The usual commercial practice is to mix the rubber on a mill with the
diamine derivative, together with filler (such as carbon black or silica) and a
metal oxide. The compounded stock is then given a short press cure (typically, 0.5 hours at 150??C) and a long oven cure (typically, 24 hours at 200??C).
The reactions which occur in the vulcanization of vinylidene fluoridehexafluoropropylene copolymers have been extensively studied and a
three-stage process is thought to be involved. The first step is postulated as
the elimination of hydrogen fluoride from the polymer upon treatment with a
base. Probably the tertiary fluorine atom of the hexafluoropropylene unit is
the most readily removed. The initially formed double bonds then activate
the elimination of hydrogen fluoride from neighbouring groups to give a
conjugated system:
Elimination is thought to be catalysed by basic materials, such as magnesium oxide, which are commonly included in commercial formulations.
Elimination takes place rapidly and probably occurs during milling when a
diamine curing system is used. Evidence for the presence of unsaturation in
amine-treated vinylidene fluoride-hexafluoropropylene copolymer comes
from the infrared spectrum of the material.
The second step in the vulcanization process is regarded as involving
addition of the curing agent at the sites of unsaturation. With a diamine
This reaction occurs during the press cure operation.
The third step in the vulcanization process is considered to involve the
elimination of hydrogen fluoride to form a diimine:
If this is the case, it would be necessary to remove all the water present in
order to force the equilibrium completely toward diimine formation and the
establishment of permanent cross-links. Experimental evidence for this proposition is that a full state of cure is not attained if the long heating process is
carried out in the mould, i.e. in a virtually closed system. Hydrolysis is also
demonstrated by the fact that much of the diamine present in a vulcanizate
can be extracted by treatment with water. It may be noted here that
vinylidene fluoride-hexafluoropropylene copolymers which are peroxidecurable do not lead to water formation and so give vulcanizates with reduced
porosity.
Vulcanized vinylidene fluoride-hexafluoropropylene copolymers show excellent resistance to oils and fuels, both at room temperature and at elevated
temperature. Vulcanizates also have good resistance to most solvents, although polar solvents such as esters and ketones cause high swelling.
Vinylidene fluoride-hexafluoropropylene copolymer vulcanizates have excellent thermal stability, withstanding long periods at 250??C without serious
deterioration. Low temperature performance is limited and the elastomers
are not generally suitable for sub-zero use. Applications of the elastomers
include seals and hose in contact with fuels and lubricants, pump components
and tank linings.
A terpolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene is also available and gives vulcanizates with improved resistance to heat ageing.
