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EPDM is an abbreviation for Ethylene Propylene Diene Monomer, and is a combination of Ethylene (C 2 H 4 ), Propylene (C 3 H 6 ) and Butadiene (C 4 H 6 ). The ASTM D1418 designation for EPDM is EPDM, and it is used in a wide range of services where hydrocarbons are not present.


Buna-N is a copolymer of butadiene (C 4 H 6 ) and acrylonitrile (C 3 H 3 N). The butadiene is processed with natrium, which is an old pharmaceutical term for sodium. (In fact, the elemental symbol for Sodium is still "NA".) The name Buna is a combination of the first two letters of butadiene and natrium, and the -N suffix indicates the addition of nitrile. The ASTM D1418 designation for Buna-N is NBR, and it is commonly used for hydrocarbon service.


Fluorocarbon Elastomers are usually referred to by manufacturer's trade names such as Viton (Du Pont) or Fluorel (3M). These are copolymers of vinylidene fluoride (CF 2 -CH 2 ), hezafluoropropylene (CF 2 -CF), and tetrafluoroethylene (CF 2 -CF 2 ). The high fluorine content enhances the chemical resistance of these Elastomers similar to the manner fluorine helps the chemical resistance of fluoroplastic materials such as Teflon (Du Pont). The ASTM D1418 designation for fluorocarbon Elastomers is FKM, and they are commonly used in high temperature or corrosive chemical service.


In general, the materials used by the rubber compounders can be classified into nine major categories:

  • Elastomers
  • Processing aids
  • Vulcanizing agents
  • Accelerators
  • Accelerator activators
  • Age resistors
  • Fillers
  • Softeners
  • Miscellaneous ingredients



Sulfur is the most common curing agent, and creates both cross links and cyclical structures of the following type:


The S x represents a single sulfur molecule that is cross linked, or connected to two different rubber molecular chains. This type of cross link gives the rubber its elastic cured properties. The S y represents a single sulfur molecule that is cyclic, or connected to different parts of the same rubber molecule. The S z represents multiple sulfur molecules that are cross linked, or connected both to rubber molecules and other sulfur molecules. The amounts of cyclic and multiple sulfur in the cross links do not contribute elastic cured properties, and in most cases produce poor aging properties.


For most rubbers, one cross link for about each 200 monomer units is sufficient to produce a suitably cured product. In an efficient curing system, there are large numbers of single cross linked (S x ) groups, with little or no cyclic (s y ) or multiple cross linked (S z ) groups, and the final product is fully cured. In inefficient systems, there are many cyclic groups and the multiple cross linked groups can be up to 8 sulfur molecules long, and the final product is undercured.


If an undercured rubber is exposed to heat in actual field service, the sulfur in the cyclic groups and multiple cross linked groups can move to form new single cross links. This is basically a continuation of the curing process called post curing, and can cause substantial changes in the properties of the final product. Important factors (such as temperature and time) which are so carefully controlled when the product is molded, are left to random chance during a post cure in the field. The uncontrolled changes that occur during a post cure are almost always detrimental to product performance.

CURING EPDM WITH PEROXIDE (Stealth Valve method)


Organic peroxides are also used to vulcanize rubber. Eliminating the use of sulfur as a curing agent can provide several important advantages, such as lower compression set and less risk of post curing when exposed to heat.


The heat of vulcanization causes the peroxide to decompose, forming free radicals on the sides of long rubber molecule c chains. The free radicals on one rubber chain will combine with the free radicals on another rubber chain, forming a direct cross link with no intermediate molecule.


Direct cross links of this type involve only carbon-to-carbon bonds and are quite stable.


Since the peroxide decomposes in the curing process, there will be no peroxide left in the final rubber product to support post curing. In other words, heat may be added to a peroxide cured rubber part once the part is in field service, but since there is no uncured peroxide to form free radicals, additional cross links will not be formed.


Although all EPDM compounds absorb hydrocarbons and swell, peroxide cured EPDM has tighter molecular bonds than sulfur cured EPDM, and will consequently absorb hydrocarbons at a much slower rate. This decrease in absorption will not allow peroxide cured EPDM to be used in direct hydrocarbon service, but it will extend service life significantly in applications where only small amounts of hydrocarbons are present. A significant example exists in the brewing industry, where wort and mash contain small amounts of vegetable oils that will eventually swell EPDM seats no matter how they are cured, but where peroxide cured EPDM will last 2-4 times longer than sulfur cured EPDM.


The main disadvantage of using peroxide instead of sulfur as a curing agent is cost related. Organic peroxide material is 20-30 times more expensive than the rhombic yellow sulfur commonly used for vulcanizing. Even though only a relatively small amount of curing agent is used in each batch, a significant cost difference is still added to the final rubber product. However, the increase in seat life and quality make the addition of peroxide a sound investment.


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