Fluoroelastomers peroxide curing systems

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]. 

A useful paper by Watkins has confirmed some of the data given in the previous section and provides information on other elastomers. Table 9 gives some additional data on the difference between a fluoroelastomer cured with peroxide and one cured by a conventional bisphenol system. The data confirm the Du Pont results, i.e. that a peroxide cure system gives better results. 

Peroxide curing systems can be used only with those fluoroelastomers containing a peroxide cure site, for example the low temperature, PMVE containing types, the highly fluorinated types, and other specialty type such as BR 9151 or ETP. Peroxides such as Luperco lOlXL are preferred, in conjunction with TAIC coagent DIAK 7. 

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. 

All these results suppose a difference in the crosslinking mechanism of bisphenol, peroxide and diamine cured systems, that are the most important crossUnking agents for VDF-based fluoroelastomers. The crosslinking mechanisms and the properties of the resulting crosslinked polymers are the subject of the following parts of this article. 

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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