Chemical Engineering June 2018 - 37

Facts At Your Fingertips
Seal material properties
Department Editor: Scott Jenkins
luid seals for chemical process
equipment are commonly made
from a host of elastomeric and
plastic materials. Selecting a seal material
requires careful consideration of
several criteria. Presented here are
short descriptions of the major criteria
for seal selection and a table outlining
the properties of several of the most
common seal materials.
F
Seal selection criteria
There are many possible factors that
may play a role in which seal materials
would perform best in a given application,
but here are four major ones.
Temperature capabilities. Each seal
has a range of temperatures within
which it is designed to be used. Near
the service limit for each seal, the
performance becomes less certain.
At low temperatures, elastomers become
harder and less pliable. Also,
elastomer seals lose their rubber-like
properties with decreasing temperatures.
The changes experienced by
elastomer seals at low temperatures
are physical changes, and are generally
reversible.
Fluid compatibility. Seals must be
chemically compatible with the fluids
they will encounter in the process.
Significant swelling and rapid deterioration
can occur if not. In addition,
factors such as concentration, operating
pressure and temperatures,
seal geometry and design, must
be taken into account. Chemicalresistance
guides, which are offered
by most seal manufacturers, can be
good resources.
Resistance to abrasion and tearing.
Depending on the requirements,
seals may be required to resist scraping
and tearing, as well as avoid small
nicks and cuts, which can lead to
seal failure. Compounding elastomers
with internal lubricants can enhance
abrasion resistance.
Ability to accommodate differential
pressure. Many seals will experience
pressure differentials between
the process interior and the external
environment, so they should resist extrusion.
Harder materials are generally
more resistant to differential pressure.
36
Seal material class Description
Acrylonitrile butadiene
rubber
(Nitrile rubber; NBR;
Buna-N)
Hydrogenated
nitrile butadiene
rubber (HNBR)
TABLE 1. POPULAR SEAL MATERIALS AND APPLICATIONS
Properties
Copolymer of 2-propene
nitrile and butadiene
Copolymer of acrylonitrile
and butadiene, similar to
NBR but with subsequent
dehydrogenation of the
carbon-carbon double bond
of butadiene in polymer
backbone
Ethylene propylene
diene monomer
(EPDM)
Fluoroelastomers
(FKM)
Perfluoroelastomers
(FFKM)
Silicone rubbers
Copolymer of ethylene and
propylene, with a small
amount of a diene monomer
to facilitate cross-linking
A class of copolymers consisting
of vinylidene fluoride,
hexafluoropropylene and
other monomers
Similar to FKM, but higher
fluorine content
Variations of polysiloxane,
an inorganic rubber with a
silicon-oxygen backbone
and methyl groups or other
groups attached to Si atoms
Fluorosilicone
rubbers
Chloroprene rubber
(tradename:
Neoprene)
TFE/P
(tradename: Aflas)
Polytetrafluoroethylene
(PTFE)
(tradename: Teflon)
Similar to silicone rubbers,
but the functional groups
attached to Si include trifluoropropyl
groups
Polymer of 2-chlorobutadiene
Co-polymer
of tetrafluoroethylene
and propylene
Synthetic polymer of tetrafluoroethylene
Polyurethane
rubber Elastomeric urethane rubber
(not to be confused with
thermoplastic polyurethane)
is either ether- (EU) or ester(AU)
based
Polyacrylic rubber
(ACM)
Butyl rubber
A family of polymers with
blends of ethyl and butyl
acrylate repeat units. Other
monomers may be included
to adjust properties
Copolymer of isobutylene
and isoprene
Cost. Although costs can vary widely
depending on compounding and
processing, relative prices begin with
nitrile and chloroprene as the least expensive,
followed by EPDM and silicone.
Polyacrylate, butyl rubber and
HNBR are the next most expensive,
then the fluorocarbons, TFE/P and finally
fluorosilicones, which are at perGood
abrasion resistance
Grades with high acrylonitrile content have higher resistance
to petroleum-based fluids
Not for use with polar solvents
Hydrogenation eliminates routes of degradation that
can occur in conventional nitrile rubbers
HNBR outperforms conventional nitrile rubber with
respect to thermal resistance and the ability to handle
sour crude oil and other materials
Compatible with polar fluids
Not for use with petroleum-based fluids
Good thermal resistance
Suitable for high temperatures (up to 250˚C), high
pressures and harsh chemical conditions
More costly than EPDM and nitrile rubbers
Not recommended for ketones and amines
Higher fluorine content improves temperature resistance
(up to 325˚C) compared to FKM
Broadest chemical resistance of any rubber class
Good temperature resistance
Good resistance to ozone, ultraviolet radiation and
weathering
Poor abrasion resistance
Not for use with petroleum-based fluids and ketones
Good low-temperature flexibility
Good low-temperature flexibility
High-temperature stability
Cost is higher than conventional silicone rubbers
Good balance of chemical and physical properties at a
relatively lower price
Not for use with aromatic and oxygenated solvents
Excellent chemical resistance to a range of aggressive
chemicals
Not recommended for aromatic fuels and ketones
Large temperature range in sealing applications
Low coefficient of friction for dynamic seals
Broad chemical resistance
Excellent abrasion resistance and tear strength
Ester-based compound has better thermal- and abrasion
resistance, while the ether-based polymer has
better flexibility at low temperatures
Good resistance to weathering, ozone and UV light
High thermal resistance (similar to fluorosilicones at
lower cost)
Poor resistance to hot water
Good thermal stability and low gas-permeability
Not suitable for petroleum-based fluids
Good vibration- and shock-dampening abilities
haps 15 times the relative cost of the
least expensive materials.
n
References
1. Parco, Inc., Elastomer Selection Guide, Parco, 1999.
2. Rogers Corp., Technical Sealing Guide, Rogers, 2012.
3. Chemical Retrieval on the Web, Polymer Properties Database,
www.polymerdatabase.com, accessed May 2018.
4. Technical materials guide, www.allsealsinc.com, accessed
May 2018.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2018
http://www.polymerdatabase.com http://www.allsealsinc.com http://WWW.CHEMENGONLINE.COM
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