Peroxides fluoroelastomers

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. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

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. 

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]. 

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). 

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). 

Fluoroelastomer vulcanizate properties cure improved by oven post curing. This is true for diamine, bis-phenol, and peroxide cures. For example in Table 3 both bis- tenol and peroxide cured black stocks of a VFi/WE/HFP terpolyner show a 50% increase in MlOO and TB, a 50% drcrease in elongation at break, and a substanti d improvement in ccn ression set resistance. 

Uses Release coaling esp. designed for peroxide-cured fluoroelastomer and similar hard-lo-release compds. 

Precaution Violent or explosive reaction with chlorinated rubber (> 200 C), fluoroelastomers ( 200 C) incompat. with Cl, metals, nonmetals reacts violently with hydrogen peroxide, strong oxidizing agents... 

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. 

Fluoroelastomers are prepared by emulsion polymerization at elevated temperatures in the presence of peroxides as initiators. 

Fluoroelastomer dipolymer and terpolymer gums are amine- or bisphenol-cured and peroxide-cured for covulcanizable blends with other peroxide curable elastomers. They can contain cure accelerators for faster cures, and they are divided into three categories (1) gums with incorporated cures, (2) gums without incorporated cures, and (3) specialty master batches used with other fluoroelastomers. 

Viton GBL is a family of fluoroelastomer terpolymers, that is they are polymerized from three monomers, vinyl fluoride (VF2), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). Viton GBL uses peroxide cure chemistry that results in superior resistance to steam, acid, and aggressive engine oils. 

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. 

One other point noted in this chart is the performance of peroxide-cured fluoroelastomers. While one of the best overall peroxide... 

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. 

Of the different materials shown in Fig. 5 the most promising appears to be the TFE/propylene polymer Aflas. This is followed by hydrogenated nitrile (HNBR) and fluoroelastomer, which in this case was a conventional type. As we have seen, an improvement would be expected for a peroxide-cured version. 

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. 

Fluoroelastomers are now usually cured by nucleophiles such as diamines [17-31], or bisphenols [3,32-38], or with peroxides [3,35,39-43], by chemical reactions when the polymers based on VDF contain a cure-site monomer, such as a thiol function [44], or by radiation, such as an electron beam [45-52]. 

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