Inorganic elastomers

The introduction of fluorine into the elastomeric macromolecule generally produces materials exhibiting an improved retention of properties at high temperatures, an improved flexibility at low temperatures, and an improved resistance to solvents. Essentially, there are two groups of fluoroelastomers fluoro-inorganic elastomers and fluorocarbon (or fluorohydrocarbon) elastomers. 

The poorly reproducible polymerization is presumably initiated by traces of cationic impurities. The poly(phosphonitrile chlorides) or poly(dichloro-phosphazenes), II, mostly have PCI3 end groups. They depolymerize at higher temperatures to hexachlorocyclotriphosphazene, I, and octachlorocyclo-tetraphosphazene and hydrolyze even in moist air. The products obtained at high yield are cross-linked and exhibit all the properties of inorganic elastomers. Consequently, they are also called inorganic rubbers. 

Inorganic Elastomers The major commercial inorganic elastomer is poly(dimethyl siloxane), known widely as silicone rubber (see Table 9.4). This specialty elastomer has the lowest known glass transition temperature, Tg = -DO C (148) it also serves as a high-temperature elastomer. A common application of this elastomer is as a caulking material. It cross-links on exposure to air. 

Another covalently bonded inorganic elastomer class is the polyphos-phazenes (148,149),... 

Poly(dimethylsiloxane) (PDMS) is one of the most popular inorganic elastomers and can be modified with many nanofillers. Pan et al. used silanol-terminated PDMS and modified this matrix with mono-POSS and tetra-POSS cages which were physically blended into the matrix. The authors prepared another type of PDMS which had vinyl terminal groups that are also able to react with the central siloxane core by hydrosilylation. The idea was to obtain larger POSS-based fillers than silica particles typically used to reinforce elastomers (Fig. 8). 

Flame and Smoke Retardants. Molybdenum compounds are used extensively as flame retardants (qv) (93,94) in the formulation of halogenated polymers such as PVC, polyolefins, and other plastics elastomers and fabrics. An incentive for the use of molybdenum oxide and other molybdenum smoke and flame retardants is the elimination of the use of arsenic trioxide. Although hydrated inorganics are often used as flame retardants, and thought to work by releasing water of crystallization, anhydrous molybdenum oxides are effective. Presumably the molybdenum oxides rapidly form... 

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

FZ elastomers have excellent resistance to hydrocarbons and inorganic acids as expected for a fluoriaated elastomer. They are strongly affected by polar solvents, but are more resistant to amines than most other fluoriaated elastomers as showa ia Table 2. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

At present the situation in the field of inorganic polymeric materials is dominated by polysiloxanes (silicones) [14, 24-27], whose utilization as low temperature elastomers, thermally stable fluids, biomaterials etc., is definitely well established. 

Inorganic polymers based on alternating main group element-nitrogen skeletons (e.g. I - IV) are of interest for their potential as elastomers, high-temperature oils, electrical conductors, biological molecule carriers, and precursors to ceramic materials (J - 6). 

Elastomers containing fluorine, also called fluorelastomers. These combine the good physical properties of organic elastomers with the thermal stability of inorganic materials. Trade names are Viton, Fluorel, Kel-F and Technoflon. 

James M. Meyer retired in 2001 as Vice President of DuPont Central Research and Development. He joined DuPont in 1969 and held a variety of research and management positions related to elastomers and polymers. He moved to Central Research and Development in 1992 as director of materials science and engineering, and he assumed his current position in 1996. Dr. Meyer received his B.S. degree in chemistry from Indiana University and his Ph.D. degree in inorganic chemistry from Northwestern University. 

Certain inorganic tin compounds are effective flame-retardant synergists when incorporated at a 2.5% level into a 50% ATH-filled ethylene-acrylic rubber composition. Tin-containing elastomer formulations retain their flame-retardant superiority at environmental temperatures up to 250°C, and samples containing 2.5% ZnSn(0H)g do not sustain combustion in air at this temperature. 

Nomenclature, 17 384-413 basic scheme of, 17 384-385 biochemical, 17 401-402 computerized approaches to, 17 400-401 elastomer, 21 761t enzyme, 10 258-260 for ionic liquids, 26 840-841 glossaries related to, 17 404 inorganic, 17 387-394 macromolecular (polymers), 17 403 404 organic, 17 394-401 polymer, 20 390-395 pump, 21 88 quinone, 21 236-237 reactor technology, 21 358 related to mass transfer, 15 731-737 reverse osmosis, 21 674-676 Society of Rheology, 21 704 spray-related, 23 199t systematic, 17 394... 

Figure 4.8 ((a), (b), (c), and (d)) displays, in a simplified manner, the alteration of the basic properties of homopolymer (H-PP) when modified with other comonomers (co-PP), with elastomer (impact) or with an inorganic filler mineral, talc, short or long glass fibres (SGF or LGF). 

While polymeric hydrocarbons have been used as illustrations for simplicity, it is important to note that the principles discussed apply to all polymers, organic as well as inorganic, and natural as well as synthetic, and to elastomers, plastics, and fibers. 

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