Chemical interaction of fluoropolymers with transition metals

Article abstract of DOI:10.1134/S002016850907019X

Chemical interaction of transition metals (Mo, W, Ta, Nb, and Ti) with a tetrafluoroethylene-vinylidene fluoride (TFE-VDF) copolymer (21 mol % TFE + 79 mol % VDF) has been studied by differential scanning calorimetry (DSC) and mass spectrometry. The DSC c

Full text of DOI:10.1134/S002016850907019X

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 7, pp. 809–813. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.V. Tarasov, A.S. Alikhanian, I.V. Arkhangel’skii, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 7, pp. 871–876.
Chemical Interaction of Fluoropolymers with Transition Metals
A. V. Tarasova, A. S. Alikhaniana, and I. V. Arkhangel’skiib
a Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
b Moscow State University, Moscow, 119992 Russia
Received October 1, 2008
Abstract—Chemical interaction of transition metals (Mo, W, Ta, Nb, and Ti) with a tetrafluoroethylene–
vinylidene fluoride (TFE–VDF) copolymer (21 mol % TFE + 79 mol % VDF) has been studied by differential
scanning calorimetry (DSC) and mass spectrometry. The DSC curves of mixtures of the fluoropolymer with Ta,
Nb, and Ti showed exothermic peaks, and those of composites with W and Mo showed endothermic peaks.
Mass spectrometric analysis indicated that the fluoropolymer reacted with the transition metals to form the
higher fluorides TaF5, NbF5, TiF4, WF6, and MoF6. In addition, WOF4 and MoOF4 molecules were detected in
the case of tungsten and molybdenum. At temperatures above 700 K, the mass spectra of all the systems showed
ions corresponding to low-molecular hydrocarbon molecules.
DOI: 10.1134/S002016850907019X
INTRODUCTION
of the reaction products. Nor were the enthalpies of
reactions between metals and fluoropolymers deter-
mined.
Fluoropolymers are widely used to produce coat-
ings on metals, e.g., nonstick, corrosion-resistant, and
insulating coatings for various apparatus intended to
operate in aggressive media. In addition, fluoropolymer
coatings are applied in electrical engineering.
According to published data, fluoropolymers react
with a number of metals to form metal fluorides.
It is of interest to study chemical interactions
between tetrafluoroethylene (TFE) copolymers and
metals. Among such copolymers are a number of TFE
compounds with vinylidene fluoride (VDF). This
copolymer is of interest because it offers relatively high
thermal stability (depolymerization temperature above
400 K) and dissolves in a number of solvents (including
acetone and ether). It has a very low hydrogen perme-
ability, which is of importance in coating metallic parts
of electrogenerating devices, such as fuel cells, and
electrochemical devices for the preparation of various
gases, primarily hydrogen, which exhibits elevated cor-
rosion activity to many metals (so-called hydrogen cor-
rosion). Studies of reactions between fluoropolymers
and metals help to determine the working ranges and
optimize the fabrication conditions of Teflon-coated
metallic parts.
As reported by Kavan [1], metal fluorides can be
obtained by bringing polytetrafluoroethylene (PTFE)
or other fluoropolymers into contact with metals such
as Ni, Al, Cr, Cu, Pb, and stainless steel. This was, as a
rule, achieved by depositing metal vapor (in vacuum)
on a polymer substrate. The reaction products were
analyzed by X-ray photoelectron spectroscopy (XPS).
XPS spectra provide evidence of metal–fluorine bonds
at the interface between the polymer substrate and the
metallic layer.
Cadman and Gossedge [2] investigated chemical
reactions of PTFE with Cd, In, Sn, Au, Ag, and Al by
differential scanning calorimetry (DSC). Mixtures of
metal and polymer powders in the volume ratio 1 : 1
were heated to 773 K. The DSC curves of the Cd +
PTFE and Sn + PTFE mixtures showed exothermic
peaks at 683 and 733 K, respectively. The DSC curve of
the In + PTFE mixture contained no peaks, but the res-
idue in the crucible had the form of black carbonaceous
material. Using XPS, Cadman and Gossedge [2]
showed that no fluorides were formed in the case of the
Au + PTFE and Ag + PTFE mixtures. Depositing tita-
nium vapor onto a PTFE substrate, Carlo et al. [3]
obtained TiF3 and TiC.
In this paper, we present DSC and high-temperature
mass spectrometric studies of the thermodynamics of
reactions between transition metals (Mo, W, Ta, Nb,
and Ti) and a TFE–VDF copolymer (21 mol % TFE +
79 mol % VDF).
EXPERIMENTAL
Metal–polymer composite samples (mechanical
mixtures of a metal and fluoropolymer) were prepared
by hot pressing on a benchtop pneumatic press with
heated plates, using a rectangular die 50 × 10 mm in
dimensions, with a clamp heater.
Unfortunately, all of those studies were only quali-
The samples were prepared as follows: Metal and
tative and provided neither amounts nor compositions polymer powders in the molar ratio 1 : (5–6) in terms of
809

810
TARASOV et al.
Table 1. DSC results for metal–fluoropolymer systems be-
low 673 K
Standard GOST 25428-82) were measured at room tem-
perature on a Bruker Avance 300 spectrometer.
The metal powders used were cut with a diamond
needle file and consisted of rounded particles with an
average size of 80 µm.
Weight
System
Thermal events
loss, %
Ta–F42 F42 melting endotherm at 408 K, exo- 18.2
therm at 548 K
RESULTS AND DISCUSSION
Nb–F42 F42 melting endotherm at 408 K, exo- 33.3
therm at 617 K
DSC scans showed that heating of the metal–poly-
mer composites to 673 K resulted in reactions between
the fluoropolymer and metal. The DSC results are pre-
sented in Table 1 and Fig. 1.
The DSC curves of the mixtures of the fluoropoly-
mer with Ta, Nb, and Ti show exothermic peaks, and
those of the composites with W and Mo show endother-
mic peaks (Fig. 1). After each experiment, the crucible
contained black carbonaceous material. The wall and
lid of the crucible were covered with a black deposit
resembling soot, which could only be removed by
washing with petroleum ether. The reaction was accom-
panied by a sharp weight loss from about 3 to 35%.
For comparison, Fig. 1 shows the DSC curve of pure
F42. Up to 680 K, the curve has only one peak (endot-
herm at 408 K), due to the congruent melting of the
polymer.
Mass spectrometric analysis showed that the fluo-
ropolymer reacted with the transition metals to form the
higher fluorides TaF5, NbF5, TiF4, WF6, and MoF6. In
addition, WOF4 and MoOF4 molecules were detected in
the case of tungsten and molybdenum. At temperatures
above 700 K, the mass spectra of all the systems
showed ions corresponding to low-molecular hydrocar-
bon molecules.
Ti–F42 F42 melting endotherm at 408 K, exo- 12.2
therm at 633 K
W–F42 F42 melting endotherm at 408 K, exo-
therm at 633 K
4.6
2.8
0
Mo–F42 F42 melting endotherm at 408 K, exo-
therm at 617 K
F42
F42 melting endotherm at 408 K
Note: Peak temperatures were determined at a heating rate of
10 K/min.
fluorine were poured into the die and heated to 423 K.
Next, a load of 20 MPa was applied for 5 min. The die
was then cooled to 353 K, the load was removed, and
the sample was withdrawn. The top die and bottom
were lined with Teflon after each pressing cycle to
avoid sample sticking and burning. The hot-pressed
bodies had the shape of plates 50 × 10 × 1–1.2 mm in
dimensions . The plates were cut into rectangular sam-
ples (3 × 2 mm) for mass spectrometric analysis and
calorimetric characterization.
DSC and thermogravimetric (TG) curves were
obtained on a Netzsch STA 449C Jupiter simultaneous
thermal analyzer, equipped with a top-loaded electro-
magnetic compensation balance (0.1-µg accuracy). The
sample weight was determined with a relative uncer-
tainty of 0.5%. In those studies, we used a high-sensi-
tivity sample holder with Pt/Pt–Rh thermocouples.
Measurements were performed in the temperature
range 313–673 K in flowing argon (50 ml/min) at heat-
ing rates of 5, 10, and 20 K/min. The weight of all the
samples was 10–15 g. The samples were enclosed in a
covered gold or aluminum crucible. Immediately
before measurements, the chamber was evacuated and
then filled with argon (volume fraction of argon
≤0.99993; volume fraction of water vapor ≥0.000009).
As an example, Fig. 2 shows a portion of the mass
spectrum of the gas phase over the W–F42 system at
630 K. To interpret the spectrum, we utilized the ioniza-
tion efficiency curves of the WOF+3 , WF4+, , and WF5+
ions and the mass spectra of individual WF6 and WOF4
[4]. The results indicate that the partial pressure ratio
pWF /pWOF in the gas phase over the W–F42 system at
6
4
582 K is ~3.
The exotherms in the DSC curves of the M–F42
(M = Ta, Nb, Ti) systems can readily be accounted for
by the fact that the average enthalpy of the M–F bonds
in the higher fluorides of these metals substantially
exceeds the average enthalpy of the C–F bond in any
fluorohydrocarbons. For this reason, reactions of fluo-
ropolymers with metals are always accompanied by
heat release.
Gaseous reaction products were analyzed on an MS-
1301 mass spectrometer using molybdenum effusion
cells. The ratio of the vaporization area to the effusion-
orifice area was 600. The temperature was measured
with a Pt/Pt–Rh thermocouple and was maintained with
an accuracy of 1 K. Reactions between fluoropoly-
mers and metals were studied in the temperature range
573–623 K.
Table 2 lists the average enthalpies of the M–F bond
evaluated as
EM–F = ∆atH0(298 K)/n,
where ∆atH0(298 K) is the enthalpy of atomization of
1H nuclear magnetic resonance (NMR) spectra of
F42 fluoropolymer powder (TFE–VDF copolymer of the higher metal fluoride, and n is the number of fluo-
composition 21 mol % TFE + 79 mol % VDF) (RF State rine atoms per formula unit of the compound. The aver-
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CHEMICAL INTERACTION OF FLUOROPOLYMERS WITH TRANSITION METALS
811
DSC, mW/mg
DSC, mW/mg
10
0.4
exo
exo
0.2
8
2,4
Ti–F42
Mo–F42
W–F42
6
4
2
0
2,3
2,2
2,0
0
T
= 408 K
m
400
500 600
–0.2
F42
–0.4
–0.6
퇖F42
Nb–F42
350 400 450 500 550 600 650 700
Temperature, K
350 400 450 500 550 600 650 700
Temperature, K
Fig. 1. DSC curves of the F42 copolymer and M–F42 systems.
age enthalpy of the C–F bond was determined from the tantalum, niobium, and titanium systems, the water
enthalpy of CF4 atomization.
content is low compared to the amount of the forming
fluorides, the fluorides hydrolyze only slightly, and the
DSC curves show a sum exotherm. If this reasoning is
true, then reducing in one way or another the amounts
of tantalum, niobium, and titanium fluorides forming in
the M–F42 systems we will observe endotherms in
their DSC curves. Such an experiment was performed
in the Ta–F42 system, with tantalum in the form of foil
instead of powder. The sharp reduction in the area of
the metal–fluoropolymer interface led to a reduction in
the amount of forming tantalum fluoride and, as a con-
sequence, the DSC curve of the Ta–F42 system showed
a sum endotherm (Fig. 4).
The endothermic peaks in the DSC curves of the
W−F42 and Mo–F42 systems may appear inconsistent
with the average enthalpies of the bonds involved, but
the presence of tungsten oxyfluoride molecules in the
gas phase over the W–F42 system leads us to assume
that the exothermic tungsten fluorination process
W(s) +1/n(C33H25F43)n(s)
is accompanied by the endothermic hydrolysis process
WF6(g) + H2O(l) WOF4(g) + 2HF(g). (2)
WF6(g) + X(s) (1)
Here, (C33H25F43)n is the gross formula of the F42
copolymer inferred from its composition (21 mol %
TFE + 79 mol % VDF), and X(s) stands for the copol-
ymer defluorination product(s).
Water was, most likely, present in the polymer, since
F42 is commonly prepared in an aqueous medium [9].
The presence of water was confirmed by NMR data.
The 1H NMR spectrum of F42 powder (Fig. 3) shows a
broad (~30 kHz) signal at 4.1 ppm and two narrow sig-
nals (~300 Hz) at 2.6 and 0.9 ppm. The broad signal
A simple thermodynamic estimate from the known
standard enthalpies of formation of the above reactants
indicates that the hydrolysis endotherm may exceed in
magnitude the fluorination exotherm if the fluoropoly-
mer defluorination products are aliphatic polyenes. It is
reasonable to assume that similar reactions occur in the
Ta–F42, Nb–F42, and Ti–F42 systems, but the fluorina-
tion process in these systems must be more effective
because of the high enthalpies of bonds in tantalum,
niobium, and titanium fluorides compared to molybde-
num and tungsten fluorides (Table 2).
Table 2. Average metal–fluorine bond energies calculated
from data in various reports
System
Eb, kJ/mol
References
C–F
488
[5, 6]
[5–7]
[5, 6]
[5, 6]
[5, 8]
[5, 6]
Ta–F
Ti–F
Nb–F
W–F
Mo–F
592.6 6.7
585.2 4.7
572.1 0.3
508.7 0.4
448.1 0.8
All of the systems under consideration are identical
in water content. In the case of the molybdenum- and
tungsten-containing samples, the water content is high
enough for a high degree of hydrolysis. As a conse-
quence, the DSC curves show a sum endotherm. In the
INORGANIC MATERIALS Vol. 45 No. 7 2009

812
TARASOV et al.
+
WF
5
100
80
60
40
20
0
+
+
4
WOF , WF
3
186
+
182
+
WF
WOF
4
3
245
250
255
260
265
270
m/z
275
280
285
290
295
Fig. 2. 585-K mass spectrum of the gas phase over the W–F42 system.
arises from the protons of the polymer matrix, and the known standard enthalpies of formation of niobium and
tantalum pentafluorides [5] and suggest some conclu-
sions as to the nonvolatile products of reactions
between fluoropolymers and metals.
narrow signals correspond to mobile water molecules,
presumably, occluded in the polymer matrix upon poly-
merization. From the relative integrated intensities of the
broad and narrow components, the water content in the
polymer system was estimated to be within 0.4 mol %
(0.1 wt %).
The reactions of tantalum and niobium with the flu-
oropolymer can be represented by the following
schemes:
It is worth noting that the heat effects obtained for
the fluorination reactions in the Ta–F42 and Nb–F42
systems (Table 1) are in perfect agreement with the
Ta(s) + 1/n(C22H25F43)n(s)
+ X1(s), ∆rH0(3) ≥ – 295 16 kJ/mol;
TaF5(g)
(3)
(a)
–150
ppm
150
100
50
0
–50 –100
(b)
15
10
5
0
–5
ppm
1
Fig. 3. H NMR spectrum of F42 powder: (a) survey scan, (b) region of narrow signals.
INORGANIC MATERIALS Vol. 45 No. 7 2009

CHEMICAL INTERACTION OF FLUOROPOLYMERS WITH TRANSITION METALS
DSC, mW/mg
813
2.0
exo
1.5
1.0
0.5
Ta(powder)–F42
Ta(foil)–F42
0
í
m
–0.5
350
400
450
500
550
600
650
700
Temperature, K
Fig. 4. DSC curves of the Ta–F42 system.
intended for long-term use, the temperature should not
exceed 523 K.
Nb(s) + 1/n(C22H25F43)n(s)
NbF5(g)
(4)
+ X2(s), ∆rH0(4) ≥ – 240 13 kJ/mol.
REFERENCES
Here, ï2 and ï1 are the fluoropolymer defluorination
products.
1. Kavan, L., Electrochemical Carbon, Chem. Rev., 1997,
vol. 97, pp. 3061–3082.
The greater-than signs in the above values of the
enthalpy of reaction are related to the fact that the
experimentally determined heat effects refer to the sum
heat effect of the hydrolysis of niobium and tantalum
pentafluorides and metal fluorination. Under the
assumption that the nonvolatile products ï1 and ï2
have the same composition, from the enthalpies of reac-
tions (3) and (4) we can evaluate the standard enthalpy
of the reaction
2. Cadman, P. and Gossedge, G.M., The Chemical Interac-
tion of Metals with Polytetrafluorethylene, J. Mater. Sci.,
1979, vol. 14, pp. 2672–2678.
3. Carlo, S.R., Pery, C.C., Torres, J., et al., Surface Reac-
tions of Vapor Phase Titanium Atoms with Halogen and
Nitrogen Containing Polymers Studied Using In Situ
X-Ray Photoelectron Spectroscopy and Atomic Force
Microscopy, Appl. Surf. Sci., 2002, vol. 195, pp. 93–106.
4. Malkerova, I.P., Disproportionation Reactions and Ther-
modynamic Properties of the Lower Fluorides, Oxyfluo-
rides, and Trifluorides of the Group VIB Transition Met-
als, Cand. Sci. (Chem.) Dissertation, Moscow: Kurna-
kov Inst. of General and Inorganic Chemistry, 1983,
pp. 148–150.
Nb(s) + TaF5(g)
NbF5(g) + Ta(s),
(5)
∆rH0(5) ≥ – 55 21 kJ/mol,
which is easy to do from known data [7]: ∆rH0 = 65
34 kJ/mol. The good agreement between these values
suggests two important conclusions: first, the reactions
of metals (at least transition metals) can be described by
similar schemes; second, our assumption that hydroly-
sis makes an insignificant contribution to the total
enthalpy of reactions of the fluoropolymer with nio-
bium and tantalum is true.
5. Termodinamicheskie konstanty veshchestv: Spravochnik
(Thermodynamic Constants of Substances: A Hand-
book), Glushko, V.P. et al., Eds., Moscow:VINITI, 1974,
issue 7.
6. Osnovnye svoistva neorganicheskikh ftoridov: Sprav-
ochnik (Main Properties of Inorganic Fluorides), Galkin,
N.P., Ed., Moscow: Atomizdat, 1976.
7. Gotkis, I.S., Mass Spectrometric Vaporization Studies of
Niobium, Tantalum, and Tungsten Pentafluorides, Cand.
Sci. (Chem.) Dissertation, Ivanovo: IKhTI, 1975,
pp. 74–87, 1140–116.
CONCLUSIONS
8. Medvedev, V.A., Ftornaya kallorimetriya (Fluorine
The present results demonstrate that, in contrast to
what is commonly believed, fluoropolymers are rather
aggressive compounds and can be used as inert coatings
only at relatively low temperatures. If coatings are
Bomb Calorimetry), Moscow: Nauka, 1978.
9. Panshin, Yu.A., Malkevich, S.G., and Dunaevskaya, Ts.S.,
Ftoroplasty (Fluoroplastics), Leningrad: Khimiya, 1978.
INORGANIC MATERIALS Vol. 45 No. 7 2009