Fluoroelastomers composition

Hetherington, R. Fluoroelastomer compositions, their preparation and use, U.S. Patent 7,244,789 (July 17, 2007) to Illinois Tool Works Inc. 

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C—H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at 1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer. 

Fluoroelastomers Novikova et al. [32] reported unproved physico-mechanical properties of fluoro mbbers by reinforcement with chopped polyamide fibers. Other fiber reinforcements are covered by Grinblat et al. [33]. Watson and Francis [34] described the use of aramid (Kevlar) as short fiber reinforcement for vulcanized fluoroelastomer along with polychloroprene mbber and a co-polyester TPE in terms of improvement in the wear properties of the composites. Rubber diaphragms, made up of fluorosilicone mbbers, can be reinforced using aramid fiber in order to impart better mechanical properties to the composite, though surface modification of the fiber is needed to improve the adhesion between fluorosUicone mbber and the fiber [35]. Bhattacharya et al. [36] studied the crack growth resistance of fluoroelastomer vulcanizates filled with Kevlar fiber. 

In practical applications, fluoroelastomer parts often involve contact with hazardous fluids at elevated temperatures. Failure of parts such as seals may result in personal injury in some cases. Care should be taken by users of parts to assure that proper fluoroelastomer grade and composition are used. Detailed information from the custom compounder or the supplier of the fluoroelastomer should be obtained to minimize such hazards. 

Fluoroelastomers are copolymers containing fluorine in their structure. There are different kinds of fluoroelastomers depending on the chemical composition and on the production in which the comonomers are found in the chain. As examples of comonomers, we can mention vinylidene fluor-ide-hexafluoropropylene and vinylidene fluoride-chlorotrifluoroethylene. The vulcanization process is performed using peroxides, diamines, and bisphenol. 

The preparation of composite materials in general is a very important appHca-tion of the mechanical properties of nanodiamond. With many polymers like caoutchouc, polysiloxanes, fluoroelastomers polymethacrylates, epoxy resins, etc., composites with markedly improved mechanical characteristics have already been obtained from the noncovalent incorporation of nanodiamond by simple admixing during polymerization. The modulus of elasticity, the tensile strength, and the maximal elongation of the material all increase upon this modification. Depending on the basic polymer, just 0.1-0.5% (w/w) of nanodiamond are required to achieve this effect (Table 5.3). Polymer films can also be reinforced by the addition of nanodiamond. For a teflon film with ca. 2% of nanodiamond added, for example, friction is reduced at least 20%, and scratches inflicted by mechanical means are only half as deep as in neat teflon. 

Fluoropolymers have been used as processing aids because small quantities can reduce signih-cantly the overall viscosity and thus facilitate extrusion. Feng et al. [1996] examined the mechanism of viscosity reduction in the capillary flow of HDPE/fluoroelastomer blends. X-ray photoelectron spectroscopy, used to characterize the composition of the extmdates surface, indicated only very small traces of the fluoroelastomer on the extrudate, pointing to the fact that the viscosity reduction is due to adhesive failure between the fluoropolymer layer and HOPE. 

Furno, J. S. and Nauman, E. B. (1991) A novel heat resistant blend produced by compositional quenching a thermoplastic polyimide impact modified with a fluoroelastomer. Polymer, 32,87-94. 

Kalrez Dupont s trade name for fluoroelastomers made from tetrafluoroethylene, perfluorovinylmethyl ether, and a small percentage of crosslinkable monomer. These elastomers combine the rubbery properties of Viton with the thermal stability, chemical resistance, and electrical characteristics of tetrafluoroethylene resin. (Handbook of plastics, elastomers and composites. 4th edn. Harper CA (ed). McGraw-Hill, New York, 2002 Ash M, Ash I (1982-1983) Encyclopedia of plastics polymers, and resins, vols I-III. Chemical Publishing, New York). 

Pyrolant compositions based on finely divided Mg particles and fluorocarbons are susceptible to chemical degradation. This degradation also termed ageing is known to have at least two main causes reaction of magnesium with moisture and an inherent reactivity between Mg and its basic compounds with fluoroelastomer binders prone to eliminate HF, such as Viton A or polyvinylidene fluoride (PVDF). 

The use of very fine Mg powder in composition 2 requires a higher percentage of fluoroelastomer binder to assure complete coating. 

Figure 11.18 gives plots of t] versus composition for binary blends of a fluoroelastomer (Viton) and ethylene-propylene-diene-monomer (EPDM) rubber at... 

Fluoroelastomers were introduced in the late 1950s and have been developed over the years with various compositions from dipolymers [1,2] to pentapolymers [3-7],... 

First, fluoroelastomers were cured with amines. However, the scorchy nature of those cure systems as well as a rather poor compression set resistance led to the development of bisphenol cure systems [13,14]. Peroxide curing is performed thanks to the addition of a cure site monomer [4,15-18] for compositions that cannot be cured with bisphenol, such as high fluorine compositions, low-temperature polymers where HFP has been substituted by PMVE, and non-VF2 containing polymers such as FEPMs (TFE/P and FTP polymers), unless a bisphenol cure site has been added to the polymer [19,20]. 

The first step is to select the right composition for the fluoroelastomer since httle can be achieved through compounding. Then, the right viscosity and the right type must be selected. Finally, the formulation will tackle properties such as hardness. 

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