A Review of Recent Progress in the Design and ... - Neue Verpackung

ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
Fluoroelastomers Base resistance
Competitive dehydrofluorination
F-19 NMR 3,3,3-trifluoropropene
Increasingly basic formulations of
automotive lubricants have required
that new heat- and oil- resistant elastomers
be developed. The reactivity of
VF2-based elastomers in solution is
examined in an NMR-monitored study
and compared to that of VF2-free
fluoroelastomers. It was found that
that a VF2-free elastomer based on TFE
and P that contains curesite levels of
3,3,3-trifluoropropene responds very
well to basic vulcanization with bisphenols
and yet affords outstanding
resistance toward base in service and in
competitive dehydrofluorination
studies in solution. This desirable
behavior is attributed to the high reactivity
of the curesite monomer and
to its limited concentration.
Ein Fortschrittsbericht zum
Aufbau und den Reaktionen
Laugenstabiler Fluorelastomere
Fluorelastomere Laugenstabilität
Dehydrofluorierung F-19 NMR
3,3,3-Trifluorpropen
Die zunehmend basische Rezepturierung
von Schmierstoffen im Automobilbereich
erfordert die Entwicklung
neuer hitze- und ölbeständiger
Elastomere. Die Reaktivität
von Flurelastomeren die auf VF2-
Basis hergestellt wurden, ist vergleichend
mit Hilfe von NMR untersucht
worden. Es wurde gefunden,
dass VF2 – freie Elastomere die
TFE und TFP enthalten sehr gut mit
basischen Bis – Phenolen vernetzt
werden können und zudem einen
außergewöhlichen Widerstand gegen
Basen im Gebrauch oder gegen
Dehydrofluorierung in Lösung aufweisen.
Dieses erwünschte Eigenschaftsbild
wird der hohen Reaktivität
des Curesite-Monomeren und
seiner geringen Konzentration zugeschrieben.
A Review of Recent Progress in
the Design and Reactions of
Base-Resistant Fluoroelastomers
The ease of proton abstraction from carbon-hydrogen
compounds that are partially
fluorinated is a specific function of structure.
Poly(vinylidene fluoride) (VF2) and
isomeric, alternating tetrafluoroethylene
(TFE)/ethylene (E) are opposites in terms
of polarity, solubility and base-resistance.
The homopolymer is highly reactive and
soluble in donor solvents like acetone,
while the copolymer is essentially inert to
bases and only soluble in fluorocarbons.
Elastomeric examples of such reactivity differences
are the family of VF2-based copolymers
of hexafluoropropylene (HFP) or
perfluoro(methyl vinyl ether) (PMVE), or
their terpolymers with TFE, and the family
of VF2-free copolymers of TFE and propylene
(P) or terpolymers of TFE, PMVE, and
E. Clearly, fluorine content alone is a poor
predictor of base resistance. Despite their
long demonstrated outstanding heat and
oil resistance generally, and good acid resistance
when crosslinked by peroxides,
VF2-based fluoroelastomers are intrinsically
highly reactive toward soluble bases. It is
an inescapable fact that amorphous fluoroelastomers
of the VF2/HFP/TFE and
VF2/PMVE/TFE families can not be made
resistant to gross dehydrofluorination
and subsequent embrittlement due to
overcrosslinking on contact with strong
nucleophilic bases. This is a consequence
of the high VF2 levels required to achieve
acceptably low glass transition temperatures
and the high levels of HFP or PMVE
required to prevent VF2, or mixed VF2/
TFE, crystallinity. Even tetrapolymers that
use E to partially replace VF2 are not
base-resistant but merely possess a somewhat
reduced, yet still too-high, concentration
of highly base-sensitive Rf – VF2 –
Rf sequences. It should not be surprising
that elastomers that can be crosslinked
in less than one minute at 200C and
have a large excess of such Rf – VF2 – Rf
curesites due to morphological requirements,
are not able to withstand the attack
of similarly basic species in service. Instead,
the hydrogen content of affordable baseresistant
fluoroelastomers required for flexibility
in low temperature service must be
provided by hydrocarbon olefins like E or P.
Such structures then confer the inertness
toward _bases that characterize TFE/E rather
than poly(VF2). Elastomeric compositions
that possess this superior level of
base-resistance are TFE/P/curesite monomer
and TFE/PMVE/E/curesite monomer.
Development of
Fluoroelastomers
The early development of fluoroelastomers
is described in a general reference [1]. Research
in the mid 1950s by DuPont [2] and
M. W. Kellogg [3] (later acquired by 3M)
led to a major advance in the thermal stability
and oil resistance of elastomers useful
for sealing applications under severe conditions.
Attempts to cure these early compositions
of VF2/HFP [4] and VF2/CTFE [3]
showed that they were unresponsive to
peroxides but reactive toward soluble bases.
Initial curing systems for VF2/HFP
used tertiary amines as dehydrofluorinating
agents and dithiols as bisnucleophiles.
Primary diamines were found to be reasonably
scorch resistant curatives when used
in the form of their carbamate or Schiff’s
base derivatives in the presence of inorganic
acid acceptors such as MgO and
Ca(OH) 2 . Commercial curatives were
developed from the carbamates of ethylene
diamine and 1,6-hexamethylenediamine
and the bis-cinnamaldehyde condensation
product of the hexamethylene
diamine [4]. To further increase fluorine
content and raise fluids resistance, DuPont
developed and commercialized TFE-contai-
W. W. Schmiegel, Wilmington (USA)
Corresonding author:
Walter W. Schmiegel
DuPont Dow Elastomers LLC
DuPont Experimental Station
Wilmington, DE 19800-0293 USA
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004 313

ning terpolymers of VF2/HFP soon after the
dipolymers were commercialized [5]. Barred
from using HFP, Montecatini Edison
entered the fluoroelastomer field with
VF2 copolymers of 1-H-pentafluoropropene
[6, 7]. However, these were less thermally
stable and solvent resistant than
corresponding HFP dipolymers and terpolymers
and were subsequently replaced by
HFP analogs upon expiration of original
patents [8].
A major advance in curing chemistry was
made in the late 1960s when it was found
that, in the presence or inorganic acid acceptors,
alkali salts of bisphenols could be
used as bisnucleophilic crosslinking agents
in the presence of a non-ionic phase transfer
catalyst (18-crown-6) [9] and ultimately
as free bisphenols with quaternary ammonium
and phosphonium salts as phase
transfer catalysts [10]. The change from aliphatic
primary diamines to bisphenol
anions as the active crosslinking species resulted
in vast improvements in processing
safety and high temperature compression
set resistance, as well as in major improvements
in the stability of tensile properties
on heat aging, and in greatly improved hydrolytic
stability. Bis-phenyl ether crosslinks,
even if linking unsaturated sites of
the fluoroelastomer, are far more stable
than bis-imine crosslinks that are sensitive
to electrophilic oxidants and that can also
regenerate the active crosslinking agent on
hydrolysis when exposed to steam or boiling
water for extended periods. The diamine
networks are therefore relatively unstable,
undergoing a ’marching modulus’
process on heat aging in ambient air and
suffering from a loss of crosslink density
on exposure to low pressure steam. Even
bisphenol-cured vulcanizates are considerably
softened by the swelling of inorganic
bases in contact with steam or aqueous
acids and the best curing systems for resistance
to such exposures are based on peroxides
and radical traps when used with
fluoroelastomers that contain a bromine
or iodine cure site. This is not necessarily
the result of a more stable crosslink and
a saturated backbone, however. Instead,
it appears also to be a strong function of
the reduced level and basicity of the inorganic
acid acceptors that are required for
peroxidic vulcanization, as for example
the replacement of Ca(OH) 2 or MgO by
ZnO. Indeed, addition of the same high levels
of acid acceptors commonly used for
bisphenol cures confers a similarly high degree
of swelling when added to a peroxide
curing fomulation.
Another advance in the curing chemistry of
VF2/HFP/(TFE) elastomers was the introduction
of “prereacted” curatives that
are organic salts of the phase-transfer
agent and the bisphenol. It was found
that the elimination of chloride as the
counter ion of the quaternary phosphonium
cation and its replacement by the
mono-anion of the bisphenol afforded
better processing compounds, especially
with respect to mold fouling and sticking.
This has been attributed to the reduced
steel corrosion.
Peroxide cures were developed by DuPont
[11] in the early 1970s and commercialized
in 1977. These systems generally require a
bromine-containing monomer, although
more recent polymers rely on iodine introduced
in the form of either curesite monomers
or chain transfer agents. Among VF2-
based elastomers, bromine or iodine-containing
polymers that are crosslinked by
peroxides and radical traps represent the
upper limit of base resistance. Daikin entered
this market with conventional products
in the early 1970s and in 1978 patented an
elegant living radical emulsion polymerization
process [12] that produces highly monodisperse
terminal di-iodide polymers
that are crosslinked by peroxides using a
conventional polyfunctional radical trap.
To increase base resistance further and
maintain the fluorine content for excellent
oil resistance has not been possible with
elastomers based on VF2/HFP/(TFE). The
emerging markets for seals that will tolerate
the increasingly alkaline formulations of
automotive engine and axle lubricants required
that truly base-resistant elastomers
be developed. In this connection it must be
remembered that an engineer’s first priority
in designing a long running internal
combustion engine is to optimize lubrication
and minimize steel corrosion from
acidic oxides of sulfur and nitrogen in
cylinder blow-by gases. This is why some
modern high performance lubricants
contain as little as two thirds petroleum
and large amounts of additives, most of
them quite basic.
Early fluoropolymer research at DuPont
had already indicated by 1949 that elastomeric
compositions of TFE could be obtained
by copolymerization with hydrocarbon
alpha-olefins [13], but these experimental
elastomers attracted little attention after
more highly fluorinated VF2-based elastomers
were developed. The TFE/P composition
again gained attention in the late
1960s [14] because of its potentially lower
monomer costs but the required polymerization
process was unattractive to DuPont
and construction of facilities never proceeded
beyond the pilot plant scale. Base resistance
was not a major issue for automotive
engine seals at the time and the general
chemical process industry had not yet
demanded elastomers that were highly
base-resistant and thermally stable.
Asahi Glass began to offer highly base-resistant
TFE/P dipolymers in 1979 [15]; however,
these elastomers proved quite difficult
to process. In 1985 less base-resistant
but more processable terpolymers that included
VF2 were patented by Asahi Glass
[16] and later marketed by Asahi Glass and
3M [17]. In the mid 1980s a highly baseresistant
polymer of ethylene, TFE, PMVE
and a peroxide-sensitive bromine-containing
curesite monomer, was developed by
DuPont [18]; however, the high level of
very expensive PMVE required to break
up TFE-E crystallinity elevated this product
to the status of a high-cost, high-performance
speciality. DuPont-Dow Elastomers,
LLC, a wholly owned joint venture of Du-
Pont and Dow, re-investigated the potential
of TFE/P-based polymers in the late
1990s and has now prepared VF2-free,
highly processable, bisphenol-curable variants
on a commercial scale using a proprietary
curesite monomer. The final development
of these polymers and their recommended
curable formulations are
now in the pre-commercial stage [19].
VF2/HFP/(TFE) Chemistry
To understand how to design a base-resistant
hydrofluoroelastomer it is necessary
to understand how conventional VF2-based
fluoroelastomers react with strong bases
and ultimately fail in severe accelerated
exposure tests designed for increasingly
demanding applications. It should be noted
that field failures of existing FKM parts
are rare in present applications, but longer
automotive warranties and the introduction
of ever more aggressively formulated
lubricants designed for long service intervals
has forced suppliers to develop fundamentally
new fluoroelastomer compositions.
VF2/HFP elastomers are derivatives of poly-
VF2 in the sense that sufficient HFP is copolymerized
to eliminate the 165 8C melting
point of the homopolymer and to prevent
the formation and crystallization of lower
melting, shorter sequences of contiguous
VF2 units. This requires at least
35 wt.% of HFP. Incorporation of
40 wt.% HFP in standard dipolymers af-
314 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004

fords a non-crystalline elastomer with a
glass transition temperature of about
-18 8C. A consequence of having to introduce
over 25 mole % HFP is the introduction
of substantial levels of HFP-VF2-HFP
sequences, which we have previously identified
as selectively base-sensitive sites that
undergo both dehydrofluorination and
crosslinking in the curing process. The advantage
of such a high curesite concentration
is that very high cure rates can be
achieved. The disadvantage is that overcrosslinking
can occur in severe service in
basic environments because difunctional
agents are not required to couple unsaturated
fluorocarbons. Thus, fluoride ion can
add to one double bond and form a carbanion
that then attacks the second double
bond nucleophilically and forms an undesired
crosslink. The carbanion adduct then
stabilizes itself by eliminating its charge as
a fluoride ion and forms a fluoroalkylated
double bond at the point of attachment.
The ease of this reaction in model compounds
is evident from dimerization of perfluorocyclopentene
when treated with CsF
in sulfolane at 125 8C (Eqn. 1 [20]).
Admittedly this reaction is highly favored
stereochemically, but it occurs under mild
conditions. It is believed that a similar process
is responsible for the roughly 10% of
crosslink density [21] that results from a
pseudovulcanization in which the bisphenol
is omitted from an otherwise fully functional
curing system. In this case the function
of the CsF solution is assumed by the
unsolvated quaternary ammonium or
phosphonium ion and its hydroxide counterion
from exchange with the surface of
the metal oxide or hydroxide.
The normal crosslinking process in phasetransfer
catalyzed bisphenol systems that
contain metal oxide or hydroxide acid
acceptors like MgO or Ca(OH) 2 can be
viewed as follows (Eqn. 2 [22]).
Comparison of F-19 FT-NMR spectra of
VF2/HFP solutions in d8-THF before and after
reaction with the highly basic cyclic
amidine base DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene)
shows that this
base, when added at a molar ratio to polymer
that corresponds to the bisphenol
level (2.0 phr Bisphenol AF) in commercial
cures and assumes that three moles of
base are required to attach one phenolic
OH group (one to form the first double
bond, another to neutralize bifluoride
and a third to ionize the phenol to become
nucleophilic), selectively attacks VF2 units
that are flanked by HFP. The amount of
reaction of contiguous VF2 sequences is
Eqn. 1. Fluoridecatalyzed
dimerization
of perfluorocyclopentene
[20]
Eqn. 2. Proposed dehydrofluorination
of
3.5VF2/HFP elastomer
by 1,8-diazabi-
cyclo[5.4.0]undec-7-
ene (DBU) in solution
[22]
minor. The DBU/polymer ratio is based
on the ionization of Ar(OH) 2 as a bis-nucleophile
and the formation of a diene
structure at each end of the bis-ether
crosslink. Polymer concentrations of the
solutions subjected to dehydrofluorination
were about 1% in order to achieve a quantitative
NMR response and T 1 experiments
were done to verify that the polymer and
internal standard had sufficiently relaxed
between excitation pulses to assure full absorption
and hence meaningful integration.
Reactions with DBU were run on approximately,
but accurately determined,
10 mg polymer samples.
The diene structure at the crosslink was originally
proposed [22] to account for the relatively
high levels of fluoride produced on
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004 315

eaction of VF2/HFP with DBU or (n-Bu) 4-
NOH in solution. The hydroxylic base
also acts as a nucleophile toward the unsaturation
and is expected to form an unsaturated
ketone after isomerization of the
initially formed dienol if attack is vinylic, or
an isomeric unsaturated ketone if attack is
allylic. Because of the excess level of HFP-
VF2-HFP sites (originally about 0.6 moles/
kg) that remains after vulcanization with
0.06 moles bisphenol/kg, the formation
of such ketones and their further haloform
reaction on continued exposure to strong
bases in severe exposures would lead to
appreciable chain degradation. Not all unsaturation
is dienic, however, because FT-IR
also shows a vinylic C-H stretching band on
base treatment in solution and in thin films
of unfilled vulcanizates. The case for the
dienic structure in bisphenol crosslinking
is best made by considering that the initially
formed fluoroolefin is expected to undergo
a substitution reaction with phenoxide
based on reactions of model compounds.
If net addition were to occur,
the phenol adduct C-H bond would become
the most acidic polymer structure
and would therefore be rapidly dehydrofluorinated.
The resulting structure would
be an allylic ether that contains an allylic
hydrogen. This would also be expected
to preferentially lose HF and generate a
dienic ether crosslink. The details of point
of attachment of the phenol are not
known, but various reaction paths predict
a common element, a dienic phenyl ether.
A similar reaction path is proposed for the
dehydrofluorination and bisphenol-crosslinking
processes of TFE/P/VF2 polymers.
As such it is particularly relevant to re-examine
the evidence for the proposed
high reactivity of isolated VF2 units in copolymers
with perfluoroolefins.
It has been proposed by Ausimont workers
[23] that the HFP-VF2-HFP site cannot be
the cure site of VF2/HFP elastomers because
as HFP content is raised, and more
HFP-flanked VF2 occurs, the vulcanization
rate is well known to decrease. This view
results from a confusion between the
intrinsic reactivity of solvent-separated
chains as described here and in earlier
work [22] and reactions in bulk, where
reactant and reaction matrix are the
same. As fluorine content rises, the rates
of ionic reactions of bulk fluoroelastomer
systems decrease. This is a result of the
reduced solubility of curatives and the
reduced ability of the reaction medium
to support ionic intermediates. The ratio
of CH 2 groups to CFþCF 2
þCF 3 groups
(averaged to CF 2 ) of typical VF2/HFP dipolymers
is 0.54 and of a typical TFE-containing
terpolymer is 0.40. Expressed this
way, the expected solubility differences between
these two polymers are more easily
appreciated than when considering their
fluorine contents (66.0 vs 68.5 wt.% F).
It is well known that the more highly fluorinated
of these two polymers requires about
twice the accelerator level to achieve a
similar cure rates. However, it was shown
in our earlier study [22] that, on addition
of DBU, gelation rates of Bisphenol AFcontaining
polymer solutions in DMAC
are indeed 2.7 times higher for the more
highly fluorinated terpolymer. Even despite
the fact that this example is complicated by
the simultaneous formation of TFE-VF2-
TFE(HFP)-derived unsaturation that is quite
unreactive toward crosslinking but equally
readily formed by dehydrofluorination, it
further illustrates the divergence between
intrinsic reactivity in a common medium
and reaction within a given polymer matrix
as a function of the concentration of the
isolated-CH 2 mole fraction. However,
even a polymer with a high mole fraction
of CH 2 groups, such as poly (VF2), resists
efficient dehydrofluorination and crosslinking
in solution because of the absence
of RfCH2Rf sites, where Rf is a two-carbon
or longer perfluoroalkyl unit, as shown
below.
Additional evidence for the HFP-VF2-HFP
sequence as curesite comes from examination
of the NMR signal of the normally
head-to-tail polymerized HFP’s CF 3 group
on treatment with DBU in solution
(Figs. 1 and 2). The high resolution of a
modern spectrometer (377 MHz for F-19)
now makes it possible to resolve this CF 3
signal into a major peak with a shoulder
and a fully resolved peak that has about
32% of the intensity of this pair. The
more intense band, at 76.41 ppm, (all chemical
shifts reported here are relative to the
internal standard 3-trifluoromethyl anisole
in d-8 THF set at 63.93 ppm on the CF 3 Cl
scale) is due to isolated HFP that is flanked
by more than one VF2 group on either
side. This is obvious from the 3.5 VF2/
HFP molar composition and the fact that
HFP does not form contiguous units or homopolymerize.
The other CF 3 band at
76.84 ppm is due to the first HFP unit of
the HFP-VF2-HFP sequence (viewed as polymerized
left to right) and the shoulder at
76.5 ppm is due to the second HFP unit,
whose chemical shift environment is almost
the same as that of the isolated
HFP. The 76.5 ppm shoulder and the
Fig. 1. 377 MHz 19 F NMR spectrum of CF 3 region
of 3.5 VF2/HFP polymer in d 8 THF solution
(ppm). Internal standard 3-trifluoromethyl
anisole set to 63.93 ppm relative to CF 3 Cl)
Fig. 2. Spectrum of solution of Fig. 1 after
treatment with 1x DBU for 20 h at 20C (see
text)
76.84 peak are indeed the signals that
are massively and selectively reduced on
reaction with DBU and shifted to new positions
due to the influence of nearby unsaturation.
It would be logical to conclude
that this reflects the specific transformation
of the HFP-VF2-HFP sequence. The intensity
of the major band at 76.41 changes
very little but unfortunately cannot be accurately
integrated because it is incompletely
resolved.
Based on this evidence, it is reaffirmed that
the originally proposed curesite of VF2/HFP
polymers is indeed the HFP-VF2-HFP monomer
sequence and that, despite the presumed
proper calculation of the concentration
of this monomer sequence as a
function of HFP mole fraction in VF2/HFP
dipolymers and its apparent negative correlation
with cure rates of such composititional
variants [23], it is inappropriate to relate
these observations to intrinsic reactivity
differences of various structural elements
within a given polymer. This kind
of comparison is equivalent to extracting
intrinsic reactivity correlations by comparing
reaction rates of a series of model
compounds with a common reagent in dif-
316 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004

ferent solvents. The literature abounds
with profound solvent effects on reaction
rates.
Better approaches to assess the reactivity
of various structures are to measure the relative
rates of dehydrofluorination and
crosslinking of model polymers in a common
medium and to perform competition
experiments in which two polymers have
equal access to the same base in the
same medium. We have already reported
such data before and now present additional
results obtained under conditions
where DBU/polymer ratios can be limited
to the crosslinking stoichiometry of practical
curing systems, rather than having to
use excess base in order to boost NMR
signal strength as in the original work
[22], which was done on a continuous
wave 94.1 MHz instrument.
Fig. 3 shows the entire 377 MHz F-19 FT
NMR spectrum of a 3.5 VF2/HFP
(60 wt.% VF2) polymer in d8-THF solution.
Fig. 4 shows the same polymer solution
20 hrs after addition of DBU. Fluoride
ion is seen at 162 ppm and some original
CF 3 intensity is shifted to higher and lower
frequencies as a result of nearby unsaturation.
The peak due to contiguous VF2 sequences
at 91 ppm is essentially unchanged.
The 110 ppm CF 2 peak due to isolated
VF2 is decreased as is the CF 2 peak due
to the adjacent HFP unit. The fluoride evolution
at 20 h at 25 8C can be expressed as
0.69 lmol F/0.375 lmol DBU/mg pol in
138 ll THF, which substantially supports
a double dehydrofluorination of the isolated
VF2 unit, complete reaction of DBU to
form DBUHþ FHF , and therefore the proposed
diene structure.
Figs. 5 and 6 show the corresponding
spectra of a 4.7 VF2/TFE (75 wt.% VF2)
polymer before and after DBU treatment.
The extent of fluoride evolution after
20 hours is only about half of that for
VF2/HFP and vinyl fluorines are prominently
visible. Reaction primarily reduces the
110 and 127 ppm bands. The fluoride evolution
can be expressed as 0.36 mol F/
0.387 mol DBU/mg pol in 142 ll THF in
20 hr at 25 8C. This reflects a single dehydrofluorination
of an isolated VF2 unit per
DBU and indicates that the rearrangement
of the initial olefin by fluoride that occurs
with VF2/HFP to yield an easily abstractable
allylic hydrogen does not occur without
stabilization of the intermediate carbanion
by a CF 3 group. The observed stoichiometry
is consistent with complete reaction of
DBU and formation of DBUH + F .
Figs. 7 and 8 show corresponding spectra
of poly (VF2) before and after DBU treatment.
Very little fluoride is formed and
no spectral changes in the polymer are evident
after 20 hrs. This suggests that long
sequences of VF2 need activation by an adjacent
perfluoromonomer to create a highly
acidic CH 2 group in order to start the dehydrofluorination
by DBU under these conditions.
These sites, of course, are abundant
in VF2 copolymers of HFP and TFE
and in the more base-resistant copolymers
of TFE and P.
Once a double bond has formed, conjugate
dehydrofluorination is expected to
proceed easily.
To summarize, analysis of these three types
of polymers shows that high reactivity toward
DBU in VF2-containing polymers requires
CH 2 sites that are flanked by perfluorinated
two or three carbon units.
Because presently known highly fluorinated
elastomeric copolymers of VF2 are sensitive
to attack by base, it has been
attempted to reduce this sensitivity by copolymerization
with hydrocarbon olefins,
generally ethylene in the case of HFP-containing
elastomers. Thus, VF2/HFP/TFE/E
elastomers have been developed by
Ausimont [24], but disadvantages to their
Fig. 3. Entire 377 MHz 19 F spectrum of 3.5 VF2/
HFP polymer and internal standard in d 8 THF
solution
Fig. 4. Spectrum of solution of Fig. 3 after
treatment with 1x DBU for 20 h at 20C (see
text)
Fig. 5. Entire spectrum of 4.7 VF2/TFE polymer
plus std
Fig. 6. Spectrum of solution of Fig. 5 after
treatment with 1x DBU for 20 h at 20C
Fig. 7. Entire spectrum of poly-VF2 plus std
Fig. 8. Spectrum of solution of Fig. 7 after
treatment with 1x DBU for 20 h at 20C
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004 317

production are low polymerization rates
without microemulsification and an insufficient
reduction in the number of selectively
acidic VF2 sites for high resistance toward
bases. This occurs because ethylene
does not readily add to a propagating VF2
radical but instead prefers to add to a propagating
TFE radical, and the preference
for VF2 by propagating HFP radicals remains
high. Practical base resistance tests
in hot, aggressively formulated motor oils
show only moderate improvements compared
to standard FKM products by this
ethylene dilution strategy.
Fig. 9. Entire spectrum of 1.62 TFE/ 1.52 VF2/
1.00 P polymer plus std
Fig. 10. Spectrum of solution of Fig. 9 after
treatment with 10x DBU for 13 d at 20C (see
text)
TFE/P-based polymers
Because high degrees of base resistance in
practical hydrofluoropolymers can only be
achieved sharply reducing or eliminating
VF2, the best flexibility-conferring comonomers
for base-resistant elastomers are
hydrocarbon olefins. Thus, TFE/E is a truly
base resistant thermoplastic and TFE/P is
the related base-resistant elastomer. These
materials possess highly alternating structures
but can be prepared with higher fluorine
levels by forcing in additional TFE.
They gain their non-acidity by the absence
of R f CH 2 R f structures. Unlike standard VF2-
based elastomers, these polymers do not
form strong hydrogen bonds with carbonyl
oxygens in common solvents and therefore
are not soluble in these polar fluids. Hydroxylic
solvents, with the possible exception
of methanol, prefer to form hydrogen
bonds among themselves and ignore even
standard VF2-based polymers. Solvents for
TFE/P are mixtures of hydrocarbons and
perfluorocarbons, for example aromatics
and hexfluorobenzene, as well as THF,
which has well recognized but poorly
understood solvating power. Without the
ability to form strong hydrogen bonds to
weakly basic oxygen in carbonyl fluids,
TFE/P also does not readily form an actual
H-N bond to nitrogen bases, and the resultant
polymer carbanion intermediate for
loss of fluoride, on attempted dehydrofluorination.
In this connection, it should be noted that
hydrofluorocarbons have been shown to
undergo amine-catalyzed deuterium exchange
rates that are many orders of magnitude
greater than HF or DF elimination
[25]. Although the ratio of exchange to elimination
is a strong function of structure, it
is primarily a testament to the high strength
of the C-F bond. Extensive fluorination in
beta positions of model structures leads
Fig. 11. Entire spectrum of 18.2 TFE / 11.4 P /
1.00 3,3,3-trifluoropropene plus std
to the highest exchange rates or, stated differently,
the most stable carbanions.
The extraordinary base resistance of TFE/P
is a liability when attempting to cure this
polymer with a basic bis-nucleophile like
a bisphenol or diamine. Peroxide cures
do not proceed well on the native polymer
and attempts to activate the dipolymer
have included some highly discoloring
pre-treatments. Some suppliers have chosen
to offer VF2-containing TFE/P polymers
in order to increase curability. This approach
also has the desirable effect of decreasing
t g but at the cost of resistance to
polar fluids and particulary bases.
Figs. 9 and 10 show the F-19 spectra of a
32 wt.% VF2 copolymer of TFE and P (54
TFE / 32 VF2/14 P wt.% or 1.62 TFE / 1.50
VF2 / 1.00 P moles) in d8-THF before and
after a 13 day exposure to 10x of the DBU
levels used in experiments with the nonbase
resistant polymers. Relative to VF2/
HFP at 1x DBU, the evolution of fluoride
is low. However, on exposure to the 10x
standard DBU level (Fig. 8), the elimination
of HF increases markedly within the 13 day
reaction period. Examination of the spectra
reveals that reaction has occurred at
a series of TFE-VF2-TFE sites at 110.93,
112.05, 112.37 and 112.78 and at
Fig. 12. Spectrum of solution of Fig. 11 after
treatment with 10x DBU for 13d at 20C
126.87, 127.05 and 127.47 ppm. This behavior
is entirely consistent with the reactions
of VF2/TFE polymers and the reactivity
pattern and spectral assignments of
our earlier lower resolution NMR study
(22) which proposed this general site as
one of four selectively base-sensitive sites
of VF2/HFP/TFE polymers. High VF2 levels
are required in TFE/P copolymers because
low levels do not respond well to bisphenol
curing since only a fraction of the total VF2
is incorporated in TFE-VF2-TFE sites. However,
the remaining VF2 in TFE-VF2-P sequences
remains available for slower dehydrofluorination
and degradation in service.
The overall evolution of fluoride can be expressed
as 2.4 lmol F/3.86 lmol DBU/mg
pol in 138 ll THF in 13 d at 25 8C. The consumption
of DBU is incomplete.
Figs. 11 and 12 show the F-19 spectra of a
4.0 wt.% TFP (3,3,3-trifluoropropene) copolymer
of TFE and P (76 TFE / 20 P /
4.0 TFP wt.% or 18.2 TFE / 11.4 P / 1.0
TFP moles) before and after the same 13
day exposure to 10x DBU. Only about
15% of the amount of fluoride compared
to the TFE/P/VF2 polymer is formed under
these conditions. The TFP-terpolymer is
therefore more than 6 times as resistant
to the strong base DBU as the 30% VF2-
318 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 6/2004

Fig. 13. Entire spectrum of a 50/50 mixture of
polymer solutions of Figs. 9 and 11
Fig. 15. Dependence of the change in maximum
elongation on VF2 content of TFE/P/
VF2 polymers when aged in a basic oil at
150C for up to 2000 h
like TFE or HFP have been shown to be
the sites highly selectively attacked by
base in both standard VF2/HFP/(TFE) polymers
and in base-resistant TFE/P-containing
terpolymers. The high level of VF2 required
for an adequate bisphenol cure response
of the TFE/P/VF2 polymers renders
them less base-resistant than the newly developed,
more highly fluorinated TFE/P/TFP
composition because the TFP level is designed
for efficient reaction and consumption
during cure. In contrast, residual VF2
units of TFE/P/VF2 polymers, not reacted
during the cure, remain available for degradation
by base during service.
Fig. 14. Competitive dehydrofluorination for
12d at 20C by 10x DBU of a solution of the
50/50 polymer mixture of Fig. 13 (see text)
terpolymer. The CF 3 absorption at
67.05 ppm is lost entirely and appears to
reflect the only reactive environment. An
absorption at 68.15 ppm increases. Spectral
features in the range of 110 –
126 ppm are almost unchanged on DBU
addition. The overall evolution of fluoride
can be expressed as 0.38 lmol F/ 4.11
çmol DBU/mg pol in 147 ll THF in 13 d
at 25 8C. DBU consumption is incomplete.
Figs. 13 and 14 show the F-19 spectra of a
d-8 THF solution of equal amounts of the
above TFE/P/VF2 polymer and the above
TFE/P/TFP polymer before and after a 12
day exposure to 10x DBU, respectively.
This is a competition experiment in which
thebasehas equalaccesstoallsitesofeither
polymer in the same reaction medium. Therefore,
the course of this dehydrofluorination
reveals the true relative reactivity of the
active sites of the two polymer types. Excess
DBU remains, as in the exposures of the separate
base-resistant polymers.
Examination of the 111-112 and 126.5-
127.5 ppm regions indicates the complete
loss of structures that generate these absorptions
in the TFE/P/VF2 polymer. In contrast,
the spectral changes due to reaction
of the TFE/P/TFP polymer in this 50/50 polymer
mixture shows that only original CF 3 -
related structures due to the curesite monomer
have reacted. This specificity in
reaction of the TFP-terpolymer is highly desirable
because it promises a facile but
limited dehydrofluorination sensitivity
and therefore would be expected to lead
to fast bisphenol-based cures and excellent
resistance to bases in service. One could
draw the analogy to the oxidation resistance
of diene elastomers vs. EPDM.
Comparison to resistance toward
a basic oil in practical exposure
tests
Fig. 15 compares the percent loss of elongation
at break of various TFE/P/VF2 polymers
on aging in ASTM Reference Oil 105
at 150 8C as a function of VF2 content and
reveals the deleterious practical effects of
increasing its level. Comprehensive favorable
comparisons of the performance of a
new bisphenol-crosslinked TFP terpolymer
(VTX-8802) vs a typical bisphenol-crosslinked
VF2-terpolymer and a typical peroxide-curable
TFE/P dipolymer are given by
Bauerle and Tang [19]. The data also
show that, although outstandingly resistant
to attack by bases, standard TFE/P dipolymer
compositions are insufficiently oil
resistant for many applications; however,
the high TFE level of the new TFP terpolymer
reduces the 504 hr swell at 150 8C in
this oil to virtually the same low level as
that for conventional VF2/HFP dipolymers.
Conclusions
A new highly base-resistant fluoroelastomer
has been designed by DuPont Dow
Elastomers and is now undergoing final
evaluations. It incorporates a proprietary
curesite monomer, 3,3,3-trifluoropropene
(TFP), and contains no VF2. VF2 units flanked
by perfluorocarbon monomer units
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