Ethylene—propylene-diene monomer peroxide vulcanization

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

The accelerated-sulfur vulcanization of these rubbers along with the vulcanization of other rubbers, which are vulcanized by closely related technology, comprises more than 90% of all vulcanization. These rubbers include ethylene-propylene-diene-monomer rubber (EPDM), butyl rubber (HR), halobutyl rubbers, and nitrile rubber (NBR). Nevertheless, we give some consideration to vulcanization by the action of other vulcanization agents such as organic peroxides, phenolic curatives, and quinoid curatives. 

Since EPR rubber molecules do not contain unsaturation, they can be vulcanized only by organic peroxide curing systems. If a third monomer is added during the polymerization, i.e., a diene monomer (wherein only one of the two double bonds takes part in the polymerization), unsaturation can be introduced into the molecule, and it can then be vulcanized by accelerated sulfur curing systems. A chemical structure for ethylene-propylene-diene-monomer (EPDM) rubbers can be expressed as follows ... 

Because of their saturated structure, EPM rubbers cannot be vulcanized by using accelerated sulfur systems, and the less convenient vulcanization with free-radical generators (peroxide) is required. In contrast, EPDM rubbers are produced by polymerizing ethylene and propylene with a small amount (3-8%) of a diene monomer, which provides a cross-bnk site for accelerated vulcanization with sulfur. 

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. 

Like the butyls, there are basically two types of ethylene propylene. One is a fully saturated chemically inert copolymer of ethylene and propylene (EPR). The other, called EPDM, has the ethylene and the propylene, plus diene monomer. EPDM is chemically reactive and capable of sulfur vulcanization. The copolymer, EPR, is cured with a peroxide catalyst. 

In the ethylene-propylene copolymers described above there are virtually no double bonds consequently vulcanization by conventional techniques using sulphur is not possible and peroxides have to be used. This limitation may be overcome by introducing unsaturation into the polymer by use of a third monomer in the copolymerization process. The third monomer is a non-conjugated diene one of its double bonds enters into the polymerization... 

---