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ASTM
D1418 Designation: HNBR
ASTM
D 2000, SAE J200 Type / Class: DH
RELATIVE
COST: High
GENERAL
TEMPERATURE RANGE: -25° to +300° F
Though
the double bonds within nitrile’s butadiene
segments are needed for cross-linking, they are
also the predominant attack sites for heat, chemicals,
and oxidation. As part of an ongoing effort to
engineer more resistant compounds, a different
class of nitrile was developed in the 1980s. Initially
known as highly saturated nitrile (HSN), this class
is now more commonly called hydrogenated nitrile
butadiene rubber (HNBR), or just hydrogenated
nitrile (see Figure 36).
Hydrogenated
nitrile results from the hydrogenation of standard
nitrile. Hydrogenation is the process
of adding hydrogen atoms to the butadiene segments.
Adding hydrogen greatly reduces the number of carbon-to-carbon
double bonds that would otherwise be weak links
in the polymer chain. Why are double bonds weak?
It stems from valence, or the ability of an atom
to form one or more energy bonds with neighboring
atoms. A carbon atom can form four distinct covalent
bonds. Because carbon has this valence of four,
it is most “satisfied” when it has
actually formed four single bonds (a state known
as saturation) rather than two single bonds and
a double bond. A satisfied, saturated atom is more
stable, so a compound composed largely of saturated
carbons is less reactive and more resistant to
chemical attack.
As
shown in Figure 36, HNBR’s
main chain is primarily composed of highly saturated
hydrocarbons and acrylonitrile (ACN). Thanks to
their saturation, the hydrocarbon segments
impart heat,
chemical, and ozone resistance. On the down side,
increased hydrogenation also leads to decreased low
temperature elasticity. As with standard nitrile,
the ACN content of HNBR imparts toughness,
as well as fuel and oil resistance. This ACN content
can be modified for specific uses. There are also
a few remaining unsaturated butadiene segments
(typically well under 10%) to facilitate peroxide curing or,
in some instances, sulfur vulcanization.
Peroxide-cured HNBR has improved thermal properties
and will not continue to vulcanize like sulfur-cured
nitriles.
HNBR
is used in automotive
air conditioning systems where R134a refrigerant
gas has replaced the chlorofluorocarbon (CFC)-containing
R12 refrigerant. HNBR is used in fuel parts due
to its increased resistance to sour gasoline and
ozone. It is used in engine applications because
of its resistance to elevated temperatures and
oil additives.
HNBR
has excellent abrasion
resistance, making it a viable alternative
to fluoroelastomers
(FKM). HNBR also has better low temperature
properties and tear
resistance than FKM.
HNBR
PERFORMS WELL IN:
• Petroleum oils & fuels
• Silicone oils & greases
• Water & steam
(up to 300° F/149° C)
• Ozone
HNBR
DOES NOT PERFORM WELL IN:
• Chlorinated hydrocarbons
• Polar solvents
(esters & ketones)
• Strong acids
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