elastomers are based on polysiloxanes that contain trifluoropropyl methyl siloxane units. The materials are used as sealants, elastomers, and fluids for aerospace applications. The monomers are prepared according to the following scheme  

The above dichloro silane is converted to a cyclic trimer  

Fluorosilicone elastomers generally respond to ionizing radiation in a fashion similar to fhaf of silicone elastomers (polydimethylsiloxanes). One interesting application is a process of preparing blends of fluoroplastics, such as poly(vinylidene fluoride), with fluorosilicone elastomers to obtain materials having a unique combination of flexibility at low temperatures and high mechanical stiength.  

Efficiencies of radiation-induced reactions of selected elastomers are in Table 5.7. Tensile stiength data from another group are in Table 5.8. 

The most common method of preparation of PMTFPS is through the base-catalyzed ring-opening polymerization of the corresponding cyclic trimer [83,84], Details are in Chapter 2, Section 2.11.1. A specihc cnre site for peroxide curing is developed by incorporating 0.2 mol% of methylvinyl siloxane [85], Typically, fluorosilicone elastomers are copolymers of 90 mol% of trifluoropropylsiloxy and 10 mol% of dimethylsiloxy monomers, bnt the fluorosilicone content in commercial products ranges from 40 to 90 mol% [85], 

Flnorosilicones can be compounded by the addition of mineral fillers and pigments. Fillers for snch compounds are most commonly silicas (silicon dioxide), because they are compatible with the elastomeric silicon-oxygen backbone and thermally very stable. They range in surface areas from 0.54 to 400 mVg and average particle size from 100 to 6 nm. Because of these properties, they offer a great deal of flexibility in reinforcement. Thus, cured compounds can have Dnrometer A hardness from 40 to 80. Other fillers commonly nsed in flnorosilicones are calcinm carbonate, titaninm dioxide, and zinc oxide. 

Flnorosilicone compounds can be processed by the same methods nsed for silicone elastomers based on PDMS. They can be milled, calendered, extended, and molded. A large proportion of flnorosilicone componnds is nsed in compression molding. Molded parts produced in large series are made by injection molding, and parts with complex shapes are prodnced by transfer molding. Calendering is used to produce thin sheets and for coating of textiles and other snbstrates. 

Cross-linking of fluorosilicones is done by essentially the same methods as conventional silicones. A comprehensive review of this snbject is in [87], Cnrrently, there are three methods of cross-linking used in industrial practice  

For peroxide cross-linking, organic peroxides, such as dicumyl, di-i-butyl, and benzoyl peroxides, are used in amounts 1 to 3 phr. Typical cure cycles are 5 to 10 min at temperatures 115°C to 170°C (239°F to 338°F), depending on the type of peroxide used. Each peroxide has a specific use. A postcure is recommended to complete the cross-linking reaction and to remove the residues from the decomposition of peroxide. This improves the long-term heat aging properties [84]. 

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Comparison of Reinforcing Efficiency of VDF-TFE Copolymers Relative to Silica Flour in Fluorosilicone Elastomers... 

Mascia, L., Pak, S.H., and Caporiccio, G. Properties Enhancement of Fluorosilicone Elastomers with Compatibilized Crystalline Vinylidene Fluoride Polymers, Eur. Polym. J. 31(5), 459 65, 1995. 

Some other silicon elastomers include vinyl silicone elastomers, phenyl silicon elastomers, nitrile silicone elastomers, and fluorosilicone elastomers. 

Natural Rubber and Synthetic Polyisoprene Polybutadiene and Its Copolymers Polyisobutylene and Its Copolymers Ethylene-Propylene Copolymers and Terpolymers Polychloroprene Silicone Elastomers Fluorocarbon Elastomers Fluorosilicone Elastomers Electron Beam Processing of Liquid Systems Grafting and Other Polymer Modifications... 

In this context, by the term fluorosilicone are meant polymers containing C-F bonds and Si-O bonds with hydrocarbon entities between them. Thus, the repeating structure may be generally written as [RfX (CH J (CH,)vSiOz, where Rf is the fluorocarbon group.59 Commercially available fluorosilicones are based on polymethyltri-fluoropropylsiloxane (PMTFPS), or more accurately poly[methyl (3,3,3-trifluoropropyl)siloxane]. In some cases PMTFPS is copolymerized with polydim-ethyl siloxane (PDMS) for cost/benefit balance.60 The manufacture of monomers for fluorosilicones is discussed in some detail in Reference 60. Fluorosilicone elastomers... 

Fluorosilicone elastomers have an excellent heat resistance, although they have slightly lower high-temperature stability compared with PDMS.72 The ultimate temperature stability depends on cure conditions and environment. A typical cured... 

Fluid and Chemical Resistance of Fluorosilicone Elastomers (ASTM D 471)... 

The glass transition temperature of PMTFPS is -75°C (-103°F). Moreover, it does not exhibit low-temperature crystallization at -40 C (-40°F) as PMDS does. Because of this and the low Tg, fluorosilicone elastomers remain very flexible at very low temperatures. For example, the brittleness temperature by impact (ASTM D 746B) of a commercial fluorosilicone vulcanizate was found to be -59°C (-74°F).62 This is considerably lower than the values typically measured on fluorocarbon elastomers. Fluorosilicones combine the superior fluid resistance of fluoropolymers with the very good low-temperature flexibility of silicones. 

Electrical properties — dielectric constant (e), representing polarization dissipation factor (tan 8), representing relaxation phenomena dielectric strength (EB), representing breakdown phenomena and resistivity (pv), an inverse of conductivity — are compared with other polymers in Table 5.14.74 The low dielectric loss and high electrical resistivity coupled with low water absorption and retention of these properties in harsh environments are major advantages of fluorosilicone elastomers over other polymeric materials.74... 

Commercial fluorosilicone elastomer compounds are made from high-molecular-weight PMTFPS (MW is typically 0.8 to 2.0 million) and are cross-linked by organic... 

Thermal degradation of fluorosilicones can occur by a reversion mechanism and is accelerated by the presence of basic compounds. The maximum long-term service of fluorosilicone elastomers depends on cure conditions and environment and is typically 200°C (392°F).19... 

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