Organic peroxides, vulcanization elastomers

For organic peroxide vulcanization, a variety of organic peroxides have been shown to be effective [7]. Such peroxides would include di-t-butyl peroxide dicumyl peroxide t-butyl cumyl peroxide l,l-di(t-butylperoxy)-3,3,5-trimeihyl cyclohexane 2,5-dimethyl-2,5-di(r-butylperoxy)hexane 2,5-dimethyl-2,5-di(r-butylper-oxy)hexyne-3 a,a-bis(t-butylperoxy)diisopropylbenzene t-butyl perbenzoate and t-butylperoxy isopropylcarbonate. In contrast to NBR elastomers where 1-3 phr of peroxide is effective, HNBRs generally require peroxide levels of 5-8 phr for effective vulcanization. As discussed earher, this is due to the very low levels of unsaturation present in the HNBR polymers. The specific peroxide chosen will depend on the process safety desired and the vulcanization temperature to be used. Generally, the most common organic peroxides used in HNBR elastomers are Dicumyl peroxide, such as Varox Dicup 40C from R.T. Vanderbilt, on an inert carrier for vulcanization below 177°C and a,a-bis(t-butylperoxy)diisopropylbenzene, such as Varox VC40KE from Vanderbilt, on an inert carrier for vulcanization above 150°C. 

As EPR and EPDM elastomers, the vulcanization process is carried out using organic peroxides because the polymer chains do not contain unsaturated bonds. If the polydimethylsiloxane chains are modified by introducing a small quantity of vinyl groups, the vulcanization is carried out using cumyl peroxide. 

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

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. 

Unmodified polyethylene cannot be converted into an elastomer because of its molecular crystallinity. Chlorination of the polyethylene polymer disrupts the crystallinity and allows the polymer to become elastic upon vulcanization. CM can be vulcanized by peroxides or certain nitrogen-containing organic compounds that cross-link the chlorine atoms. 

Most rubber is based on polymers of isoprene or butadiene and contains many reactive C=C double bonds available for cross-linking. It is cross-linked by sulfur, aided by metal oxides and organic catalysts, producing sulfide cross-links between the polymer chains. Ethylene-propylene rubber is mostly made with several percent of diene ter-monomer to inttoduce C=C double bonds, which can then be vulcanized in the same way. Similarly, butyl rubber is made with a few percent of isoprene comonomer to introduce C=C double bonds and permit sulfur vulcanization. Even saturated elastomers are sometimes cured by sulfur, using peroxides and catalysts to activate C-H bonds, and metal oxides to attack C-Cl bonds. 

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