Mailbag.

Dear editor:

During the nearly 25 years I have worked in the rubber industry, I have maintained a steady, albeit low profile, crusade to educate the uninformed regarding the true meanings of rubber nomenclature. Common terms such as SBR, NBR and EPDM are frequently misquoted. For example, EPDM is frequently referred to inaccurately as "ethylene-propylenediene-monomer."

The fact is that in 1955 a system for designating rubbers and latices was developed to replace the original government designations of GR-A, GR-I, GR-S and GR-T and is detailed in ASTM D1418, Standard Practice for Rubber and Rubber Latices-Nomen-clature. Under this practice, it is accurate that "SB," "NB" and "EP" indicate the base monomers in the polymer. The interesting point is in the final character of the designation. According to the ASTM standard, "rubber in both dry and latex form shall be classified and coded from the chemical composition of the polymer chain." The final character assigned to each abbreviation identifies this classification.

For example, a classification of "R" (as in SBR) indicates the rubber has an unsaturated carbon molecular chain containing carbon-carbon double bonds. If the unsaturation is not present in the chain, or if carbon atoms do not form the chain, another classification applies. A final character of "M" specifies a rubber having a saturated carbon chain of the polymethylene type. Thus, the term "EPDM" identifies a terpolymer of ethylene, propylene and a diene, with the unsaturated portions of the diene in a side (pendant) chain.

Under ASTM's standard practice for nomenclature, one can see that "EPDM" translates "ethylene-propylenediene-polymethylene chain." The correct designation for copolymers of ethylene and propylene is "EPM" rather than "EPR."

R.E. Benefield

DSM Elastomers Americas

Baton Rouge, LA