Amine-resistant fluoroelastomers

Applications of fluoropolymers are still growing, even decades after the discovery of the first plastic (polytetrafluoroethylene) in this family. The increasing use of fluoropolymers in such dynamic industries as wire and cable insulation, automotive, aerospace, oil and gas recovery, and semiconductor manufacture has led to significant material developments and trends in the last few years. New fluoropolymers have been introduced to the market (amorphous fluoroplastics, modified PTFE, low-temperature fluoroelastomers, and amine-resistant fluo-... 

Fluoroelastomers excel compared to aU other elastomers in heat, chemical, flame, weathering, fuel, and o2one resistance. In addition oU, oxygen, and water resistance ate very good. The fluoroelastomers, however, ate attacked by amines and some highly polar solvents. The abrasion resistance and low temperature properties ate adequate for most appHcations. 

A fluoroelastomer manufactured by Du Pont called Kalrez, has mechanical properties and resistance to oxidants which are similar to those of Viton. In contrast with Viton, Kalrez has good resistance to polar molecules such as amines, ethers, ketones, and esters. Kalrez is unique among the elastomers in its tolerance to both polar and nonpolar solvents. The cost of O-rings made from Kalrez is very high, but for certain critical applications this cost can be justified because of the outstanding range of solvent tolerance. 

Swelling resistance of fluoroelastomers is directly related to the fluorine content in the molecule. This is demonstrated by data in Table 5.3 [9]. For example, when the fluorine content is increased by mere 6% (from 65% to 71%), the volume swelling in benzene drops from 20% to 3%. Copolymers of VDF and HFP have excellent resistance to oils, fuels, and aliphatic and aromatic hydrocarbons, but they exhibit a relatively high swelling in low-molecular-weight esters, ketones, and amines, which is due to the presence of the VDF in their structure [10]. VDF-based fluoroelastomers... 

Peroxide-cured fluoroelastomers of this type are claimed to exhibit a superior resistance to steam, hot water and mineral acids than amine and bisphenol cured systems as well as providing certain advantages in the curing of thick sections. 

Fluoroelastomers are increasingly in demand to meet California Air Resources Board (CARB) requirements by 2004, which limit evaporative hydrocarbon emissions. Dy-neon fluoroelastomers, with low permeation number in alcohols and reformulated gaso-hnes, are used in the fuel delivery system, fuel lines, vapor hnes, O-rings, and custom seals. Dyneon BRE base-resistant elastomers are formulated for resistance to amine additives in automatic transmission fluids (ATF), engine oils, and gear lube oils— in addition to properties in other Dyneon fluoroelastomers including high temperature and chemical resistance. ... 

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]. 

Since about half of the applications for fluoroelastomers are O-rings and gaskets, compression set resistance is a key property compounder that typically aim for a value of 20% after 70 h at 200°C for a 75 Shore A formulation. For higher fluorine types, compression set resistance is usually a bit worse, in the 30%-40% range. The major factor for optimizing compression set is the selection of the type and level of curative. Amine curatives are the least effective and led to the development of bisphenol curatives. Until recently, peroxide curatives could not provide the excellent compression set resistance offered by bisphenol cure systems. However, recent advances in development of new cure site monomers have addressed these issues, even offering the capability to reduce the post-cure cycles to only 1 h [15-18]. 

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