Fluoroelastomers physical properties

The description of the physical properties of fluoroelastomers is necessarily less precise than that of fluoroplastics because of the major effect of adding curatives and fillers to achieve useful cross-linked materials of a given hardness and specific mechanical properties Generally, two parameters are varied increasing cross-link density increases modulus and decreases elongation, and raising filler levels increases hardness and decreases solvent swell because of the decreased volume fraction of the elastomer In addition to these two major vanables, the major determinants of vulcanizate behavior are the chemical and thermal stabilities of its cross-links The selection of elastomer, of course, places limits on the overall resistance to fluids and chemicals and on its service temperature range... 

In all these comparisons, the overall properties must be taken into account. Although fluoroelastomers have excellent compression set properties, the other poor physical properties and cost also must be considered. 

Peroxidic cure systems are applicable only to fluorocarbon elastomers with cure sites that can generate new stable bonds. Although peroxide-cured fluorocarbon elastomers have inferior heat resistance and compression set, compared with bisphenol cured types they develop excellent physical properties with little or no postcuring. Peroxide cured fluoroelastomers also provide superior resistance to steam, acids, and other aqueous solvents because they do not require metal oxide activators used in bisphenol cure systems. Their difficult processing was an obstacle to their wider use for years, but recent improvements in chemistry and polymerization are offering more opportunities for this class of elastomers [42]. 

Their physical properties, chemical stability, and resistance to temperatures above 135°C qualify these materials as competitors for sdicone rubber and fluoroelastomers. Applications include hose, tubing, gaskets, and protective covers for use in high-temperature environments. 

The fluorinated rubbers are exceptionally good for high-temperature service, but they are below silicones in this respect. They resist most of the lubricants, fuels, and hydraulic fluids encountered in aircraft a wide variety of chemicals, especially the corrosive variety and also most chlorinated solvents. They have good physical properties, somewhere near those of styrene butadiene rubber (SBR) at the higher hardness levels. FKM is valnable in automotive use for its extreme heat and oil resistance and is on a much higher level in this respect than the acrylic elastomers. It has weathering properties snperior to those of neoprene. However, fluoroelastomers are relatively expensive. 

The incorporation of nanoparticles into polymers has generated considerable excitement and activity since the interfacial effects at that scale can impart significant physical property improvements [18], The low surface energy of perfluoroplastics increases the difficulty of producing fully exfoliated nanocomposites. The incorporation is easier with partially fluorinated compounds such as polyvinylidene fluoride (PVDF) or fluoroelastomers [ 19], but even PTFE compounds have been produced for wire and cable applications [20],... 

Fluoropolymers are thermoplastic and nonrigid materials while fluoroelastomers have elastomeric properties. See Tables 2.1 and 2.2 for a comparison of physical and mechanical properties of select plastics and elastomers including fluorinated materials. 

The general physical and mechanical properties of fluoroelastomers are similar to those of other synthetic rubbers. Fluoroelastomer compounds have good tensile strengths, ranging from 188 to 2900 psi. In general, the tensile strength of any elastomer tends to decrease at elevated temperatures however, loss in tensile strength is much less with the fluoroelastomers. 

Fluoroelastomers are relatively impermeable to air and gases, ranking about midway between the best and the poorest elastomers in this respect. In all cases permeability increases rapidly with increasing temperature. This permeability can be modified considerably by the way they are compounded. Table 4.23 provides some data on the permeability as well as the physical and mechanical properties of the fiuoroelastomers. 

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