Improving the thermal properties of fluoroelastomer (Viton GF-600S) using acidic surface modified carbon nanotube

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T10501 Polymer Nanocomposite Synthesis and Irradiation

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Acid surface modified carbon nanotube (MCNT)-, Carbon nanotube (CNT)-filled fluoroelastomer (FE) and unfilled-FE were prepared (MCNT/FE, CNT/FE and FE). The compounds were subjected to thermogravimetric analysis (TGA) and heat air aging, and characterized by Energy Dispersive X-Ray (EDX). Results showed that MCNT improved the thermal properties of FE, resulting in a larger amount of FE and char remaining in the temperature range of 400-900 °C relative to unfilled FE and CNT/FE. The MCNT/FE TGA curve shifted towards higher temperatures compared to CNT/FE and FE. The same results also revealed that higher percentages of FE were undegraded or less degraded especially near MCNT in the temperature range of 400-540 °C. Energy Dispersive X-Ray (EDX) results indicated that the percentage of carbon and fluorine in the residue of TGA scans, up to 560 °C, of MCNT/FE were the same as CNT/FE, and were higher than FE. EDX results of TGA residue (run up to 900 °C) showed that most of the undegraded FE, which was not degraded at temperatures below 560 °C, was degraded from 560 °C to 900 °C in both MCNT/FE and CNT/FE, with the char in MCNT/FE being more than that in CNT/FE. EDX analysis of thermal aged specimens under air showed that, with increasing aging time, a greater percentage of C, O and F was lost from the surface of filler/FE and FE. The order of element loss after 24 hour aging time was: MCNT/FE > FE > CNT/FE.

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# S C I E N T I F I C - T E C H N I C A L

## Improving the thermal properties of fluoroelastomer (Viton GF-600S) using acidic surface modified carbon nanotube

Javad Heidarian 1,2 - , Aziz Hassan 1 and Nor Mas Mira Abd Rahman 1

_1Polymer and Composite Materials Research Laboratory, Department of Chemistry,_
_University of Malaya - UM, Kuala Lumpur, Malaysia_

_2Polymer and Science Technology Division, Research Institute of Petroleum Industry - RIPI, Tehran, Iran_

* _heidarianj@um.edu.my; heidarianj@yahoo.com_

Abstract

Acid surface modified carbon nanotube (MCNT)-, Carbon nanotube (CNT)-filled fluoroelastomer (FE) and unfilled-FE
were prepared (MCNT/FE, CNT/FE and FE). The compounds were subjected to thermogravimetric analysis (TGA) and
heat air aging, and characterized by Energy Dispersive X-Ray (EDX). Results showed that MCNT improved the thermal
properties of FE, resulting in a larger amount of FE and char remaining in the temperature range of 400-900 °C relative
to unfilled FE and CNT/FE. The MCNT/FE TGA curve shifted towards higher temperatures compared to CNT/FE and
FE. The same results also revealed that higher percentages of FE were undegraded or less degraded especially near
MCNT in the temperature range of 400-540 °C. Energy Dispersive X-Ray (EDX) results indicated that the percentage
of carbon and fluorine in the residue of TGA scans, up to 560 °C, of MCNT/FE were the same as CNT/FE, and were
higher than FE. EDX results of TGA residue (run up to 900 °C) showed that most of the undegraded FE, which was not
degraded at temperatures below 560 °C, was degraded from 560 °C to 900 °C in both MCNT/FE and CNT/FE, with the
char in MCNT/FE being more than that in CNT/FE. EDX analysis of thermal aged specimens under air showed that,
with increasing aging time, a greater percentage of C, O and F was lost from the surface of filler/FE and FE. The order
of element loss after 24 hour aging time was: MCNT/FE > FE > CNT/FE.

Keywords: _nanocomposites, fluoroelastomer, acidic surface modified, carbon nanotube, thermal properties, thermal_
_aging._

1. Introduction

Viton is a synthetic rubber and fluoroelastomer (FE).
Viton  GF-600S is a terpolymer of hexafluoro-propylene,
vinylidene fluoride and tetra-fluoroethylene with a cure
site monomer. It is a peroxide cure, 70% fluorine FE [1,2] .

Normally, in the formulation of FE, carbon black (CB)
is used. Replacing CB with surface modified CNT (MCNT)
is expected to improve the thermal properties of FE, which
is very effective in making O-rings especially for the oil
and gas industries [3-5] . MCNT is also expected to improve
thermo-oxidative degradation resistance of FE composites
when subjected to thermal aging conditioning in air.

MCNT improves the properties of fluoropolymers, such
as crystallinity, electrical response, mechanical properties,
viscoelastic behavior, etc, and therefore their thermal stability.
This finding has been reported by a number of researchers
and examples of literatures and the reasons for the changes
in these properties are mentioned bellow.

It has been reported [3] that the use of multi-walled carbon
nanotubes (MWCNTs)-embedded in fluorinated rubber
leads to a rubber nanocomposite with cellulation structure.
This structure possesses improved thermal resistance.
Pham et al. [4] observed that with increased MWCNT loading
in FE/MWCNTs nanocomposites, there is a steady increase
in the decomposition temperature. Interactions between
CNT and fluropolymers and the effect of surface area of
CNT on the active interfacial area between them were
investigated by a number of researchers, for examples:

A strong interaction between fluoropolymers and CNT was
reported by He et al. [6] . The high aspect ratio of nanotubes
also leads to a great increase in the active interfacial area
between CNT and the fluoropolymer chain as reported by
Chae et al. [7] . An energetic relationship between the surface
of CNT and fluoropolymer has been reported by Levi et al. [8] .

CNT can change the mechanical and thermo-mechanical
properties of polyvinilydienefluoride (PVDF) [9-11] or the
mechanical properties of polytetrafluoroethylene (PTFE)

[12,13]. CNT can also change the degree of crystallinity of
fluoropolymers and, therefore, their properties [14-16] .

Sementsov et al. [13] reported that in nanocomposites
of PTFE and MWCNT, the concentration and nature of
oxygen containing MWCNT surface groups influences the
strength parameters of the composite material. Wen et al. [17]
prepared (PVDF)/MWCNTs nanocomposites and studied
the melt viscoelastic behavior of the composites. The results
showed that PVDF/MWCNTs-g-OH (OH grafted)
nanocomposites exhibit more significant solid-like behavior
than PVDF/MWCNTs nanocomposites. Huang et al. [16]
showed that the interfacial interaction between the single
walled carbon nanotubes (with hydroxyl groups) and PVDF
has an effect on inducing crystallinity. The thermal stability
of the PTFE composites is also enhanced by the presence of
CNT as reported by Park et al. [18] . The temperatures recorded
at maximum decomposition rate were affected by the type
of surface modification of MWCNT. The thermograms

for PTFE/MWCNT composites also shifted towards high
temperatures.

Carabineiro et al. [19,20] prepared CNT/PVDF composites
using modified CNT samples. According to their research
results, surface modifications of MWCNTs had an effect
on the electrical response and the degree of crystallinity of
the CNT/PVDF nanocomposites. Similarly, Xu and Yang [21]
prepared FE/MWCNT composite films using different
surface modified CNTs. The better mechanical properties of
CF4 plasma-modified CNT (FCNTs) composites over other
studied CNTs is due to the better dispersion and enhanced
chemical compatibility from introducing electron-rich
fluorine atoms and also from hydrogen bonding.

The effect of thermal aging conditioning on FE in air
and/or other environments were also verified by researchers.
For example: the physical and chemical changes associated
with degradation on the surface, near the surface and bulk of
the FE seals in engine oil and additives were investigated by
Smith et al. [22] . Degradation of these materials was reported
to be limited to the near surface region of the samples to
a depth of less than approximately 50 μm. The process
resulted in the release of either F2 or HF.

Research on the verification of the effect of MCNT
on the amount of residues in TGA runs of MCNT/FE
nanocomposites is rather rare. In addition, comparison of
the amount of residue in TGA runs of MCNT/FE, CNT/FE
and FE detailing elemental composition is seldom published.
Therefore, this investigation is unique in the sense that
knowledge of the amount of residue and the composition of
the elements in the residue gives an insight into the ability of
MCNT to improve the thermal stability of CNT/FE and FE
composites. Furthermore, the presence of elemental fluorine
on the surface of MCNT/FE aged in air indicates whether
MCNT can improve the resistance of the nanocomposites
to thermo-oxidative degradation or not.

In the present work, MCNT was used as a filler for
Viton  GF-600S with the aim of improving the thermal
stability and thermal aging resistance of FE. The thermal
properties of the composites MCNT/FE were assessed by
Thermogravimetric Analysis (TGA) and compared vis‑à‑vis
those of CNT filled and unfilled FE composites in our
previous work [23] . Energy Dispersive X-Ray (EDX) was
also used to characterize the elemental composition of the
undegraded FE and char after TGA runs at temperatures
below 900 °C. This was to ascertain whether the incorporation
of MCNT could preserve more of the undegraded FE and
char compared to CNT/FE or unfilled FE. EDX was also
employed to verify the amount of Zn in the residue and to
confirm a possible reaction with MCNT. Thermal aging
conditioning in air together with EDX was also conducted
to investigate the elemental composition of the surfaces of
MCNT/FE as a function of aging time.

2. Experimental

_2.1 Materials and compounding procedure_

The materials used were Viton GF-600S fluoroelastomer,
FE; organic peroxide, Luperox 101 XL-45; acid surface modified
carbon nanotube MCNT (TNMC8, -COOH Content:0.49wt%),
carbon nanotube CNT (TNM8) both with an outside

diameter > 50 nm, inside diameter:5-15nm, purity > 95%,
and length 10-20 μm; zinc oxide; and triallylisocyanurate,
TAIC supplied by ERIKS Sdn. Bhd. (Malaysia), Arkema
Sdn. Bhd. (Malaysia), Chengdu Organic Chemicals Co. Ltd.
(Chinese Academy of Sciences, China), Texchem Materials
Sdn. Bhd. (Malaysia) and Liu Yang San Ji Chemical Trade
Co. Ltd. (China) respectively. Three formulations were
compounded, MCNT filled FE (MCNT/FE), CNT filled
FE (CNT/FE) and unfilled FE (FE). The amounts of FE,
organic peroxide, zinc oxide and TAIC in these compounds
were 70.0, 2.1, 2.1 and 2.1 g respectively. For MCNT/FE
and CNT/FE, 7.0 g filler was used.

Mixing FE with additives was done using a laboratory
scale two roll mill with a roll temperature of 48°C. FE in
the above mentioned composition was supplied to the open
roll. A uniform band was formed while three rolling cuts
from each side of the mill were made, so that the polymer
would be uniform and sufficiently warmed up. In the next
stage, pre-blended ZnO and TAIC were added uniformly
into the gum and three rolling cuts from each side of the
mill were made. After setting the roll distance to 1.1 mm,
MCNT was then fed in. The compound was then tightly
milled ten times. The roll distance was then adjusted to
1.1 mm, and the peroxide was added and, after the final
five to six rolled up end passes, the mixture was supplied
to the open roll and sheeted. After 24 hours, re-milling was
done with a roll temperature of 26 °C. A similar procedure
was used for CNT/FE and FE.

_2.2 Curing and post curing_

Curing of the FE compound was done in molds
(15 cm × 15 cm × 1 mm and 18 cm × 18 cm × 2 mm) in
a heated press, at 177 °C under a pressure of 10 MPa for
7 min. The post curing was done in an oven at 232 °C for
two hours. The conditions for curing and post curing were
recommended by the supplier.

_2.3 Thermal gravimetric analysis (TGA)_

25-35 mg of post-cured sample was subjected to TGA
runs from room temperature to 900 °C. This was carried
out on a Perkin-Elmer thermal analysis system, model Pyris
Diamond TG/DTA, at a scan rate of 20 °C.min –1 and under
nitrogen atmosphere of 20 ml.min –1 . In addition, TGA runs
from room temperature to 560 °C were carried out under the
same conditions and then cooled to room temperature at a scan
rate of 100°C.min -1 . The same procedure was repeated with the
temperature raised to 900 °C. The specimens were designated
according to temperature, for example MCNT/FE-560 °C,
where the temperature after the specimen designation is the
highest temperature of the TGA run. The same procedure
was applied for 3.5-5 mg of pure MCNT.

_2.4 Thermal aging_

Thermal aging was carried out according to ASTM
D-573 for 24, 48 and 72 hours at 250 °C. The samples
(5 mm × 5 mm × 2 mm) were named FE, FE-24, FE-48 and
FE-72, where the numbers following FE indicate the aging
time in hours. Similar types of abbreviation were used for
the MCNT/FE and CNT/FE aged samples.

_2.5 Energy Dispersive X-Ray (EDX) and Field Emission_
_Scanning Electron Microscopy (FESEM) image analyses_

EDX analysis was conducted using FESEM Philip
XL-40 (UK) coupled with EDX. The residue obtained from the
heating-cooling TGA scans was first applied on an aluminum
stub using double sided copper tape (3M Company), then
coated with gold for EDX elemental analysis to determine
the elements of C, O, F, Si, Ca and Zn.

Elemental compositions of the surfaces of the aged and
unaged samples were obtained using EDX. Un-aged and
aged samples, 5 mm × 5 mm × 2 mm in dimension, from
the molded post cured FE, CNT/FE and MCNT/FE rubber
sheets were used for EDX analysis. The major elements
characterized were C, O, F and Si. The surface images for
some of the samples were obtained by FESEM. In FESEM
imaging of MCNT/FE, the razor cut (cross-section) surface
was used. Specimens for EDX and FESEM imaging analysis
were applied on a copper tape and gold coated as mentioned
above. However, for FESEM images of residues obtained
from MCNT/FE-560 °C and MCNT/FE-900 °C, no gold
coating was used.

3. Results and Discussion

_3.1 TGA_

Figure 1 shows the TGA thermograms of MCNT, FE,
MCNT/FE and CNT/FE. The extracted data from these
plots are presented in Table 1. In Table 1, Tonset, T5%, T10%
and T50% are the onset temperature at which the compound
started to lose weight (degrade), and temperatures at which
the compound lost 5%, 10% and 50% weight respectively.
Tonset, T5%, T10% and T50% increased for MCNT/FE compared
to CNT/FE and FE. As can be seen from Figure 1 the
MCNT/FE curve shifted towards higher temperatures and the
thermal stability improved compared to CNT/FE. Figure 2
shows the amount of this temperature (thermal stability)
shift versus weight%. Figure 2 shows that for the weight%
of 100 to 40% the amount of thermal stability shift is
almost constant (≈10 °C). However, for weight% between
≈20 to 40% the thermal stability shift increases suddenly
(≈20ºC). This increase in thermal stability shift indicates that
in this weight% range compared to the 100 to 40 weight%
range, FE in MCNT/FE showed more thermal stability
improvement relative to FE in CNT/FE.

Pham et al. [4], used a SEM micrograph of the ash residue
collected after 90% degradation of the compound and showed
that less or partial degradation of the polymer chains attached
to the nanotubes occurred whereas extensive degradation
occurred in the bulk. Therefore, the higher thermal stability
shift for weight% between ≈20 to 40% represents the part
of FE which is near or attached to MCNT. All of the above
mentioned results show that FE in MCNT/FE is more

Table 1. TGA thermal properties of FE, filler/FE and pure filler.

Specimen TOnset (°C) T5% (°C) T10% (°C) T50% (°C)

FE 423.4 442.6 470.7 497.8
CNT/FE 421.1 446.2 475.4 505.1
MCNT/FE 433.1 457.7 485.4 513.7
MCNT 627.7 693.6 748.4 CNT 594.5 640.2 692.9 -

thermally stable than FE in CNT/FE under N2. Furthermore
in MCNT/FE, FE near MCNT is considerably more thermally
stable than FE in the bulk. FE near MCNT in MCNT/FE to
a larger extent is more thermally stable than both FE near
CNT and also FE in the bulk in CNT/FE.

It can be concluded that a highly filled FE with MCNT
will have a high percentage of FE near or attached to MCNT,
which can have thermal stabilities up to 540 °C. Therefore

Figure 1. Heat and cool TGA curves of pure MCNT, FE and
filler/FE: (a) zoomed and (b) unzoomed.

Figure 2. Thermal stability shift curve calculated from TGA curves
of FE and filler/FE.

it is predicted that a highly filled FE with MCNT can stay
up to 540 °C.

Figure 2 also shows the amount of this thermal stability
shift versus weight% for MCNT/FE relative to FE. Again the
MCNT/FE curve shifted towards higher temperatures and the
thermal stability improved compared to FE. Figure 2 shows
that, for the weight% of 100 to 50%, the amount of thermal
stability shift is almost constant (≈13-15 °C). However for
weight% between ≈20 to 50% the thermal stability shift
increases gradually (≈15-33 °C). This increase in thermal
stability shift shows that in this weight% range compared
to the 100 to 50 weight% range, FE in MCNT/FE has more
thermal stability improvement relative to FE in unfilled FE.

Once more, based on Pham et al. [4] work and the above
results it can be concluded that greater thermal stability shift
for weight% between ≈20 to 50% will be for the part of FE
which is near or attached to MCNT. All of the above mentioned
results show that FE in MCNT/FE is more thermally stable
than FE in unfilled FE under N2. Furthermore in MCNT/FE,
FE near MCNT is considerably more thermally stable than
FE in the bulk. FE near MCNT in MCNT/FE to a large extent
is more thermally stable than FE in unfilled FE.

By comparing the thermal stability improvement of FE
in MCNT/FE and CNT/FE, both relative to FE in unfilled
FE, it can be seen that for all weights% (100 to ≈20%)
the thermal stability improvement of FE in MCNT/FE is
considerably more than that of CNT/FE.

3.1.1 TGA: weight of residual

Figure 1 also shows that with the incorporation of MCNT,
a greater amount of the FE remained and produced char
preserve in the temperature range of 400-540 °C compared
to CNT/FE and FE. This amount was calculated as follows.

Table 2. The percentage of filler remaining in pure filler and
filler/FE at different temperatures.

Temperature (°C) MCNT MCNT/FE CNT CNT/FE
520 100.00 8.40 100.00 8.4
530 99.94 8.39 100.00 8.4
540 99.87 8.39 99.70 8.4
560 99.67 8.37 99.30 8.3
600 99.04 8.32 97.90 8.2
650 97.21 8.17 94.50 7.3
700 94.39 7.93 89.30 7.5
800 82.52 6.93 66.60 5.6
875 74.15 6.23 54.80 4.6

The percentage of “MCNT remaining” within the
nanocomposite can be calculated by multiplying the percentage
of MCNT obtained from the TGA data of pure MCNT at
that temperature by 8.4 (the initial percentage of MCNT in
FE). These amounts are shown in Table 2. From Figure 1
and Table 2, it can be seen that pure MCNT lost less weight
at temperatures up to 900 °C than pure CNT. Again, in
the TGA of the nanocomposite, at each temperature, the
percentage of “undegraded FE + char + mineral additives”
can be calculated by subtracting the percentage of “MCNT
remaining” in the nanocomposite (Table 2) from the
percentage of nanocomposite remaining in the TGA curve
extrapolated in Figure 1 at that temperature. The results
of these calculations are shown in Table 3. This amount,
minus the initial mineral filler (2.5%), gives the percentage
of “undegraded FE + char” as recorded in the same table.
The same calculations were done for CNT/FE. However, in
Table 3 the possible reactions of fillers at high temperatures
are not considered.

The results in Table 3 support the finding that, in
MCNT/FE, a greater amount of FE remained and produced
char preserve, particularly in the temperature range of
520‑540 °C, compared to CNT/FE and FE. Therefore,
the presence of MCNT enhances the thermal stability of
FE, resulting in a higher percentage of the nanocomposite
remaining undegraded at higher temperatures compared
to CNT in FE or unfilled FE. Besides, in MCNT/FE and
CNT/FE, most of the char was stable up to 900 °C.

3.1.2 EDX analysis: residual elemental

The composition of the residue from the heating‑cooling
TGA run cycle, as mentioned in the experimental section,
was characterized by EDX. The TGA curves of the samples
are shown in Figure 3. In the heating-cooling run, the
cooling was done with the highest possible cooling rate
in order to prevent further degradation and to keep the
same composition of the remaining materials at 560 °C
and at 900 °C upon cooling. Table 4 shows the elemental
compositions of the residues in heating-cooling TGA runs
for the compounds under study which were extracted from
the EDX results (Figures 4 and 5). Figures 4 and 5 also show
FESEM images of MCNT/FE-560 °C and MCNT/FE-900 °C
residues respectively.

From Figure 3 it can be seen that, at low temperatures,
after a cooling scan of the MCNT/FE-560 °C sample,
19.18% of the nanocomposite remained. If this amount

Table 3. The percentage of “undegraded FE + char + mineral additives” and “undegraded FE + Char” remained in the compounds at
different temperatures.

“undegraded FE + char + mineral additives” “undegraded FE + Char”
Temperature (°C)
FE MCNT/FE CNT/FE MCNT/FE CNT/FE
520 - 30.58 12.56 28.08 10.06
530 - 17.63 11.58 15.13 9.08
540 - 12.19 10.88 9.69 8.38
560 - 10.54 10.48 8.04 7.98
600 7.90 9.65 9.68 7.15 7.18
650 7.00 8.84 9.52 6.34 7.02
700 6.10 8.24 8.32 5.74 5.82
800 4.90 7.65 8.32 5.15 5.82
875 4.30 7.30 8.32 4.80 5.82

is multiplied by weight% of each element in Table 4, the
weight% of that element based on the initial nanocomposite
can be obtained. The same procedure was carried out for all

specimens. The results of these calculations are presented
in Table 5.

These results show that the percentage of carbon and
fluorine in the residue of the TGA scan of MCNT/FE-560 °C
are the same as CNT/FE-560 °C. This proves that MCNT is
able to preserve the same amount of FE from degradation
compared to the samples containing CNT at this temperature.
Similarly, based on the EDX results and considering the
work of Pham et al. [4] as mentioned earlier, it can be seen
that FE attached to or near MCNT at the higher temperature
range exhibits a lower degradation tendency. Considering
this and the fact that more carbon and fluorine remained in
this nanocomposite residue after the TGA run, it can also be
concluded that a greater amount of the FE remained undegraded
or less degraded especially near MCNT. The amount of
remaining fluorine represents the undegraded FE.

These results also show that the percentage of carbon and
fluorine in the residue of the TGA scan of MCNT/FE-560 °C
is higher compared to FE-560 °C. This proves that MCNT
Figure 3. Heat and cool TGA curves of FE and filler/FE up to is able to preserve more FE from degradation compared to
560 °C. the samples containing unfilled FE.

Figure 4. EDX spectra (a) and FESEM image (b) of
MCNT/FE-560 °C.

Figure 5. EDX spectra (a) and FESEM image (b) of
MCNT/FE-900 °C.

Table 4. The weight% (Wt%) and Atomic% (At%) composition of the remaining composites in TGA runs.

FE-560°C MCNT/FE-560°C CNT/FE-560°C MCNT/FE-900°C CNT/FE-900°C
Element
Wt % At % Wt % At % Wt % At % Wt % At % Wt % At %
C 27.16 50.48 69.78 84.46 68.98 80.89 83.87 92.01 65.42 83.62
O 14.00 19.53 5.34 4.86 10.05 8.85 6.14 5.06 8.95 8.59
F 11.07 13.01 9.11 6.97 10.45 7.75 1.55 1.07 2.54 2.05
Si - - - - 0.46 0.23 - - - Ca 3.05 1.70 1.45 0.53 0.83 0.29 1.20 0.40 2.11 0.81
Zn 44.72 15.28 14.32 3.18 9.23 1.99 7.23 1.46 20.98 4.93
Total 100 100 100 100 100 80.89 100 100 100 100

Table 5. Amount of element remained based on initial weight of compound.

Element remained (%)
Element
FE-560°C MCNT/FE-560°C CNT/FE-560°C MCNT/FE-900°C CNT/FE-900°C
C 3.42 13.38 13.13 10.69 6.26
F 0.85 1.75 2.00 0.20 0.24
Zn 2.08 2.75 1.76 0.92 2.00
Ca 0.27 0.28 0.16 0.15 0.20
Si - - 0.09 - -

3.1.3 EDX analysis: undegraded FE and char

At 560 °C, the “undegraded FE + char” for MCNT/FE-560 °C
can also be calculated by subtracting the percentage of
“MCNT remaining” in the nanocomposite (Table 2) from the
percentage of “C + F” in the EDX analysis at 560 °C. Thus, the
percentage of “undegraded FE + char” for MCNT/FE-560 °C
is 6.76 and for CNT/FE-560 °C is 6.98%, again showing
the same percentage of “undegraded FE + char” as in the
case of using MCNT or CNT as filler for FE.

Table 5 also shows that when MCNT/FE-900 °C and
CNT/FE-900 °C were run up to 900 °C, much less fluorine
and carbon remained compared to MCNT/FE-560 °C and
CNT/FE-560 °C respectively. These results indicate that
most of the undegraded FE at temperatures below 560 °C
was degraded from 560 °C up to 900 °C in both MCNT/FE
and CNT/FE.

At 900 °C, the “char” for MCNT/FE-900 °C can also
be calculated by subtracting the percentage of “MCNT
remaining” in the nanocomposite (Table 2) from the
percentage of “C + F” in EDX analysis at 900 °C. Thus, the
percentage of “char” for MCNT/FE-900 °C is 3.29% and for
CNT/FE-900 °C is 1.9%, showing more “char” remained
in the case of using MCNT as the filler for FE compared
to CNT. Therefore, it can be suggested that in MCNT/FE,
more char is stable up to 900 °C compared to CNT/FE.

Comparison of Zn% in CNT/FE-560 °C and
CNT/FE-900 °C (Table 5) shows that the amount of Zn
in CNT/FE was almost unchanged and did not undergo
any reaction up to 900 °C. On the other hand, the Zn%
in MCNT/FE-560 °C and MCNT/FE-900 °C shows that
a certain amount disappeared up to 900 °C. This is due
to the reaction of ZnO with carbon at high temperatures,
producing vaporized Zn. This reaction of ZnO and C at high
temperatures was reported by Gokon et al. [24] .

_3.2 Verifying thermal stability in FE composites_

The increased decomposition temperatures caused by
using MCNT in FE are due to several reasons, some of which
are the same as those mentioned for CNT in FE [4] . Firstly,

the presence of MCNT in FE makes the active centers of the
FE main chains inactive, preventing degradation, therefore
saving FE that is nearer to the MCNT surface. Secondly,
the interactions between MCNT and FE result in increased
physical and chemical cross-linking points which prevent
the degradation of the polymer chains. Besides, considering
that the degradation of FE is a radical chain reaction, it is
therefore susceptible to inhibition by reagents capable of
trapping such radicals. Furthermore, the antioxidant nature
of MCNT, attributed to its high electron affinity (≈ 2.65 eV),
enables it to act as a radical scavenger. Consequently, MCNT
helps to trap radicals and inhibits the degradation of FE
nearer to it. Moreover, Endo et al. [3] reasoned that the thermal
stability of FE near MCNT is due to the presence of bounded
rubber to MCNT and concluded that this structure could
prevent the decomposition of rubber at high temperatures
by resisting molecular mobility.

Wang et al. [25] reported that in PVDF/SiO2- grapheme
oxide (GO) composites there is a strong interaction between
many of the functional groups in the GO and PVDF chains.
In particular, the C-F bond in PVDF and carboxyl or hydroxyl
groups of grapheme oxides (GO) in the PVDF/SiO2-GO
composites form hydrogen bonds, and this supports the
enhancement of interfacial interaction. The same hydrogen
bonding can be formed between functional groups in MCNT
and C-F of FE. Ma and Larsen [26] also reported that nitric acid
treated single wall nanotubes (SWNT) compared to untreated
MWNT have better physical surface affinities with PVDF.
The same compatibility can be mentioned here between
MCNT and FE compared to CNT and FE. Therefore, there
is a strong interaction, which is from the hydrogen bonding
and compatibility described above, between MCNT and FE,
and these interactions are not present between CNT and
FE. More interactions between MCNT and FE compared
to CNT and FE result in increased physical and chemical
cross-linking points, which prevents more degradation of
the polymer chains in the case of MCNT/FE. Furthermore,
these interactions will increase the amount of bounded rubber
to MCNT in MCNT/FE compared to CNT in CNT/FE and,

as explained above, more bounded rubber will increase the
thermal stability in the case of MCNT/FE.

_3.3 Effect of thermal aging on residual elemental_
_composition_

Figure 6 illustrates representative figures of EDX
obtained for aged and un-aged samples. The elemental
weight percent extracted from these figures is presented in
Table 6. This table shows the surface composition of the
composites before and after aging. The major elements in
the composites are C, O, F and Si. The initial loss in element
is defined as the percentage lost due to aging for different
periods of time. The percentage loss can be calculated as
follows: For 100 g of un-aged FE (Table 6), there were
34.26, 4.99, 59.82 and 0.92 g of C, O, F and Si respectively.
After 72 hours of aging, if Crem, Orem, and Frem g of C, O and
F respectively remained and Clos, Olos and Flos g of C, O and
F respectively were lost. Then;

_Crem_ + _Clos_ = 34.26 ; _g_

+ _Clos_ =

+ _Olos_ =

+ _Flos_ =

34.26 ; _g_

59.82 _g_

_rem_ + _Clos_

_rem_ + _Olos_

_rem_ + _Flos_

_Orem_ + _Olos_ = 4.99 _g_ _and_

(1)

4.99 _g_

_Frem_ + _Flos_ = 59.82 _g_

Since Si is not lost during aging, the amount of Si after
72 hours of aging is 0.92 g. If T is the total weight of the
remaining elements, then; T = Crem + Orem + Frem + 0.92 g

After 72 hours of aging, the weight percentage of Si

is 2.32% (Table 6), therefore; 0.92 = 0.0232, and thus
_T_

T = 39.66 g. Using T and the percentage of C (Table 6),
after 72 hours of aging (30.89%), Crem can be calculated

as, 0.3089 = _Crem_

= _Crem_ . Thus, Crem = 0.3089 x 39.66 = 12.25 g.
_T_

Clos = 34.26 – Crem = 22.01 g. Therefore, the percentage
of Clos is,

_Clos_ (%) = _Wt ofinitialCWt ofC_ . . _los_ _x_

(2)

_Clos_ (%) = _Wt ofinitialCWt ofC_ . . _los_ _x_ 100(%)

22.01 _x_ 100(%) = 64.24%
34.26

_los_ (%) = _Wt ofC_ . . _los_

= _los_ _x_ 100(%) =

with CNT/FE was observed. However, as the aging time
increased, there was a drastic increase in percentage loss.
FE underwent thermal oxidation and dehydrofluorination
during hot air aging [27] . Due to the dehydrofluorination
reaction, HF was produced [27] and the carboxylic acidic groups
on the surface of MCNT reacted with HF. Wang et al. [28]
reported a similar reaction between dispersed graphene oxide
(GO) and hydrofluoric acid (HF) during the synthesis of
fluorinated grapheme sheets (FGS). The authors mentioned
that fluorine was grafted onto the basal plane of graphene.
Furthermore, the oxygen-containing groups in GO played
a major role in FGS formation, and the fluorination degree
could be easily controlled by varying the reaction temperature,
times and amounts of HF. They also stated that fluorination

Figure 6. EDX spectra of (a) MCNT/FE and (b) MCNT/FE-72.

_x_

This means 64.24% of the initial C was removed from
the surface after 72 hours of aging at 250 °C. The same
calculations were done for F and O in all other aged
composites. The results of these calculations are presented
in Table 7. These results show that for each composite, with
increasing aging time, the percentage loss of C, O and F
increased. This is because, as the aging time increased, more
degradation including dehydrofluorination, defluorination
and carbon oxygen reactions occurred especially at the
surface [27] . The results also show that at each aging time, for
CNT/FE the loss of elements was lower than for MCNT/FE.
Therefore, CNT can preserve a larger portion of the elastomer
surface from degradation compared to MCNT.

For CNT/FE, up to 24 hours of aging, the initial percentage
loss of each element slightly increased. Nevertheless, as the
aging time increased, even a decrease in percentage loss was
observed. This means that the slight degradation happened
mainly in the first 24 hours and did not increase after that.
For MCNT/FE, up to 24 hours of aging, the same trend as

Table 6. The surface composition of the composites before and after aging, obtained by EDX.

Wt % At % Wt % At % Wt % At % Wt % At %
Element
FE FE-24 FE-48 FE-72
C 34.26 44.95 - - 30.65 41.04 30.89 41.37
O 4.99 4.92 - - 3.94 3.96 04.75 04.78
F 59.82 49.62 - - 64.02 54.20 62.03 52.52
Si 0.92 0.52 - - 1.40 0.80 2.32 1.33
Total 100 100 - - 100 100 100 100
Element MCNT/FE MCNT/FE-24 MCNT/FE-48 MCNT/FE-72
C 34.06 44.64 30.46 40.63 31.46 42.11 26.58 36.95
O 6.41 06.31 7.36 7.37 7.07 7.10 12.95 13.51
F 58.5 48.48 60.51 51.04 56.99 48.23 47.77 41.99
Si 1.02 0.57 1.68 0.96 4.48 2.56 12.69 7.55
Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
Element CNT/FE CNT/FE-24 CNT/FE-48 CNT/FE-72
C 35.43 46.32 36.75 47.92 31.99 42.60 30.90 41.39
O 3.60 3.53 2.79 2.70 3.53 3.53 3.69 3.71
F 59.99 49.60 58.56 48.32 62.92 52.98 63.63 53.89
Si 0.98 0.55 1.91 1.07 1.56 0.89 1.78 1.02
Total 100 100 100 100 100 100 100 100

occurred at the original sp 3 C sites in GO, that is, the C sites
connected with the oxygen-containing groups. In the case
of MCNT/FE, this reaction caused increased degradation
particularly at aging times higher than 24 hours, leading to
more loss of F and C from the surface compared to CNT/FE.

Comparing the results of FE and MCNT/FE at aging
times longer than 24 hours, it can also be seen that for FE
without MCNT, the loss of elements was lower than for FE
with MCNT. This could be due to the same reasons as those
given above in the comparison of MCNT/FE and CNT/FE.

The reasons adduced for the saving of a higher percentage
of undegraded FE and char in the TGA runs are also valid
for the thermal aging of MCNT/FE in the first 24 hours.
Besides, the antioxidant nature of MCNT enables it to act
as a radical scavenger [4] especially for the oxide radicals
produced due to the presence of air. This also helps to trap
radicals, thereby inhibiting the degradation of nearby MCNT,
and results in an increased percentage of undegraded FE.

A considerable depletion of F and C on the surface and
to a depth of around 10-15 μm of the cross-section of the
CB/FE seals aged in oil containing amine based dispersant
at 150 °C was also reported by Smith et al. [22] . They also
reported the catalytic effect of amine on the degradation of
FE. The authors reported that polytetrafluoroethylene (PTFE)
also began to soften and release F at around 200-220 °C.
This may explain the apparent susceptibility to heating in
air shown by elastomers with a high TFE content. A drastic
decrease in C and F due to thermal aging in air at 200 °C
from the surface of CB/FE (up to a depth of 5 nm) was also
reported by Wang et al. [27] .

Figure 7 shows the FESEM image of MCNT/FE razor
cut surfaces. The very good distribution and dispersion of
MCNT in FE can be seen in this figure.

_3.4 Other works on FE and fillers/FE_

In our before study [29], we verified the crystalinity of
some of the above mentioned FE and fillers/FE, by Dynamic
mechanical analysis (DMA), Differential Scanning Calorimetry

Table 7. Initial element loss percentage after aging compared to
un-aged samples.

Initial Element loss%
Element
FE-24 FE-48 FE-72
C - 41.21 64.25
O - 48.11 62.25
F - 29.67 58.88
Si - 0 0
Element MCNT/FE-24 MCNT/FE-48 MCNT/FE-72
C 45.70 78.97 93.73
O 30.29 74.89 83.76
F 37.20 77.82 93.44
Si 0 0 0
Element CNT/FE-24 CNT/FE-48 CNT/FE-72
C 46.73 43.28 51.98
O 60.19 38.40 43.57
F 49.86 34.11 41.60
Si 0 0 0

Figure 7. FESEM image of MCNT/FE (the razor cut cross section
surface).

(DSC) and X-ray diffraction (XRD). Verifying thermal
properties of fluoroelastomer using carbon nanotubes in
presence of air and under nitrogen flow is the other study [30] .

4. Conclusions

TGA shows that the thermal stability of FE in a N2
atmosphere increased with the presence of MCNT relative to
when either CNT was used or while FE was unfilled. A higher
percentage of this composite in the form of undegraded FE
or char remained within the temperature range of 400‑900 °C
compared to CNT/FE or FE. The MCNT/FE TGA curve
shifted towards higher temperatures compared to CNT/FE or
FE. The same results also revealed that a higher percentage
of FE was undegraded or less degraded especially near
MCNT in the temperature range of 400-540 °C.

EDX analysis shows that the percentage of carbon
and fluorine in the residue of TGA runs up to 560 °C of
MCNT/FE was the same as CNT/FE and higher than that
of FE. EDX of the residue of TGA runs up to 900 °C shows
that most of the undegraded FE which was not degraded at
temperatures up to 560 °C was degraded from 560° up to
900 °C in both MCNT/FE and CNT/FE. However, the char
in MCNT/FE was more than that in CNT/FE at 900 °C.

The EDX results of aged specimens under air indicate
that the percentage of C, F and O lost on the surface of
filler/FE and FE increased with increasing aging time.
Again, at 24 hour aging time, MCNT preserved a higher
percentage of these elements compared to CNT. The order
of percentage loss in elements after 24 hour aging time was
MCNT/FE > FE > CNT/FE.

5. Acknowledgements

We thank the University of Malaya as the work reported
in this paper was funded under the grant numbers UMRG
(RG322-15AFR) and BKP (BK005-2014).

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_Received: Dec. 27, 2014_

_Accepted: Apr. 29, 2015_

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