Fluoroelastomers curing with

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. 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

With the exception of implications regarding solubility, a feature not yet apparent is any recognized trend in the emissions from sulphur cures with variations in the base polymer. This is not the case with peroxide cures, where the reactivity of the polymer can influence both the quantity and type of emissions. A well-studied example is that of NR which carries an abundance of abstractable allylic hydrogens to favour alcohol formations (eqn (29)). Thus when DTOP (R = Me) is the peroxide, fert-butanol (BP 82°C) is obtained, whilst cumyl alcohol (2-phenyl-2-propanol BP 202°C) is obtained from Dicup (R = Ph). Ketone formation (eqn (30)) competes with hydrogen abstraction and can predominate in the presence of a different polymer emissions from formulations based on EPDM, silicone and a fluoroelastomer have been characterized. Other by-products include alkenes from alcohol dehydration, although numerous other reactions can occur. 

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 preferred way to cure fluoroelastomers is with Bisphenol AF and accelerator salts. They are usually incorporated in the commercial product but can be purchased separately (e.g., Viton Curative 20, 30, and 50) to adjust the cure state or cure rate. 

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

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. 

Generally, as discussed previously, the mechanism involved in the cross-linking of fluoroelastomers is the removal of hydrogen fluoride to generate a cure site that then reacts with diamine [39], bisphenol [40], or organic peroxides [41] that promote a radical cure by hydrogen or bromine extraction. Preferred amines have been blocked diamines such as hexamethylene carbamate (Diak No. 1) or bis(cinnamylidene) hexamethylene diamine (Diak No. 3). Preferred phenols are hydroquinone and the bisphenols such as 4,4 -isopro-pylidene bisphenol or the corresponding hexafluoro-derivative bisphenol AF. 

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]. 

This pe r discusses the chaniced reactions involved in the curing of fluoroelastomers with bis- henols and with peroxides. The mechanism of enuring with peroxides is based on cxir work where bromine was introduced in the polymer by cxjpolymeriza-tion. The experimental procedures and materials have been described in detail in previous publiceticxis (5,6c). 

Curing of Vinvlidene Fluoride Ba Fluoroelastomers with Bis-Phenols. We have Investigated the reactions of basic nucleo-... 

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