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

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