Fluoroelastomers vulcanization

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. 

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. 

These steps are typical for most of the synthetic elastomers. The use of sulfur for vulcanization is common for the production of most elastomers. Magnesium and zinc oxides are often used for the cross-linking of polychloroprene (CR). Saturated materials such as EPM and fluoroelastomers are cross-linked using typical organic cross-linking agents such as peroxides. 

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 PFAP(I) selected for this study is an amorphous fluoroelastomer that consists of pendant trifluoroethoxy and octafluoropentoxy groups (II). A small quantity of cross-link site was incorporated also to facilitate vulcanization via conventional methods, i.e., organic peroxides, sulfur-accelerator, and radiation (high-energy electrons). 

Other Types of Vulcanization. There are still other types of vulcanization systems based on other types of chemistry. These are applied to elastomers such as acrylates, fluoroelastomers, chlorosulfonylpolyethylene, and epichlorohydiin type elastomers. These are very specific curing systems. Some of them wiU be dealt with later sections of this chapter. 

Other comonomers such as vinylidenefluoride and chlorotrifluoroethylene are used, generally in smaller amounts. In addition, some fluoroelastomers incorporate bromine-containing curing-site monomers and can be vulcanized with peroxides. 

FKMs are coextruded with lower-cost (co)polymers such as ethylene acrylic copolymer. 1 They can be modified by blending and vulcanizing with other synthetic rubbers such as silicones, EPR and EPDM, epichlorohydrin, and nitriles. Fluoroelastomers are blended with modified NBR to obtain an intermediate performance/cost balance. These blends are useful for underhood applications in environments outside the engine temperature zone such as timing chain tensioner seals. 

After the dehydroliuorination of the poly(VDF-co-HFP) copolymer, the amine can add across the unsaturated center. Addition can be carried out with primary and secondary diamine, and less readily with primary and secondary monoamines. Vulcanization of VDF-based fluoroelastomers is induced by secondary and tertiary monoamines [20]. 

Once considerations involving chemical resistance and low-temperature flexibility have narrowed down the selection of fluoroelastomers, there are still quite a few parameters to consider such as finer aspects of chemical resistance, complexity of the part to make, and the forming and vulcanization processes envisioned to make the part. 

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