Fluoro-Rubber (FKM) Copolymers, terpolymers or
tetrapolymers with various composition and with fluorine contents from 65
to 71%, which thereby have varying resistance to surrounding media and
varying cold flexibility, can be made by polymerisation of vinylidene
fluoride (VF) and variable amounts of hexafluoropropylene (HFP),
tetrafluoroethylene (TFE), 1-hydropentafluoropropylene (HFPE) and
perfluoro(methylvinylether) (FMVE). Cross-linking is achieved either with
diamines and bisphenols or with organic peroxides. Materials based on FKM
are especially important due to their resistance to high temperatures and
their chemical stability. Their permeability to gases is low. FKM
elastomers display minimal weight loss in high vacuum. Resistance to ozone
and the crack resistance in the presence of light are very good, as is the
fire retardancy. Amines can react to destroy the material and necessitate
a choice of suitable types and special blends. One particular group of
materials is represented by copolymers of TFE and propylene having a
relatively small fluorine content (57%). Materials based on this elastomer
have excellent resistance to hot water and to steam, as well as to amines
or amine-containing media, but also have reduced resistance to swelling in
mineral oils.
Good resistance to swelling in: mineral oils and greases
(even with most additives), fuels, aliphatic and aromatic hydrocarbons,
some fire retardant hydraulic fluids and synthetic aero-engine oils. In
addition, newly developed materials which are cross-linked by peroxides
have good resistance to media which can only be tolerated to a small
extent, if at all, by conventional FKM. Such media are for instance
alcohols, hot water, steam, alcohol-containing alternative fuels, and SF
mineral oils.
High swell in: polar solvents and ketones, fire retardant
hydraulic fluids of the Skydrol type, and brake fluid, eg, ATE.
Temperature range for use: ca. -20°C/-4°F to +200°C/390°F (for short periods
to + 230°C/445°F). Special grades: -35°C/-31°F to +200°C/390°F. By suitable design, and
by using material compositions developed especially for such applications,
seals and mouldings can also be used at still lower temperatures.
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