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Nitrile Rubber Applications
Nitrile rubbers material properties make it an excellent
solution for sealing applications. The popularity of nitrile
material results from its
excellent resistance to petroleum products and its ability to be
compounded for service of temperatures up to 250F. With these
temperature resistances, the right nitrile rubber compounds can
withstand all but the most severe automotive applications.
Other
applications that benefit from nitrile rubbers properties which
can be custom compounded and molded include:
Oil resistant applications Low temperature applications
Automotive, marine and aircraft fuel systems
Nitrile roll covers Hydraulic hoses Conveyer belting Nitrile
tubing
Nitrile Rubber Chemistry and Compounding
The production of nitrile material takes place through an
emulsion polymerization system. This emulsion process begins when
the
monomers (butadiene and acrylonitrile) are introduced along with
water, emulsifier/soap, radical generating activator and other
ingredients into polymerization vessels. The result of this
emulsion process is a polymer latex yield that is coagulated using
various
materials such as calcium chloride and aluminum sulfate to form
the crumb rubber that is then dried and compressed into bales.
Like most other unsaturated thermoset elastomers, Nitrile
compound formulations require added ingredients and further
processing to
be rendered useful in different applications. The additional
additives typically include plasticizers, reinforcement fillers,
protectants, and
vulcanization packages.
The differing amount of additive materials and levels of
acrylonitrile will determine the differences in the various
compounds that can be
formulated. Timco rubber has the capabilities to compound
nitrile rubber specifically to your individual needs. We are able
to vary
polymerization temperatures to create "hot" and "cold" polymers.
Also, by varying the Acrylonitrile and butadiene ratios we can
specifically formulate your nitrile compound for the necessary
fuel resistance and low temperature requirements. Nitrile
elastomer
compounds can also be hydrogenated in order to reduce the
chemical reactivity of the polymers backbone, thereby
significantly
improving the materials heat resistance.
Acrylonitrile Content
By adjusting the acrylonitrile (ACN) content in a nitrile
compound, a number of different results can be achieved. Because of
its polarity,the ACN level determines several key properties of the
nitrile material such as; oil and solvent resistance, abrasion
resistance, and also
low-temperature flexibility/glass transition temperature. A
higher concentration of ACN will provide improved solvent, abrasion
and oil
resistance along with a higher glass transition temperature. The
table below summarizes properties common to many conventional
NBR
polymers. The direction of the arrows signifies an improvement
in the values.
Nitrile Rubber with Lower
Acrylonitrile Content
Nitrile Rubber with Higher
Acrylonitrile Content
Processability
Cure Rate w/Sulfur Cure system
Oil/Fuel Resistance
Compatibility w/Polar Polymers
Air/Gas Impermeability
Tensile Strength
Abrasion Resistance
Heat-Aging
Cure Rate w/Peroxide Cure System
Compression Set
Resilience
Hysteresis Low Temperature Flexibility
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General Types of Nitrile Rubber
Hot Nitrile Rubber
Hot NBRs are polymerized at temperatures ranging from 30 to 40C
in a process that yields highly branched polymers. This
branching
supports good tack and strong bonding in adhesive applications.
By processing this kind of polymer and creating a physically
entangled
structure, the end product also exhibits a significant
improvement in hot tear strength when compared to a
cold-polymerized counterpart.
Hot NBR has a natural resistance to flow, making it an excellent
candidate for compression molding and sponge applications.
Other
applications where Hot Nitrile Rubber proves useful include thin
walled or complex extrusions where shape retention is
important.
Cold Nitrile Rubber
Depending on the balance of linear-to-branched configuration
desired, cold nitrile is polymerized at temperatures ranging from 5
to
15C. These lower polymerization temperatures will yield more
linear polymer chains. The reactions are conducted in processes
known
as continuous, semi-continuous, or batch polymerization.
Cold NBR compounds span a wide variety of compositions. They can
contain Acrylonitrile in ranges from 15% to 51%. The Mooney
values of these Nitrile materials range from a very tough 110 to
pourable liquids, with 20-25 as the lowest practical limit to
maintain a
solid material. These Nitrile compounds are made with an array
of chemical compositions, coagulants, molecular weight
modifiers,
stabilizers and emulsifier systems. Third monomers can also be
added to the polymer background to provide advanced
performance;
with each variation of the compound providing a specific
function to the overall Nitrile material.
Crosslinked Hot Nitrile Rubber
Crosslinked Hot NBRs are branched polymers that become further
cross-linked through the addition of a di-functional monomer.
These
nitrile rubber compounds are typically used in molded parts to
provide sufficient molding forces, or back pressure, in order to
eliminate
any trapped air. An additional use is to provide increased shape
retention or dimensional stability for extrusion. As a result,
crosslinked
hot nitrile proves to be more efficient in the extrusion and
vulcanization of intricate shaped parts. These NBRs will also add
impact
resistance, dimensional stability and flexibility for PVC
modification.
