Nitrile NBR rubber
Nitrile NBR
To the chemist this rubber is known as acrylonitrile butadiene rubber, to others it is
called Buna-N but to many people in the industry, simply, nitrile. It is the workhorse of
the marketplace for its oil resistant properties. The grades offered differ in the percentage
of acrylonitrile (ACN) in the polymer chain as well as the overall viscosity of the polymer.
The higher the amount of ACN in the elastomer the better the oil resistance; the lower
end of the ACN distribution range being approximately equivalent to the oil resistance
of CR and therefore only having a moderate level of oil resistance. NBR also has superior
fuel resistance. The terms oil and fuel used here refers loosely to those products derived
from petroleum. The weather resistance of NBR is poor, similar to NR and SBR, although
it can be enhanced by blending with the plastic, polyvinyl chloride (PVC), at some 'cost'
to its low temperature properties. This latter attribute of NBR also varies with ACN
content; the lower the percentage of ACN in the polymer, the better the low temperature
flexibility, and the poorer the oil resistance. A compound which has a nitrile raw gum
elastomer in it, with a medium (33%) ACN content, would have good oil resistance and
low temperature resistance down to the region of -40 "C. A low ACN (18%) nitrile
would be useful down to -55 "C. NBR has better heat aging resistance than CR and is in
the region of 107 "C for continuous (defined approximately as 1,000 hours) use. Special
compounding ingredients can be added to increase heat aging resistance. Like SBR, NBR
needs reinforcing fillers to give good mechanical properties.
NBR use is dominant in the oilfield, used in blow out preventors, packers and seals.
However, sour gas wells containing hydrogen sulfide and amine corrosion inhibitors
have been a problem for NBR based components because both chemicals can degrade
nitrile elastomers causing embrittlement. The other major use for NBR is in the automotive
sector. However, as 'under the hood' temperatures increase with reduced airflow and
smaller engine compartments, NBR producers are searching for ways of increasing this
elastomer's upper region of heat aging resistance. The term sour gas is also used in
automotive applications, but here it means hydroperoxides (rather than hydrogen sulfide),
which sometimes form in unleaded gasoline; this can be damaging to the 'average' NBR.
Very high ACN NBR or NBWPVC might be used in this situation.
The use of alcohol (methanol and ethanol) in gasoline, the so called oxygenated fuels,
has made NBR processors look very closely at the effect of such a mixture on this elastomer;
since alcohol concentrations at a certain level (around 10%) could be a problem. The
alcohol causes NBR in contact with the gasoline alcohol blend to swell significantly.
Blends above 5% should be treated with caution unless a resistant rubber type is used. It
is worth noting that many applications of NBR exclude contact with oxygen (from the
surrounding air), since the product is immersed in oil. This can increase the heat aging
resistance temperature mentioned above. NBR is a distinctly polar rubber.

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