Polymer Degradation and Stability 41 (1993) 109-116
Effect of compatibiliser, curing sequence and
ageing on the thermal stability of silicone
rubber, E P D M rubber and their blends
S. Kole, T. K. Chaki, Anil K. Bhowmick & D. K. Tripathy*
Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721 302, India
(Received 17 July 1992; accepted 6 August 1992)
The thermal stability of the vulcanisates of silicone rubber, EPDM, their
blends and aged specimens of these systems has been studied by nonisothermal thermogravimetry. The activation energy for the degradation
process suggests that a different route is followed for the degradation of
EPDM after ageing. The effect of a silane-grafted EPR and also the effect of a
programmed two-stage curing process on thermal stability has also been
studied. Both are found to contribute moderately to the thermal stability of
the blends.
INTRODUCTION
stability of E P D M is attributed to its saturated
main chain structure. 5 As far as high-temperature
stability is concerned, silicone rubber is superior
to EPDM, 6 but the insulation characteristics of
E P D M seem to be better. 6'7 Thus, a suitable
blend of silicone-EPDM can give a product with
generally superior properties to either polymer.
However, silicones are known to be incompatible, in general, with other polymers. 8 This
explains why simple blending and subsequent
vulcanisation do not yield a material with
improved properties. The present work aims at
improving the heat-resistance properties of
silicone-EPDM blends, either by the use of a
compatibiliser or a programmed sequence of
mixing and vulcanisation. The effect of ageing on
the thermal stability of the blends has also been
studied. Sometimes, specifically when a fire
breaks out, polymeric materials undergo degradation in a restricted supply of oxygen or in a
closed chamber, and the entire process can be
visualised as an initial thermal oxidation followed
by subsequent thermal degradation. Such a
degradation process is gradually attracting more
attention because it generates mt;ch smoke and
toxic gases which endanger lives. The controlled
laboratory simulation of such a degradation
sequence can be represented best by initial
ageing of the specimens in a restricted supply of
The stability of rubber vulcanisates depends
largely upon the environment, because degradation is primarily dependent upon thermal or
thermal oxidative degradation, or both. Thermal
stability becomes an important factor when the
polymer is highly resistant to oxidation, or when
it is heated in an inert atmosphere, or when the
rubber is in the form of a thick section, so that
oxidation becomes diffusion-controlled. The
structure of the main chain, the energy of the
main chain bonds, the nature of the crosslinks
and the presence of any extra-network material,
all affect the thermal stability of rubber
vulcanisates. Thermogravimetric analysis, together with other supporting experiments like
efffluent-gas analysis, IR and X-ray diffraction can
all help in understanding the degradation
mechanism and must assist any effort to enhance
the thermal stability of a polymeric material.~ 3
The thermal stability of polysiloxane elastomers is ascribed to the high strength (110 kcal)
of its main chain bonds (Si--O). 4 The thermal
* To whom correspondence should be addressed.
Polymer Degradation and Stability 0141-3910/93/$06.00
© 1993 Elsevier Science Publishers Ltd.
109
S. Kole e t al.
110
oxygen, followed by thermal degradation in an
inert atmosphere.
EXPERIMENTAL
Materials
Silicone rubber. Silastic 1625 (Dow Corning),
type VMQ (i.e., vinyl-methyl-based silicone),
specific gravity 1.25 (25°C), and E P D M rubber.
Keltan 520 [ethylene content 55 mole %, diene
content 4.5 mole % (DCPD), density 0.86 g/cm 3]
were both supplied by Fort Gloster Industries
Ltd., India.
Compatibiliser. Silane-grafted E P R was developed in our laboratory following the procedure of Sen et al.9
Peroxide. Dicumyl peroxide was procured
from Hercules Incorporated, USA.
Mixing and vulcanization
Table 1 shows the composition of the mixes used
in the present investigation. Blending of silicone
rubber with E P D M was carried out in a
Brabender plasticorder (PLE 330) at 120°C for 5
min at a rotor speed of 100 rpm. Compatibiliser
was mixed, whenever applicable, with silicone
rubber in the plasticorder under identical
conditions, and the masterbatch thus obtained
was blended with the desired amount of E P D M
in the Brabender plasticorder at the same
temperature and speed for 5 min as before. In
the single-stage curative mixing process as
generally followed, the curative was mixed in a
cold two-roll mill. In a few cases, part of the
curative was mixed in the Brabender at 120°C
and at a rotor speed of 100rpm for various
periods of time to ensure a certain degree of
cure, and the balance at room temperature in the
roll mill. Curing was carried out in a hydraulic
press at a pressure of 5 MPa at 170°C for 10 min.
Post curing was done at 150°C for 2 h in a
temperature-controlled air-circulated ageing oven
(model No. FC 712, Blue M. Electric Co., Blue
Island, Illinois, USA).
Agemg
Ageing was carried out in a Test Tube Ageing
Tester (Seisaku-SHO, Toyoseiki, Japan) at 175°C
for 72 h.
Thermogravimetric analysis
Thermogravimetric analysis was done in a 951
Thermogravimetric Analyser fitted to a 9000
model Thermal Analyser from DuPont. Thermal
degradation of aged and unaged vulcanisates was
studied non-isothermally from ambient temperature to 800°C at a programmed rate of 20°C/min
Table 1. Compositions of the mixes
Mix
Silicone
(parts)
EPDM
(parts)
Silanegrafted
EPR
(parts)
DCP
(parts)
A
B
C
D
E
F
G
H
I
100
75
50
25
-50
5O
50
50
-25
50
75
100
50
50
50
50
-----8
10
15
--
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0-5+1
J
50
50
0.25 + 1.25
K
50
50
0.13 + 1-37
(1)
(3)
(3)
Samples I, J and K were cured in a two-stage process as described in the experimental
section. The second n u m b e r under D C P indicates the a m o u n t added in the second
stage of mixing. The figures in parenthesis indicate the curative mixing time (in mins)
in the first stage.
Effect of compatibiliser, curing sequence and ageing
111
position step only. The order of the decomposition process for silicone rubber, EPDM and for
the first decomposition step of the blends was
found to be approximately unity.
in a nitrogen atmosphere. Thermogravimetric
and differential thermogravimetric curves were
obtained from the plotter.
Analysis of thermograms
RESULTS AND DISCUSSION
Order and activation energy for the decomposition reaction were determined using the standard
kinetic equation
dw
- -
=
Degradation of the base elastomers
Ae-e/grw .
Figures 1 and 2 show the thermal degradation
behaviour of the vulcanisates of silicone rubber
(mix A), EPDM rubber (mix E) and a
representative blend (mix C, 50/50 blend of
silicone/EPDM). From these data, it is clear that
both the pure vulcanisates undergo a one-step
decomposition process. Further, silicone rubber
decomposes over a wide temperature range and
EPDM decomposes over a narrow temperature
range. The thermal stability of silicone rubber is
found to be superior to that of EPDM rubber as
reflected by its significantly higher values of
IPDT and Tmax (Table 2). However, the kinetic
parameters apparently contradict this. The
activation energy (Table 2) for decomposition of
silicone rubber (25 kcal/mole) is lower than that
of EPDM (78 kcal/mole) and the pre-exponential
factor is very much lower, 105 compared with
1022 , which indicates relatively few degradation
sites for silicone rubber compared to EPDM.
dt
where w denotes the weight fraction of the active
material remaining, and A, E, T, n and t have
their usual meanings. 1° The equation was solved
graphically for E and n using the thermogravimetric (TG) and the differential thermogravimetric (DTG) curves as described by
Freeman and Carroll. '1 The pre-exponential
factor, A, was determined from the original
equation using the known values of E and n, and
the range of values obtained over the entire
thermogram is reported.
Integral procedural decomposition temperatures (IPDT) were determined from the TG
curves in the usual way. Tmax,the temperature of
the maximum decomposition rate, was read from
the DTG curve.
The degradation parameters, E, n and A, for
the blends were calculated for the first decom-
~oo
-
~
~
~
.....
-- . . . . .
x~ ~ .
EPDM (mix E)
5 0 1 5 0 blend ( m i x C)
Silicone (mix A )
80
l ! \\\.\.
~ 6O
o°
\
r:
\.~.~
4O
~'N%,N
%\
\
\
\
20
0
I
0
200
I
400
T e m p e r a t u r e ( °C )
~
i
600
I
800
Fig. 1. T h e r m o g r a v i m e t r i c curves of silicone, E P D M and 50/50 blend of silicone and E P D M .
112
S. Kole et al.
60
.....
.....
EPDM (mix E)
50•50 blend (mix C)
Silicone (mix A)
40
r-
E
i/i
o
20
¢0A
" \,l.f~ .--.\"
i t if I
*6
rr
o
-20
0
I
2o0
I
I
4oo
Temperature (*C)
60o
I
8uo
Fig. 2. Differential thermogravimetric curves of silicone, EPDM and a 50/50 blend of silicone and EPDM.
the second Tmax value (Tm~,2) gradually shifts to
lower temperatures from the Tm~ value of
silicone as the E P D M concentration is increased.
A t the peak decomposition t e m p e r a t u r e of
E P D M (488°C), silicone decomposes at a rate of
2.65% per min c o m p a r e d with a value of 55%
per min for E P D M . Thus in the absence of
strong specific interactions between the components, which is true for an incompatible system
like s i l i c o n e - E P D M , the peak decomposition
rate and t e m p e r a t u r e for the first decomposition
will be controlled by E P D M so long as its
concentration does not fall drastically. The
detailed morphological study of s i l i c o n e - E P D M
blends clearly confirms the incompatible nature
This peculiarity in behaviour of the silicone
vulcanisate can be u n d e r s t o o d if we recall the
degradation mechanism of silicone which consists
of oligomer elimination from the chain end.
Thus, the actual degradation route, which
determines the n u m b e r of available sites that can
participate in the degradation process, is also
very important in dictating the thermal stability
of a specimen.
Degradation of the blends
Blends degrade in two steps (Figs 1 and 2 and
Table 2). The first Tmaxvalue (Tmax,1) in the D T G
curve coincides with the Tmaxvalue of E P D M and
Table 2. Degradation behaviour of unaged specimens of silicone, EPDM and their blends
Mix
Pre-exponential
factor
Activation
energy
(kcai/mole)
IPDT
(°C)
Tm,x.ta
(°C)
T,~ax.2a
(°C)
A
B
C
D
E
(1.9-2.9) x 105
(1-2-3-3) x 10'2
(1-1-3.6) x 1015
(2-3-5) × 1015
(1.4-2.7) x 1022
25
46
55
55
78
547 + 1
505 + 1
490 -t- 2
485 + 1
485 + 1
-488 + 2
488 + 2
488 + 2
488 + 2
581 + 1
575 + 2
560 + 1
545 + 2
--
" Tm,x.~= T,,~ for EPDM or for 1st decomposition of the blends. Tin,x.2= Tmaxfor silicone or for
2nd decomposition of the blends.
Effect of compatibiliser, curing sequence and ageing
113
change of activation energy with composition is
also assumed by other workers. 13 IPDT is found
to decrease with increase in E P D M content.
Figure 3 shows the effect of ageing on thermal
degradation behaviour. Results for aged specimens are shown in Table 3. The same trend is
shown as in the case of unaged specimens. The
main differences are reductions in activation
energy, pre-exponential factor, IPDT and Tmax
for E P D m and E P D M containing blends. The
behaviour of silicone rubber does not alter with
ageing. A lowering of the IPDT value indicates
some loss in thermal stability on ageing.
Reduction in activation energy implies that
ageing offers an alternative degradation route.
Reduction in pre-exponential factor suggests that
new degradation sites are generated only at
selected points on ageing. Thus it appears that
ageing incorporates new potential degradation
sites of lower activation energy and so leads to
reduced thermal stability of E P D M and its
blends.
of the blend, and this is in line with the
theoretical considerations of Sanchez on
miscibility. 12'8 The second decomposition zone is
possibly interfered with by the last trace of
EPDM remaining and this explains the gradual
shift of Tm~.2 towards lower temperatures with
E P D M loading. Both the elastomers are cured
with DCP, and curing proceeds through free
radicals. Thus at the interface of the two
polymers, some inter-crosslinking can occur,
which may temporarily retain some of the E P D M
fragments with the silicone even when the bulk of
EPDM has volatilised. These inter-crosslinked
fragments, at some elevated temperatures, will
volatilise either by further fragmentation or by
scission of the inter-crosslinks. This explanation
seems to be very reasonable in view of the very
low value of the peak decomposition rate of
silicone and the correspondingly very high value
of EPDM, so that retention of a little E P D M will
be enough to modify the overall degradation rate
in this zone. Values of IPDT, Tmax, and other
kinetic parameters for silicone rubber, E P D M
and their blends are shown in Table 2. Calculated
values indicate the decomposition processes to be
of first order. Values of E and A increase with
EPDM concentration. Change in activation
energy is simply a dilution effect and linear
Effect of compatibiliser
Figure 4 shows the effect of compatibiliser (mix
F, 8 parts per hundred parts of rubber
compatibiliser) on the thermal degradation
12(3
60
- -
EPDM ( u n a g e d m i x E )
.....
EPDM ( a g e d mix E )
10(3
4O
t
80
E
~6c
2O _o
40
nO
20
I
I
. . . . . . . . . . .
I
200
400
--
600
T e m p e r a t u r e (°C)
Fig. 3. TG and DTG curves of aged and unaged EPDM.
__.
1-20
800
114
S. Kole et al.
Table 3. Degradation behaviour of the specimens of silicone, E P D M and their blends aged for 72 h at 175"C
Mix
A
B
C
D
E
Preexponential
factor
Activation
energy
(kcal/mole)
(1.2 x 4.2) x 106
(4.8 x 4.9) x 1 0 4
(1.8 x 2.3) x 106
(3.3-9) x 109
(0.9-3.4) X 1012
28
21
24
35
43
IPDT
(°C)
553
500
465
465
465
±
+
+
+
+
1
1
2
2
2
T,....,"
7",....~"
(°C)
(°C)
-476 +
476 +
476 ±
476 ±
575 +
565 ±
540 +
530 +
--
2
2
1
2
1
1
1
2
= See footnote to Table 2.
Apart from the interaction mechanism at the
interface, the finer morphology generated by
compatibiliser can lead to slower diffusion of
reaction products by offering a more tortuous
diffusion path owing to the physical barrier
imposed by silicone to the diffusion of the
volatiles. This can lead to a pseudo-stability of
the blend.
behaviour of a 50/50 silicon-EPDM blend.
Results of degradation kinetics for specimens
containing compatibiliser (mixes F, G and H) are
shown in Table 4. Silane-grafted EPR acts as a
physical compatibiliser that controls the domain
sizes by reducing interracial energy. 12a4 As the
interaction is present in the interface and the
bulk of polymer remains uninfluenced, individual
components are likely to follow their own
degradation route. This explains why the
compatibiliser does not improve the thermal
stability significantly. In fact, at compatibiliser
concentrations up to 10 phr, the improvement is
only marginal and at 15 phr there is a decrease in
this property. This reflects the dilution effect of
EPR in the blend, whose thermal stability is
identical to EPDM and inferior to silicone.
Effect of restricting the domains by two-step
curing
These specimens were prepared with a view to
generating a co-continuous structure. During
mixing, severe stratification of the phases and
partial coalescence of the stratified structures can
30
120
.....
13
.......
100
50150 blend ( m i x C )
50150 blend with 8 p hr
compatibiliser ( m i x F)
A t w o st age cur ed
p r o d u c t (mix K)
20
E
I
80
t~
o
/,ii i!
~. 6C
c~
lY
10
~""
4£
n~
2C
O
I
200
I
400
Temperature
I
600
I
8OO
-10
(*C)
Fig. 4. T G and D T G curves o f a 50/50 blend, a specimen containing compatibiliser and a restricted-domain specimen.
115
Effect of compatibiliser, curing sequence and ageing
Table 4. Role of compatibiliser on thermal stability of silicone-EPDM blends
Mix
C
F
G
H
Preexponential
factor
Activation
energy
(kcal/mole)
(1.1-3-6) x 10 ~5
(4-5-4) × 1012
(5-9-4) x 109
(0.7-2-4) × 10 L3
55
47
36
46
Tm~x.,a
Tmax.2a
(°C)
(°C)
488 + 2
498 + 1
490 5:1
485 + 2
560
568
560
538
IPDT
(°C)
490
497
493
473
+2
:t: 2
5=2
± 3
See footnote to Table 2.
Table 5. Effect of restricted-domain structures on thermal stability of silicone-EPDM blend
Mix
C
I
J
K
Pre-
Activation
IPDT
T.~ax.,"
T.,ax.2a
exponential
energy
(°C)
(°C)
(°C)
factor
(kcal/mole)
(1.1-3-6) x 10 ~5
(2-1-3.9) x 1013
(2-3.6) x 10 t4
(3"7-8-4) x 10 ~
55
49
53
44
490 + 2
490 :t: 2
505:1:2
500-t-2
488 + 2
505 + 2
507 + 2
502+2
560
568
585
591
u See footnote to Table 2.
lead to a co-continuous structure, and it was
thought that partial crosslinking of the phases
during this stage might retain this morphology
and also the continuous structure of the silicone
phase would lead to improved heat ageing
properties. ~2 Degradation behaviour and kinetics
of restricted-domain specimens (mixes I, J and
K) are shown in Fig. 4 and Table 5. The
specimens do not show any significant change in
activation energy. However, the pre-exponential
factor decreases to some extent, whereas IPDT
and Tmax,~show some improvement. This can be
ascribed either to the difficulty of diffusion of the
degradation products, or to some interfacial
bonding between the two phases. During
dynamic vulcanisation, a considerable amount of
interface is generated and this ultimately
enhances the possibility of inter-crosslinking.
One surprising observation is that Tm~x,~for these
specially prepared blends are higher than the T~,ax
of silicone rubber, leading to the apparent
conclusion that the process strengthens the
silicone phase further.
CONCLUSIONS
(i)
Degradation of silicone, EPDM and their
blends follow first-order kinetics.
(ii) Degradation of the blends shows the
effects of the degradation of the individual
components.
(iii) Finer morphology generated by compatibiliser leads to higher thermal stability
and this is attributed to two reasons,
namely the difficulty of diffusion of the
products and an inter-crosslinking reaction
between the two elastomers.
(iv) Restricted-domain specimens have higher
thermal stability and the silicone phase
seems to be more stabilised.
(v) Partial oxidation can reduce the thermal
stability drastically, since it creates points
that are highly susceptible to degradation
and provides an altogether different route
for degradation.
REFERENCES
In Developments in Polymer
eds. G. Scott. Applied Science
Publishers, London, 1979.
Grassie,
N.
(ed.),
Developments in Polymer
Degradation--1. Applied Science Publishers, London,
UK, 1977.
Kenyon, A. S., In Techniques and Methods of Polymer
Evaluation, Vol. 1, eds P. E. Slade Jr & L. T. Jenkins.
Marcel Dekker, New York, USA, 1966, p. 217.
Yilgor, I. & McGrath, J. E. In Advances in Polymer
Science-86, ed. H. Benoit, Springer Verlag, 1988, p. 6.
Brydson, J. A., Rubber Chemistry. Applied Science
Publishers, London, UK, 1978, p. 323.
Technical Literature, JSR JENIX E. Japan Synthetic
Rubber, Tokyo, Japan, 1990.
Technical Literature, Royalene ~m EPDM for IM and
RPO Applications. Uniroyal Chemical, Naugatuck,
USA, 1989.
1. Stuckey,
J.
E.,
Stabilisation--1,
2.
3.
4.
5.
6.
7.
116
S. Kole et al.
8. Sanchez, I. C., In Polymer blends, Vol. 1, eds. D. R.
Paul & N. Newmann. Academic Press, New York,
1978, Ch. 3.
9. Sen, A. K., Mukherjee, B., Bhattacharya, A. S., De,
P. P. & Bhowmick, A. K., J. Appl. Polym. Sci., 44
(1992) 1153.
10. Reich, L. & Levi, D. W. In Encyclopedia of Polymer
Science and Technology, Vol. 14, ed. N. M. Bikales.
Interscience Publishers, New York, 1971, p. 7.
11. Freeman, E. S. & Carroll, B., J. Phys. Chem., 62
(1958) 394.
12. Kole, S., Bhattacharya, A. K. & Bhowmick, A. K.,
Piast. Rubb. Comp. Proc. Appln., 19 (1993) 117.
13. Feldman, D. & Rusu, M., Eur. Polym. J., 10 (1974) 41.
14. Kole, S., Bhattacharya, A. K., Tripathy, D. K. &
Bhowmick, A. K., J. Appl. Polym. Sci. (in press).