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Materials of Construction - Fluoro
Rubber
November 15, 2003
Description
| Common Name |
FKM, FPM, FFKM |
| Chemical Name |
Fluoro Rubber |
| Trade Names |
VITON® (registered
trademark of the DuPont Dow
Elastomers)
FLUOREL® (registered trademark of the 3M
Company) |
| General |
FKM is a M Class Rubber
- Contains a saturated chain of the polymethylene type
- Fluoro rubber of the polymethylene type having substituent
fluoro and perfluoroalkyl groups on the polymer chain
Fluorocarbon elastomers are inherently
compatible with a broad spectrum of chemicals. Because of this extensive chemical
compatibility, which spans considerable concentration and temperature ranges,
fluorocarbons have gained wide acceptance as a material of construction for butterfly
valve o-rings and seats.
Excellent resistance to mineral acids,
salts solutions, oils and many aliphatic and aromatic hydrocarbons.
Attacked by caustic (sodium hydroxide) and
low molecular weight organics.
There are several different grades of
material available. Care should be taken in selecting the right grade for the
application. |
| Application |
General
- Not suitable for steam service
- Gaskets
- O-rings
- Bellows
Sulphuric Acid Plants
- Seals and gaskets in the gas cleaning system handling weak sulphuric
acid.
- Plate heat exchanger gaskets
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Viton
DuPont produces several different grades of Viton. There are three major families of standard Viton Fluoroelastomers: A, B and
F. The designation refer to their relative
resistance to attacks by fluids and chemicals. The
differences in fluid resistance that exist are the result of different levels of fluorine
in the polymer which is determined by the types and relative amounts of monomers that
comprise the polymer.
| Type |
Chemical
Composition |
Fluorine
Content |
Description |
| A |
-CH2-CF2-CF3CFCF2- |
66% |
The most common
fluoroelastomer composition used in O-rings. Comprised
of vinylidene fluoride (VF2) and hexafluoropropylene (HFP). |
| B |
-CH2-CF2-CF3CFCF2-CF2-CF2- |
67% |
Improved
acid and alcohol resistance over A type. Comprised
of vinylidene fluoride (VF2), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). |
| GF or F |
|
70% |
Improved
fuel and alcohol resistance. Comprised of vinylidene fluoride (VF2),
hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). |
| GLT |
|
65% |
Often referred to as
“Good for Low Temperature”.
Good performance down to –40oC. Introduced in 1976, this type
was the first commercial fluoroelastomer to use the fluorinated vinyl ether monomer. |
| GFLT |
|
68% |
Often referred to as
“Good for Fluids and Low Temperature”. Good performance down to –35oC. |
| AFLAS |
-CF2-CF2-CH3CHCH2- |
55 to
60% |
Excellent resistance
to bases, amines and solvents. |
| Super FKM |
-CF2-CF2-CF3OCFCF2-CH2-CH2- |
72 to
74% |
Resistance to fluids
is increased with the higher fluorine content |
| Perfluoroelastomers
FFKM |
-CF2-CF2-CFOCFCF2- |
75% |
Family of
fully-fluorinated fluorocarbon elastomers |
The chemical make-up of
the polymer contributes to the differences in physical properties and chemical resistance
between the materials. As well, the
compounding method, particularly the cross-linking or curing system used imparts certain
physical characteristics to the material.
GLT types are peroxide
cured which provides fast cure rates and imparts excellent physical properties. As such, only the G type of fluoroelastomers are
suitable for service in sulphuric acid.
Viton B is the original grade of Viton used in plate heat
exchangers when they were first introduced into sulphuric acid plants.
Viton G is Alfa Laval's recommended material for hot
concentrated sulphuric acid. The material Vton G contains substantially more
fluorine than other grades which improves its resistance to hot concentrated sulphuric
acid. The gasket material is also peroxide cured which provides the lowest
temperature relaxation and the lowest acid absorption.
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Naming
With the wide range of
materials available a naming system was created to identify the material more accurately.
Viton A-401C
1st Character
(letter)
Represents the type of
viton: A, B or F
A “G” prefix,
in addition to a type prefix, indicates that the polymer can be crosslinked with the
peroxide cure system.
An “L”
designation indicates that the polymer provides slightly improved low temperature
flexibility.
An “LT”
designation indicates a more significant improvement in low temperature performance
criteria.
2nd Character
(Number)
Represents nominal
Mooney Viscosity of the product – ML 1 + 10 at 121oC (250oF)
3rd Character
(Number)
Represents the relative
level of curative in a precompound on a scale of 10 to 1 (10 is represented by 0)
0 – High curative
level (for optimum compression set)
9 to 2 –
Intermediate, decreasing levels of curative (increased elongation at break, tear
resistance)
1 – Low curative
level (for optimum tear, flex resistance)
4th Character
(Number)
Represents a slightly
different version of a particular compound (e.g. Viton A-402C contains process aid, A-401C
does not).
5th Character
(Letter)
No letter suffix
indicates that the product is a gum polymer only and containes no curatives (may contain
process aid).
“C” indicates
that the product is a precompound, containing accelerator and curative.
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