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In this work, the kinetics of the reaction of tungsten with
TFE–VDF was studied by thermogravimetry, calorimetry, and high-
temperature mass spectrometry.
basis of the discrepancies between the calculated and experimental
values.
The Peak Separation program was used for decomposing
polymodal DSC peaks into components, processing TG curve
derivatives, and searching for a best-fit function describing the peak
shape.
2. Experimental
The commercially available TFE–VDF copolymer F42 (Rus-
sian State Standard GOST 25428-82) containing 21 mol% TFE and
79 mol% VDF was used. A tungsten metal powder was prepared by
filing a metal ingot with a diamond file. The average particle size
was 80 m. The W content was 98 mol%.
The DTA/DSC Corrections program was used for recovering the
true DSC signal shape, determining the thermal resistance and time
constants of the sample-holder system on the basis of reference
data (Hg, In, Bi, Pb), and applying corrections to the original DSC
curve shape after the determination of heat transfer parameters.
Samples of metal–polymer composites were molded by hot
pressing of mechanical metal–polymer mixtures on a table-top
pneumatic press with heated plates (Gibitre Instruments). A rect-
angular mold, 50 mm × 10 mm, with a heater was used. The metal
and fluoropolymer powders were taken in amounts such that the
molar ratio of tungsten to the fluorine in the polymer sample was
1:6. They were placed into the press mold and heated to 423 K;
then a load of 200 atm was applied for 5 min, the press mold was
cooled to 353 K, the pressure load was removed, and the sample
was taken from the mold. The punch and die of the press mold
were covered with a Teflon film after each hot pressing event to
avoid sticking of samples. The final composite specimen was a plate
50 mm × 10 mm with a thickness of 1–1.2 mm. From this plate, rect-
angular specimens 3 mm × 2 mm were cut for mass spectrometric
and calorimetric studies.
3. Results and discussion
3.1. Mass spectrometry
Thermodynamic analysis showed that the fluorination reactions
1
n
M(solid)
+
(C33H25F43
)
→ MFx(gas) + X(solid)
n(solid)
involving different metals presumably follow the same mechanism.
various temperatures were obtained by mass spectrometry (Fig. 1a
and b).
The curves are best fitted by the Prout–Tompkins nth-order
autocatalytic equation [5]
Simultaneous TG–DSC was carried out on a NETZSCH STA
449C Jupiter thermoanalyzer equipped with an electromagnetic
microbalance with top loading. The TG resolution was 0.1 g. The
relative error of weight determination was 0.5%. A highly sensitive
sample carrier with Pt/Pt–Rh thermocouples was used. Measure-
ments were taken in the temperature range 313–873 K under an
argon flow (50 ml/min) at a heating rate of 5, 10, 15, and 20 K/min.
Sample weights were 10–15 mg. Gold crucibles with lids were used.
Before measurements, the chamber with a sample was evacuated
and filled with argon (Ar, ≥99.993 vol%; water vapor, ≤0.0009 vol%).
Calorimetricexperiments werecarriedout on aNETSCHDSC 204
Phoenix calorimeter in a dynamic argon atmosphere (50 ml/min).
Temperatures were calibrated against the melting points of Hg, In,
Zn, Bi, and Pb and the temperature of the phase transition in CsCl.
The gaseous reaction products were identified by mass spec-
trometry on a MS-1301 mass spectrometer. Molybdenum effusion
cells with an evaporation/effusion surface area ratio of 600 were
measured with a Pt/Pt–Rh thermocouple and maintained with an
accuracy of 1 K. The interaction of the fluoropolymer and the
metal was studied in the temperature range 550–585 K. Kinetic
curves were recorded by monitoring a change in the intensity of the
d˛
dt
E
ln
= ln A −
+ n · ln(1 − ˛) + a · ln ˛,
RT
˛ = 1 value in Fig. 1b is the maximum value of the WF5+ concentra-
tion. For n = 1, the kinetic curves in the coordinates of this equation
are linearized with R ≥ 0.997 at ˛ = 0.2–0.8, which is evidence that
it formally corresponds to the mechanism of the process of the
physical model chosen (Fig. 1c). Thus, we determined the kinetic
parameters (E/R, n, a) of the reaction for the conversion ˛ = 0.2–0.8
and for the entire kinetic range (Table 1).
In this case, E is the activation energy of formation of a mole of
tungsten hexafluoride from tungsten metal and the F42 fluoropoly-
mer. In the ˛ range of 0–1, the activation energy is considerably
higher than that for ˛ from 0.2 to 0.8 since the former involves
energy-consuming stages, such as the induction period, the reac-
tion acceleration onset, and the deceleration of fluorination where
diffusion processes are superimposed.
The interaction involves not only the chemical reaction but also
mass and heat transfer processes, which can considerably distort
the early and late stages. At these stages, the formation of WF5
is not necessarily the rate-limiting stage. Therefore, the chosen ˛
range 0.2–0.8 corresponds with high probability to the formation
The autocatalytic character of the reaction is explained by the
formation of unsaturated bonds in the carbon backbone of the poly-
mer since some of its fluorine atoms bind to tungsten, which leads
to a decrease in the C–F bond energy of neighboring CF2 groups and
an increase in reaction rate [6].
184WF5 ion produced from the WF6 molecule [3] as a function of
+
time and temperature. The intensity is similar to the reaction rate,
i.e., the concentration per unit time. To determine the dependence
of concentration on time, the intensities were integrated over time
and the resulting values were normalized: c(ti)/c(tmax) = ˛i(t) Here,
tmax is the time when the WF5+ ion intensity is zero, i.e., when WF6
was released and completely removed from the system. The result-
ing isothermal curves were processed with the Origin program. The
kinetic data obtained in the dynamic experiments were processed
using Netzsch software.
The Proteus Thermal Analysis program was used for processing
the raw TG and DSC curves, determining heats and weight changes,
and smoothing and differentiating the curves.
The Thermokinetics program was used for processing kinetic
dependences and solving the direct and inverse kinetic problem.
There were tried 18 different models. For all models under consid-
eration, a set of statistical criteria of adequacy was used according
to the Netzsch procedure. The optimal model was selected on the
According to the Prout–Tompkins model, solid-state hetero-
geneous reactions follow a branched mechanism; i.e., cracks
and boundary lines along which the reaction propagates are
Table 1
+
Kinetic characteristics of WF5 formation.
System
W–F42
˛
n
a
E/R × 103, K
0.2–0.8
0–1
1
1.6
1
0.8
2.0 0.4
10.0 2.5
Tarasov
