FFKM: the 'designer' rubber
DP Seals technical director Andrew Piper provides this technical
insight into FFKM and its advantages for sealing applications
Every so often, a material moves from the realms of esoteric and
expensive into the commercially viable; FFKM is one that is making this
transition. Andrew Piper, Technical Director at DP Seals, provides an
insight into this esoteric designer rubber.
FFKM - fully-fluorinated perfluoroelastomer - is not new, it has been
around for some 40 years. However, it is only recently that this high
performance rubber compound has moved from the high cost, low volume
oilfield exploration and aerospace sectors into markets where it needs to
be more commercially-competitive.
The appeal of this material to an engineer is its exceptional combination
of temperature and chemical performance. It has found application in a
wide variety of very demanding applications, from the harsh down-hole
conditions of well drilling to the ultra-pure environments demanded by
semiconductor manufacturing. To appreciate why FFKM offers a performance
envelope beyond that of most other rubbers used in seals, gaskets and
mouldings, it is necessary to understand its pedigree and chemistry.
Pedigree
Fluoroelastomers, commonly referred to as FKM or by the trade name Viton,
have been around for years. Initially formulated for use in aerospace
applications, FKM's exceptional resistance to chemicals and oil, its low
gas permeability and resistance to climatic factors such has ozone and
sunlight extended its application into areas such as automotive
engineering, domestic appliances, fluid power and chemical processing.
Three factors contribute to this performance:
? The bonds between the carbon atoms in the FKM polymer backbone and the
attached fluorine atoms are extremely strong, which means the chain will
not break down into smaller segments (scission),
? With fluorine atoms rather than hydrogen atoms fulfilling the majority
of the available bonds, FKM is extremely stable and hence less inclined
to react with, or be broken down by, the environment,
? FKM has a fully saturated backbone with no covalent double bonds,
rendering it impervious to chemical attack by harmful agents like oxygen,
ozone and ultraviolet light.
However, FKM offers limited resistance to acids, alkalis and amines,
making it unsuitable for some environments. For example, in many
oil-field applications, corrosion inhibitors are amine based which will
cause FKM to harden, decreasing its tensile strength and causing surface
cracks to form which can develop into leak paths. These effects are
exacerbated under high temperature conditions, reducing FKM's attraction
to modern, high performance engine developers. The higher compression
ratios and faster rotational speeds of today's engines mean fuels and
lubricants are at significantly higher temperatures, shortening the
operating life of many seals and gaskets.
Chemistry
Standard FKM is a copolymer of vinyldene fluoride and
hexafluoropropylene. Many compounds of FKM have been developed in the
drive to produce a more base resistant fluoroelastomers. These FKM
compounds typically add tetrafluoroethylene - or replace the vinyldene
fluoride with tetrafluoroethylene - increasing the percentage of fluorine
atoms in the final material, hence improving overall performance. In
almost all cases, however, any improvement in base resistance is at the
expense of temperature performance or chemical resistance. It was not
until rubber chemists achieved a fully-fluorinated polymer - FFKM or
perfluoroelastomer - that these problems were overcome.
Today's commercial FFKM is a terpolymer of tetrafluoroethylene,
perfluoromethylvinylether, and a cure site monomer. Using these three
fully-fluorinated monomers creates the extremely strong carbon=fluorine
bonding that ensures a virtually unbreakable chemical structure,
providing superior chemical resistance while adding resistance to harsh
chemicals, such as the acid and alkali bases.
FFKM has now found applications n a wide variety of industries:
? Petrochemical - mechanical seals, pumps and pipe work in all kinds of
chemical processing plant
? Semiconductor - manufacturing equipment, chemical delivery/recovery
pipe-work, component transport, analytic/process instrumentation
? Automotive - engine gas sensors and similar near-engine components
? Energy - down-hole electronics during oil and natural gas exploration,
pipelines and valves
? Aerospace - jet and rocket engine components
At DP Seals, we have delivered FFKM parts to oilfield, pipeline and
automotive customers' specifications, enabling us to get to grips with
this material.
If it's so good...
...why is it so difficult to get? Although FFKM has an impressive
performance envelope, it is not perfect. For example, compression set
increases with temperature. A material's compression set is expressed as
the percentage of deformation (held for 22 hours at 70°C) that does not
return to the original height; the smaller the number, the better. Early
perfluoroelastomers made use of bisphenol cross-links but this caused a
high degree of compression set.
FFKM is available from different pre-compounding suppliers in a variety
of grades, each tailored to suit the temperature range and chemical
resistance needed for the target application. DP Seals creates its custom
mouldings primarily from peroxide cross-linked FFKM which gives less
compression set and higher strength at lower cost. Importantly, these
compounds also offer benefits to DP Seals, particularly as no hydrogen
fluoride gas is produced during the moulding process - although it can
occur during high temperature post curing.
Other problems include:
? gland volume - the space allowed in the design to retain the seal - may
need to be increased to allow for expansion at higher temperatures
? it can swell from contact with uranium hexafluoride, fully halogenated
Freon and some fluorinated solvents, reducing its usage in nuclear,
refrigeration and some chemical engineering processes
? it is expensive and processing is difficult.
These last points are why most rubber moulding manufacturers do not offer
FFKM as one of the materials they provide. At DP Seals we have overcome
these processing problems and appear to be highly competitive on price
compared with our competition. FFKM is transitioning from the esoteric to
the almost ordinary, finding increasing application across the oil, gas,
petrochemical, pharmaceutical and semiconductor industries. Although it
is still a difficult material to mould, some rubber moulders have started
to master FFKM's production parameters and offer parts at more
competitive prices. DP Seals is at the forefront of providing custom
mouldings in this exciting material.
About DP Seals
DP Seals continues to set new quality standards in the design and
manufacture of standard and custom rubber seals and mouldings for
multiple applications in such diverse industries as aerospace, brewing,
automotive and electronics.