PRÜFEN UND MESSEN
TESTING AND MEASURING
Silicone rubber Fluororubber Fluorosilicone rubber Limiting oxygen index Thermogravimetric analysis Hot oil resistance
The paper reports the results of Limiting
oxygen index (LOI) measurements, thermogravimetric (TG) analysis and hot oil resistance (ASTM #1 and IRM 903 oils both at
150 8C) of silicone rubber, fluororubber
based on tetrafluoroethylene/propylene/ vinylidene fluoride terpolymer and their
blends. Fluorosilicone rubber is included in
the studies as a control. The results of LOI
measurements reveal that the fluororubber
has higher flame resistance than that of the
silicone rubber, while the blends show
flammability characteristics of the silicone
rubber, which forms a continuous phase in
the blend. As expected the fluororubber
exhibits greater hot-oil resistance than that
of the silicone rubber, but the blend of the
two rubbers, when immersed in hot oil,
shows retention of mechanical properties,
which are either higher or in between that
of the constituent polymers. The initial decomposition temperatures (corresponding
to 3 % mass conversion) of silicone rubber/
fluororubber blends are higher than that of
the constituent rubbers. It is evident that
the flammability characteristics of the
blends are determined by the blend morphology, while the blend composition and
the interaction between the blend components control the oil resistance and the
thermal stability of the blends. Fluorosilicone rubber behaves like silicone rubber in
flammability, but in respect to oil resistance
and thermal stability it is similar to that of
the silicone rubber/fluororubber blends.
Untersuchungen zur Entflammbarkeit,
thermischen Stabilität und Heißölbeständigkeit von Silikonkautschuk,
Fluorkautschuk und deren Verschnitte
Silikonkautschuk Fluorkautschuk Fluorsilionkautschuk Flammfestigkeit Ölbeständigkeit thermische Beständigkeit
Es wird über Messungen der Flammfestigkeit (LOI), der thermischen Zersetzung und
der Heißölbeständigkeit an Silikonkautschuk, Fluorkautschuk und deren Verschnitte sowie an Fluorsilikonkautschuk
berichtet. Die Ergebnisse der LOI- Untersuchung zeigen eine höhere Flammbeständigkeit bei Fluorkautschuk als bei Silikonkautschuk. Verschnitte in denen Silikonkautschuk die kontinuierliche Phase bildet, liegen auf dem Niveau von reinem Silikonkautschuk. Wie erwartet ist die Ölbeständigkeit von FKM höher als von Silikonkautschuk. Verschnitte der beiden Kautschuke
haben öfter höhere mechanische Eigenschaften als der arithmetische Mittelwert.
Die thermische Zersetzung der Verschnitte
setzt bei höheren Temperaturen ein als bei
den Reinkomponenten. Fluorsilikonkautschuk verhält sich wie Silikonkautschuk
bezüglich der Flammfestigkeit, ist jedoch
bezüglich der Ölbeständigkeit und der thermischen Stabilität den Verschnitten ähnlich.
96
Studies on Flammability,
Thermal Stability and Hot Oil
Resistance of Silicone Rubber,
Fluororubber and Their Blends
A. Ghosh, P. P. De, S. K. De, Kharagpur (India), M. Saito,
Chiba (Japan) and V. Shingankuli, Bangalore (India)
The stability of rubber vulcanizates depends upon the type of rubbers and
the service environment. The effects of
oil on rubber depend on a number of factors that include type of the rubber compound, composition of the oil, temperature and time of exposure. Polymer
blending and modification are important
routes for achieving oil resistance, as demanded in the automotive industries [1 –
5]. Thermal stability of silicone rubber is
ascribed to the high bond strength
(110 Kcal/mole) of the main chain bonds
(Si-O) [6]. The thermal stability of the
fluororubber is attributed to the presence
of ÿ CF2 groups in the main chain backbone [7]. Fluororsilicone rubber, which
has similar polymer structure as that of silicone rubber with additional fluoro substituent groups on the polymer chain,
has good fuel, oil and solvent resistance
in addition to the properties of silicone
rubbers, such as wide service temperature range.
Blends of silicone rubber and fluororubber were chosen in the present investigation with the objective of finding out if
such blends, could act as a replacement
of the fluorosilicone rubber. Ghosh et al
[8] have reported that the blends of silicone rubber and fluororubber are technologically compatible and the low viscous
silicone rubber forms a continuous matrix, with the highly viscous fluororubber
forming the dispersed phase. Mechanical
properties of the blends either follow the
additive rule or show synergism, indicating co-crosslinking and consequent
technological compatibility. Dielectric
properties measurements [9] and dynamic mechanical spectra [8] reveal
that the blends of silicone rubber and
fluororubber are thermodynamically immiscible.
The present paper aims at studying
flammability, thermal stability and hot oil
resistance of silicone rubber, fluororubber
and their blends. For comparison, a
fluorosilicone rubber has been included
in the studies.
Experimental
Materials
Fluororubber [AFLAS 200], based on tetrafluoroethylene/propylene/vinylidene
fluoride terpolymer with a specific gravity
of 1.55, was provided by Asahi Glass Co.,
Yokohama, Japan. Silicone rubber with a
specific gravity of 1.21, and fluorosilicone
rubber (FS) with a specific gravity of 1.40
were supplied by GE Bayer Silicone Pvt.
Ltd., Bangalore, India. Dicumyl peroxide
[DCP] with a purity of 98 % and melting
point of 39 – 41 8C and 2,4,6-triallyloxy1,3,5-triazine [TAC] of 97 % active were
obtained from Aldrich Chemicals Company, Inc., Milwaukee, WI, USA. Laboratory grade calcium hydroxide [Ca(OH)2] in
powder form was supplied by s.d.fine
Chem. Ltd., Mumbai, India. ASTM # 1
oil with aniline point of 124 1 8C and
IRM 903 oil (used as a replacement for
ASTM # 3 oil) with aniline point of
70 1 8C were provided by GE Bayer Silicone Pvt. Ltd., Bangalore, India [10].
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 3/2003
Studies on Flammability, Thermal Stability . . .
Table 1. Formulations
Ingredients
0/100
Silicone rubber
–
Fluororubber
100
Fluorosilicone rubber
–
DCP
2
TAC
5
Ca(OH)2
5
Optimum cure time (min)
5.0
a
Table 2. LOI of different rubbers and rubber blends.
Mix Symbol
a
25/75
a
50/50
25
75
–
2
3.75
3.75
3.2
50
50
–
2
2.5
2.5
1.5
a
75/25
75
25
–
2
1.25
1.25
1.0
a
100/0
a
100
–
–
2
–
–
1.0
FS
–
–
100
2
–
–
1.9
Mix Symbol
LOI
0/100a
25/75a
50/50a
75/25a
100/0a
FS
39
28
25
25
23
22
a
Silicone rubber/Fluororubber, by weight
Silicone rubber/Fluororubber, by weight
Result and discussions
Limiting oxygen index (LOI)
Preparations of rubber vulcanizates
Thermogravimetric analysis
The formulations are given in Table 1. The
compounds were prepared by using a
Brabender plasticorder (model PLE330; Brabender OHG, Duisburg, Germany) at 80 8C and a rotor speed of
60 rpm. For the preparation of the silicone
rubber compound, first silicone rubber
was charged and sheared for 2 min and
then DCP was added and mixed for another 2 min. Similarly, for the preparation
of the fluororubber compound, fluororubber was first charged into the plasticorder
and sheared for 2 min and then DCP, TAC
and Ca(OH)2 were added and mixing continued for another 3 min. In the case of
blend preparation, first fluororubber was
sheared and then silicone rubber was
added and mixed for 2 min and finally
DCP, TAC and Ca(OH)2 were added and
mixed for additional 3 min.
Thermogravimetric analyses were performed in a TG/DTA 220 from Seiko Instruments Inc. Japan in the temperature
range between 0 and 600 8C at a heating
rate of 10 8C/min under dynamic mode.
The sample weight was inbetween 15
and 20 mg. All the experiments were performed under 200 mL/min of blowing air
atmosphere.
Molding
Percent change in mass
Thin sheets of approximately 2 mm thickness were prepared by molding the rubber compounds according to the respective optimum cure times (given in Table 1)
in a hydraulic press at 170 8C and at a
pressure of 5 MPa. The optimum cure
times were determined at 170 8C, using
a moving die rheometer (Monsanto model MDR 2000; Monsanto, St. Louis, MO,
USA). After molding, the samples were
post-cured at 200 8C for 24 h.
¼
Limiting oxygen index
Limiting oxygen index (LOI) measurements were carried out in a Oxygen Indexer manufactured by Toyoseiki, Ltd.,
Japan using the UL method according
to ASTM D 2863.
Hot oil ageing
The oil-ageing test was conducted according to ASTM D 471. The test specimens were immersed in ASTM oil #1 and
IRM 903 oil at 150 8C for 210 h. The test
specimens were then removed from the
oils, wiped with tissue paper to remove
the excess oil from the surface and
weighed. The percent mass swell was
then calculated as follows:
w2 ÿ w1 Þ
100
w1
ð1Þ
where w2 and w1 are the masses of the
swelled and initial dried sample respectively.
The tensile strength (TS) and elongation at break (EB) of the samples before
and after oil ageing were measured according to ASTM D412-98 using dumbbell-shaped test pieces in a Zwick Universal Testing Machine (UTM, model 1445;
Zwick GmbH & Co., Ulm, Germany) at
25 8C. The hardness was determined
as per ASTM D2240 (1997) and expressed in Shore A units.
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 3/2003
The LOI values of different compositions
are shown in Table 2. The fluororubber
shows higher flame resistance than that
of silicone rubber, while flame resistance
of fluorosilicone rubber is closer to that of
silicone rubber. The flame resistance of
the blends is similar to the silicone rubber.
It has been observed earlier in scanning
electron microscope (SEM) studies that
in silicone rubber/fluororubber blends
the fluororubber is dispersed in a continuous matrix of the silicone rubber [8]. It is
believed that the flammability is controlled
by the morphology of the blend and the
continuous silicone rubber phase overshadows the flame retardance effect of
the fluororubber. It has also been reported [8] that the domain size of the
75/25-fluororubber/silicone rubber blend
is higher than that of the other blends,
and accordingly the limiting oxygen index
value is slightly greater than that of other
blend ratios.
Thermogravimetric analysis
The TG and DTG curves of different compositions are shown in Figures 1(a – f). The
degradation temperatures obtained from
these thermograms are summarized in
Table 3. The initial decomposition temperature (Ti ) corresponding to 3 % mass
decomposition of the silicone rubber is similar to that of the fluororubber and the
fluorosilicone rubber. The Ti of the blends
are marginally higher than that of the constituent rubbers, which is presumably
due to interphase crosslinking between
the two rubber phases [9].
The temperature range of different decomposition stages and the corresponding weight loss of the rubber vulcanizates
are summarized in Table 3. The structure
97
Studies on Flammability, Thermal Stability . . .
Figure 1. TG and DTG thermograms at the heating rate of 10 8C/min: (a) 100/0, (b) 0/100, (c)
75/25, (d) 50/50, (e) 25/75silicone rubber/fluororubber and (f) FS.
of the silicone rubber is based on vinyl
methyl siloxane. The silicone rubber exhibits two-stage decomposition (Fig. 1a):
the first and minor stage decomposition
at lower temperature range (i. e., in the
temperature range of 338 to 428 8C),
where weight loss corresponds to 8 %,
may be assigned to that of breakage of
the vinyl group and the second-stage decomposition (i. e., in the temperature
range of 424 to 533 8C) is due to the dimethyl siloxane unit of the silicone rubber,
where weight loss is about 37 %. The
fluororubber is degraded upto 7 % conversion very slowly in the temperature
range of 303 to 420 8C beyond which
the rubber is degraded in three different
stages (Fig. 1b). The first major stage
decomposition (420 to 499 8C), which
corresponds to the weight loss of 67 %,
may be ascribed to the decomposition
of the main chain backbone. The second
and third stage-decompositions at higher
temperature, which result in lower
amount of weight loss (11 and 10 % respectively), may be ascribed to the presence of some block segments of the
monomers. The blending of the silicone
rubber and the fluororubber by interphase crosslinking results in merging of
different decomposition stages and the
resultant blends exhibit three-stage decomposition (Fig. 1c – e) after the mass
conversion of 5 %. It is observed that
the thermal behaviour of the blends is
controlled the fluororubber phase. The
TG thermograms (Fig. 1a – f) also exhibit
that the decomposition rate of the blends
and the fluororubber are higher than that
of the silicone rubber and the fluorosilicone rubber. The liberation of hydrofluoric
Table 3. Different stages of decomposition and the residues of the rubber vulcanizates
Mix
Symbol
100/0a
75/25a
50/50a
25/75a
0/100a
FS
a
b
Ti b
371
399
390
385
373
403
First–stage
Second–stage
Third–stage
Fourth–stage
Temperature
range ( 8C)
Weight
loss (%)
Temperature
range ( 8C)
Weight
loss (%)
Temperature
range ( 8C)
Weight
loss (%)
Temperature
range ( 8C)
Weight
loss (%)
338 – 424
350 – 414
314 – 410
314 – 410
303 – 420
372 – 412
8
5
5
5
7
5
424 – 533
414 – 469
410 – 476
410 – 488
420 – 499
412 – 495
37
54
61
71
67
52
–
469 – 481
476 – 493
488 – 533
499 – 525
495 – 518
–
11
12
13
11
10
–
481 – 510
493 – 494
533 – 534
525 – 540
–
–
3
8
6
10
–
Residue
(%)
55
27
13
5
5
33
Silicone rubber/fluororubber, by weight.
Ti corresponds to 3 % conversion.
98
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 3/2003
Studies on Flammability, Thermal Stability . . .
Figure 2. (a) Retention of tensile strength (%) (i) ASTM oil #1 and (ii) IRM 903 oil. (b) Retention of elongation at break (%) (i) ASTM oil #1 and (ii)
IRM 903 oil.
acid (HF) from the fluororubber during
thermal degradation promotes the rate
of decomposition of the fluororubber
and the silicone rubber/ fluororubber
blends.
The residues obtained after decomposition of the rubber vulcanizates are also
summarized in Table 3. The silicone rubber yields about 55 % of residue on decomposition, which is silica (SiO2). This
silica corresponds to the silica, which is
present as filler and the silica obtained
from the silicone polymer on thermal degradation at higher temperature. The
fluororubber results only about 5 % of residue on thermal degradation. The
blends of silicone rubber and fluororubber, however, on thermal degradation
give residues of lesser amount than expected. It is envisaged that during thermal degradation HF is liberated from
the fluororubber, which reacts with SiO2
obtained from the silicone rubber, resulting in the formation of volatile silicon tetrafluoide (SiF4). The fluorosilicone rubber
gives about 33 % of residue on thermal
degradation, primarily due to the presence of silica filler in the rubber.
Table 4. Physical properties of rubber vulcanizates
Physical properties
Tensile strength (MPa)
Elongation at break (%)
Hardness (Shore A)
a
100/0a
0/100a
7.9
212
74
6.3
834
40
Silicone rubber/Fluororubber, by weight
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 3/2003
Mix Symbol
50/50a
6.6
302
70
FS
8.7
226
57
Hot oil ageing
The mechanical properties of different
compositions before oil ageing are summarized in Table 4. Figure 2a shows that
the percent retention of tensile strength
(TS) of fluororubber is always higher
than that of silicone rubber after immersion in oil. The tensile strength of the
50/50 silicone rubber/fluororubber blend
does not change significantly when exposed to hot ASTM oil #1 for 210 h at
150 8C. Irrespective of the nature of the
oil and ageing period, the 50/50 silicone
rubber/fluororubber blend possesses
greater retention in tensile strength than
that of the constituent rubbers and the effect is more pronounced in the case of
ASTM oil # 1. The greater retention in tensile strength of the 50/50 blend may be
due to the interphase crosslinking between silicone rubber and fluororubber
phases [9]. The retention of tensile
strength ( %) of fluorosilicone rubber is in-
99
Studies on Flammability, Thermal Stability . . .
Table 5. Effect of hot oil ageing on hardness and mass of rubber vulcanizates
Mix Symbol
100/0a
0/100a
50/50a
FS
a
ASTM # 1 oil
Hardness
(point change)
Mass change
( %)
IRM 903 oil
Hardness
(point change)
Mass
change ( %)
ÿ4
ÿ6
ÿ5
ÿ2
þ5
0
þ3
0
ÿ 29
ÿ5
ÿ 16
ÿ3
þ 35
þ4
þ 15
þ3
Silicone rubber/ fluororubber, by weight.
between that of the fluororubber and silicone rubber.
Figure 2b shows the percent retention
of elongation at break (EB) for various
compositions. The EB of the fluororubber
in hot ASTM oil # 1 and IRM 903 oil at
150 8C for 210 h remains almost constant. On the other hand, silicone rubber
shows poorer EB after immersion in different oils. EB retention of the 50/50 silicone rubber/fluororubber blend is almost
similar to that of fluororubber in ASTM oil
#1. It is also observed that the percent retention of EB on hot oil ageing is greater in
the case of the blend, as compared to the
fluorosilicone rubber. However, in IRM
903 oil the percent retention of EB of
the 50/50 blend is inbetween of the constituent rubbers.
Table 5 shows the mass swell ( %) of
different compositions in ASTM oil #1
and IRM903 oil. It is observed that the
fluororubber is highly resistant to mass
swell in both the oils, as compared to
the silicone rubber. Furthermore, the
swelling behaviour of the 50/50 silicone
rubber/fluororubber blend is inbetween
of the corresponding constituent rubbers. It is also evident that the oil resistance of fluorosilicone rubber is similar
to that of the fluororubber. Fluororubber
is highly polar due to the presence of
fluorine (57 – 59 %) whereas silicone rubber is less polar. Thus, fluororubber vulcanizate is highly resistant to oil attack leading to reduction in mass swell. The IRM
100
903 oil is more polar than ASTM oil #1
which results in greater mass swell of different compositions after immersion in
IRM 903 oil.
Table 5 also shows the effects of hot oil
ageing on the hardness of different compositions. In ASTM oil #1 the hardness
(point) changes of silicone rubber, fluororubber and their 50/50 blend are more or
less same. Fluorosilicone rubber shows
comparatively low hardness change. In
IRM 903 oil, the decrease (point change)
in hardness of silicone rubber is higher
than that of fluororubber. The 50/50 blend
shows the hardness point change, which
is inbetween the two constituent rubbers.
Conclusions
(1) The flame resistance of the fluororubber is higher than that of the silicone rubber and the flame resistance of the blends
is closer to silicone rubber, which is the
continuous phase in the blend.
(2) The initial decomposition temperature
of the blends is higher than that of the
constituent rubbers. The thermal behaviour of the blends is controlled by the
fluororubber phase and the blends
show the thermal stability similar to that
of the fluorosilicone rubber. The formation
of volatile silicon tetrafluoride results in the
low amount of residues of the blends during thermal degradation.
(3) Fluororubber shows greater hot oil resistance than silicone rubber. Retention
of mechanical properties such as tensile
strength and elongation at break of the
blend is closer to that of fluororubber.
Mass and hardness changes of the blend
are inbetween the constituent rubbers.
(4) The fluorosilicone rubber exhibits
flame resistance and thermal stability similar to that of the silicone rubber. But
the hot oil resistance of the rubber is better than the silicone rubber and closer to
the fluororubber.
(5) The blends of silicone rubber and
fluororubber can substitute the fluorosilicone rubber.
Acknowledgement
Authors wish to express their sincere gratitude to Asahi Glass Co., Yokohama, Japan and GE Bayer Silicone Pvt. Ltd., Bangalore, India for providing fluororubber
and silicone rubber respectively.
References
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[5] A.Y. Coran and R. Patel, Rubber Chem. Technol., 56 (1983) 1045.
[6] I. Yilgorand and J.E. McGrath, „Advances in
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[7] Technical literature on „AFLAS“, Asahi Glass
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[8] A. Ghosh, P. Antony, A.K. Bhattacharya, A.K.
Bhowmick and S.K. De, J Appl Polym Sci. 82
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West Conshohocken, PA, vol. 09.01, p. 86.
Corresponding author
Prof. S.K. De
Indian Institute of Technology
Rubber Technology Centre
Kharagpur 721302, India
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 3/2003