ELASTOMERE UND KUNSTSTOFFE
ELASTOMERS AND PLASTICS
EPDM SBR Blends Peroxide
cure Sulfur cure Coagent
The objective of this study is to address the replacement of up to 30
parts of EPDM with SBR to reduce
the cost of finished products and to
improve selected blend properties
using more efficient cure systems.
Effects of sulfur, peroxide, and coagent curing systems on blend properties were studied. It was found
that the addition of a small amounts
of sulfur as a coagent to the peroxide cure system in EPDM/SBR compounds remarkably improved mechanical properties of the blends.
Important compound properties including: mechanical properties,
compression set, heat aging, and
ozone resistance of the EPDM/SBR
blends are also compared with
those of EPDM compounds.
Eigenschaften von EPDM/SBR
Mischungen vernetzt mit Peroxid und Schwefelcoagenzien
EPDM SBR Mischungen Peroxidvernetzung Schwefelvernetzung Coagentien
Die Untersuchung zielt auf den Ersatz von 30 Teilen EPDM durch SBR,
um die Kosten von Fertigprodukten
zu senken und die Wirkung eines
effizienten Vernetzungssystems auf
die Eigenschaftsverbesserung zu
bewerten. Die Wirkungen von Vernetzungssystemen wie Schwefel,
Peroxid und Coagentien wurden untersucht. Es wurde gefunden, daû
die Zugabe geringer Mengen
Schwefel als Coagens von Peroxidsystemen zu einer beachtenswerten
Verbesserung der mechanischen Eigenschaften von EPDM/SBR-Compounds fuÈhrt. Letztlich wird ein Vergleich der wichtigen Vulkanisateigenschaften wie Druckverformungsrest, Hitzealterung und
OzonbestaÈndigkeit von EPDM/SBRund EPDM-Compounds vorgestellt.
Properties of EPDM/SBR Blends
Cured with Peroxide and Sulfur
Coagent
J. Zhao, G. Ghebremeskel and J. Peasely
Port Neches (USA)
Crosslinking with peroxides has been
known for a long time [1]. It became commercially important only with the development of the saturated and highly saturated polymers, such as EPM and
EPDM. Curing of EPDM (EPM) rubbers
have been accomplished largely by the
use of peroxide alone or in conjunction
with co-curing agents [2 ± 4]. Peroxide
curing of EPDM (EPM) elastomers not
only can improve performance and longer service life, but can also improve hightemperature resistance, and reduce
compression set. Peroxide curing, however, has been confined to special applications because of the limited compound
and processing flexibility and typically
higher cost relative to sulfur cure systems.
EPDM like SBR can be vulcanized with
sulfur cure systems. However, differences
in the solubility of sulfur and the level of
unsaturation in the two elastomers create
cure incompatibility in blends of these
elastomers [2, 5 ± 7].
The purpose of this study is to investigate the effects of peroxide and coagent
on the curing behavior and mechanical
properties of 70/30 EPDM/SBR blends.
The effect of cure system on important
compound properties including: mechanical properties, compression set,
heat aging, and ozone resistance were investigated.
Experimental
Materials
The SBR used in this study is SBR 1502
type from the Ameripol Synpol Coporation. EPDM with differing levels of diene
and ethylene content were obtained
from various suppliers. Carbon Black,
N330, was obtained from Engineered
Carbons, Inc. (ECI). Oil (Sunpar 2280)
was obtained from Sun Company. Dicumyl peroxide, ZnO, sulfur, accelerators,
stearic acid, and stabilizers used in this
study were of commmercial grade and
sources.
Formulations and mixing
Table 1 and Table 2 show the general recipe used in this study. Compounding was
carried out in a small-scale laboratory
Brabender Plasti-corder or a laboratory
size Banbury mixer. The speed of the rotor in the Brabender and Banbury mixers
was set at 80 rpm. The total volume of
each mix in the Brabender mixer was
kept constant at about 60 cm3 . Recipes
Table 1. General formulation
Sample #
P (phr)
S (phr)
C (phr)
EPDM
SBR1502
CB (N330)
Sunpar 2280
ZnO
Stearic Acid
Sulfur
TMTD
MBT
DCP40
70
30
80
50
5
Ð
Ð
Ð
Ð
2.0
70
30
80
50
5
1
1.5
1.0
0.5
Ð
70
30
80
50
5
1
0.3
0.10
0.12
3.0
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001
223
Properties of EPDM/SBR Blends . . .
in the Banbury mixer were adjusted to
give equal mixing volumes of 1200 cm3
times the specific gravity for each compound. Mixing was done in two stages.
All ingredients except for curing agents
were mixed in the Banbury or Brabender.
Curatives were added to the batch on a
mill.
Cure characterization was carried out
with a Monsanto ODR 2000E Rheometer
in accordance with ASTM 2084. Samples
were compression molded at 160 8C for
an optimum curing condition. The procedure used to determine tensile, hardness,
tear, compression set, and heat aging
has been described in detail in previous
work [8]. The procedure used to determine brittleness temperature and ozone
resistance is discribed below.
Brittle point measurement
Brittleness temperature was measured
by a Scott Tester according to
ASTM D 746-79. A modified T-50 specimen was die punched. A solution of
mixed solid carbon dioxide with acetone
was used to achieve low temperatures
and an electric immersion heater was
used for raising the temperature. When
the test temperature reached equilibrium,
the specimens were installed and immersed into the bath for 3 minutes at
the test temperature. After immersing
specimens for 3 minutes, the temperature was recorded and a single impact
blow was delivered to the specimens.
Brittleness temperature was calculated
as follows:
S
1
1†
ÿ
Tb ˆ Th ‡ DT
100 2
ozone at 40 8C. The specimens were
kept under a surface tensile strain. The
time elapsed for visible cracking to occur
was determined using a magnification
X10.
Dynamic ozone resistance was
performed in accordance with ASTM
D 3395-86 method A using die C dumbbell specimens. The cracking resistance
of samples was estimated by exposing
test samples to 50 pphm of ozone at
40 8C under dynamic strain conditions
(from 0 to 25 % strain at a rate of
0.5 Hz). The time elapsed for the first visible crack to appear and the changes in
the stress-strain curves due to the ozone
exposure were monitored.
Curing behavior
Table 3 shows the cure characteristics of
the EPDM/SBR blends cured with dicumyl peroxide (P), sulfur (S), and coagent (C). Scorch time, T50, T90, and
the cure rate showed the following
trends:
Scorch time, T50, and T90: P > C > S
Cure rate:
P<C<S
The compound cured with the coagent
system showed higher maximum torque
(34 dNm) than those cured with peroxide
or sulfur cure system (26 dNm). The addition of a relatively small amounts of sulfur (0.26 mole of sulfur/mole of peroxide)
to the peroxide cure system improved the
peroxide efficiency significantly.
Results and discussions
The purpose of this study was to evaluate
the effects of sulfur, peroxide, and coagent on the curing, mechanical and physical properties of EPDM/SBR blends.
Performance of the end-products cured
with peroxide and sulfur coagent as determined by mechanical properties,
ozone resistance, heat aging, and compression set was also investigated.
Cure systems
In this section, a comparative study of
sulfur, peroxide and coagent cure systems is presented.
Mechanical properties
Figure 1 shows the stress-strain curves of
EPDM/SBR compounds cured with peroxide cure system (PCS), sulfur cure system (SCS) and coagent cure systems
(CCS). Mechanical properties and hardness of the compounds are given in Table 4. Comparison of the stress-strain
curves of PCS, SCS, and CCS show
that there is no significant difference in
the modulus of CCS and PCS at the lower strain region (up to 200 %). The modulus of SCS was slightly higher than that
of CCS and PCS. This observation can
be explained by the fact that the crosslink
Table 2. General formulation of EPDM and EPDM/SBR blends
Sample #
EPDM (phr)
EPDM/SBR (phr)
where Tb is the brittleness temperature;
Th is the highest temperature at which
failure of all the specimens occurs; DT
is a temperature increment; and S is
the sum of the percentage of the broken
specimens at each temperature.
EPDM
SBR1502
CB (N330)
Sunpar 2280
ZnO
Stearic Acid
Sulfur
TMTD
MBT
DCP40
100
Ð
80
50
5
0±1
0.3 ± 0.5
0.18 ± 0.3
0.12 ± 0.2
3±5
70
30
80
50
5
0±1
0.3 ± 0.5
0.18 ± 0.3
0.12 ± 0.2
3±5
Ozone resistance
Table 3. Cure characterization of the SBR/EPDM blends with varying cure systems
Ozone resistance was carried out under
both static and dynamic conditions. In
the static test, ozone resistance was
performed
in
accordance
with
ASTM D 1149-91.
The cracking resistance of samples
was determined by exposing the samples
to an atmosphere containing 50 pphm
Sample #
P
S
C
Max. Torque, dNm
Min. Torque, dNm
Delta Torque, dNm
Scorch time, minutes
T50, minutes
T90, minutes
Cure rate index, 1/min
26.7
5.9
20.80
2.46
7.12
18.17
6.37
25.9
6.2
19.68
1.90
3.10
7.62
17.48
33.9
6.7
27.24
2.05
5.03
15.42
7.48
224
P: peroxide system, S: sulfur system, C: coagent system
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001
Properties of EPDM/SBR Blends . . .
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001
225
Properties of EPDM/SBR Blends . . .
Figure 1. Stress-strain curves of EPDM/SBR (70/30) blends with different cure systems
density in the SBR domains is significantly higher than of the EPDM domain
in the SCS. This behavior is due to the diffusion of the accelerator into the more polar and/or faster curing phase of the elastomer blend. In support of this conclusion, Woods and Davidon [9] have found
sulfur and accelerators from the EPDM
phase diffuse to the NBR phase during
the vulcanization process.
The modulus at higher strain for the
PCS was higher than SCS because the
EPDM matrix of the blends cured with
sulfur did not have a high enough crosslink density to support the higher stress.
The accelerator loss from the EPDM
phase results in lower crosslink density
in this phase resulting in decrease of elongation at high the stress.
The tensile strength and the energy at
break of the CCS was about 18 MPa and
12.2 J, while the tensile strength and the
energy at break of the compound PCS
was 15 MPa and 10.7 J. The tensile
strength and the energy at break of the
Figure 2. Stress-strain curves of EPDM and EPDM/SBR blends with
peroxide and sulfur coagent
SCS were found about 11.5 MPa and
9.18 J, respectively. The ultimate elongation of CCS, PCS, and SCS were found to
be almost identical. The higher tensile
strength and the higher energy at break
for CCS indicates that the compounds
cured with peroxide and sulfur have
stronger and better network properties.
The addition of a relatively small amount
of sulfur as a coagent to the peroxide cure
system (0.26 moles of sulfur/mole of peroxide) showed a remarkable influence on
the mechanical properties of the blends.
Tensile strength, energy at break, elongation at break and modulus at high strain
increased significantly (Figure 2). The
coagent cure system (combination of sulfur and peroxide cure system) covulcanized the continuous phase (EPDM), the
dispersed phase (SBR), and phase
boundary of the elastomers. This result
in increase in the interfacial strength so
that the blend properties are similar to
those attributed to a single polymer.
The polysulfidic crosslinks are generally
Table 4. Mechanical Properties of the EPDM/SBR compounds
Sample #
P
S
C
Tensile Strength (MPa)
Elongation at Break (%)
100 % Modulus (MPa)
200 % Modulus (MPa)
300 % Modulus (MPa)
Hardness ªShore Aº
Energy at Break (J)
15.0
428
2.69
6.58
11.3
68
10.7
11.5
428
3.43
6.37
9.22
72
9.18
18.0
432
2.61
6.77
12.3
67
12.2
226
stronger than the C-C or monosulfide
crosslinks [10 ± 15]. Vulcanizates with
the appropriate mix of crosslink types
have superior strength and fatigue resistance compared to networks containing
only the stronger monosulfide or C-C
bonds. Polysulfidic bonds are weaker
and more readily broken than C-C bonds,
and thus, high stresses in the molecular
network are relieved by fracture of at least
some of these crosslinks before backbone chains are broken. The broken ±
S-S- bonds may either reform again under load [16] or link up with the carbon
black to form either chemical carbongel bonds or physical carbon-gel linkages. The reformed bonds can continue
to support stress and to generate more
energy dissipation due to chain slippage
as the deformation is increased.
Hardness and the modulus at low
strain of the compounds cured with the
coagent were not affected by the addition
of the sulfur. Since the amount of sulfur
added to the peroxide cure package is
very small, only the C-C bonds (peroxide
cure) play the major role in determining
the hardness and modulus at low strain.
In addition, the coagents improve the efficiency of peroxide crosslinking by suppressing unwanted side reactions of
polymer radicals [17].
The cure behavior and mechanical
properties of the blends cured with coagent and sulfur cure systems are discussed below in further detail.
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001
Properties of EPDM/SBR Blends . . .
Peroxide and sulfur coagent
In the previous section, it was reported
that EPDM/SBR blends cured with peroxide and sulfur coagent have better curing and mechanical properties. Compression set, heat aging, mechanical
properties and ozone resistance of the
blend compounds were also compared
to those of the EPDM compounds. The
results are summarized below.
Curing behavior
Table 5 above shows the cure characteristics of EPDM and EPDM/SBR (70/30)
compounds cured with peroxide and sulfur coagent. Two levels of curatives were
investigated (Table 2). The curing behavior of the blend compounds was not significantly different than that of the EPDM
compounds. Due to the higher Mooney
viscosity of the EPDM relative to that of
the SBR used in this study, the maximum
torque of the EPDM compounds was
higher than that of the EPDM/SBR compounds.
sion, both physical and chemical stress
relaxation can occur simultaneously. At
room temperature, the physical stress relaxation dominates over the chemical
stress relaxation, while at a higher temperature the relaxation process is dominated by chemical reactions.
Heat aging
Changes in the mechanical properties of
EPDM and EPDM/SBR compounds
cured with peroxide and sulfur coagent
after 7 days aging at 100 and 140 8C
are given in Table 8 and Table 9. At
100 8C, tensile strength and ultimate
elongation of both compounds did not
change significantly. Modulus and hardness increased slightly.
At 140 8C, the mechanical properties
of the EPDM/SBR compounds were
found to be significantly worse than those
of the EPDM compounds. This is due to
the fact that the heat resistance of EPDM
Sample #
EPDM1
Figure 2 shows the stress-strain curves of
the EPDM and the EPDM/SBR compounds cured with peroxide and sulfur
coagent. The mechanical and physical
properties of the compounds are given
in Table 6. Replacing 30 parts of the
EPDM by SBR decreased the tensile
strength and ultimate elongation by about
18 % and 15 % respectively. The tear
strength decreased, while the modulus
increased at the low strain region due
to the increase in the crosslink density
in the SBR domains. The SBR domains
in the blends played a reinforcing role.
Cure system
Max. Torque, dNm
Min. Torque, dNm
Delta Torque, dNm
Scorch time, minutes
T50, minutes
T90, minutes
Cure rate index, 1/min
46.1
7.1
39.0
1.72
3.89
12.4
9.35
The compression set of the EPDM and
EPDM/SBR compounds cured with peroxide and sulfur coagent were measured
according to Method B of ASTM D39589. The compression set of the EPDM/
SBR compounds was not significantly
different from that of the EPDM compounds (Table 7).
The compression set of EPDM and
EPDM/SBR compounds was higher at
room temperature than at 100 8C.
When a sample is subjected to compres-
Ozone resistance
The EPDM and EPDM/SBR blends were
exposed to ozone under dynamic strain
conditions (from 0 to 25 % at a rate of
0.5 Hz) in an atmosphere containing
50 pphm of ozone at 40 8C. Optical microscope analysis of the surface of all
the samples after 12 days of dynamic
and 14 days of static ozone aging
showed no cracking.
Other properties
Table 10 shows that replacing 30 parts of
EPDM with SBR decreased the compound Mooney by up to 10 points. This
is a definite plus in the production of extruded goods. Other physical properties,
such as brittleness, hardness, abrasion
loss, rebound and heat build-up were
Table 5. Cure behavior of EPDM and EPDM/SBR (70/30) blends with peroxide and sulfur
coagent cure system
Mechanical properties
Compression set
vulcanizates is better than that of the SBR
vulcanizates.
High level
EPDM/SBR1
EPDM2
41.9
7.1
34.9
1.68
3.88
13.2
8.67
38.8
6.7
32.1
2.20
5.37
15.5
7.55
EPDM/SBR2
Low level
34.8
6.6
28.2
1.98
4.77
14.9
7.73
The compounds contain 80 phr N330 and 50 phr Oil.
Table 6. Mechanical properties
Sample #
Cure system
Tensile Strength, (MPa)
Elongation at Break, (%)
100 % Modulus, (MPa)
200 % Modulus, (MPa)
300% Modulus, (MPa)
Tear Strength, (kN/m)
EPDM1
EPDM/SBR1
High level
Stress-Strain
23.7
19.2
391
338
3.03
4.15
9.21
10.5
17.3
17.1
Die C-Tear
49.2
44.3
EPDM2
EPDM/SBR2
Low level
22.2
529
2.54
6.34
11.6
18.4
443
3.04
7.49
12.6
54.8
44.9
Table 7. Compression set
Sample #
Cure system
EPDM1
EPDM/SBR1
High level
EPDM2
Test Conditions
23 8C for 70 hours, %
100 8C for 70 hours, %
35.8
20.5
42.4
30.6
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001
31.9
20.9
EPDM/SBR2
Low level
37.3
27.3
227
Properties of EPDM/SBR Blends . . .
Table 8. Heat aging* studies
Sample #
Cure system
EPDM1
EPDM/SBR1
High level
Tensile Strength Retention, (%)
Strain at Break Retention, (%)
100 % Modulus Retention, (%)
200 % Modulus Retention, (%)
300 % Modulus Retention, (%)
ÿ 2.53
0.52
1.28
2.37
ÿ 10.23
ÿ 21.20
ÿ 5.97
ÿ 8.57
ÿ 3.47
ÿ 3.51
Hardness
ÿ 2.86
ÿ 4.17
EPDM2
EPDM/SBR2
Low level
ÿ 5.41
4.54
ÿ 17.7
ÿ 21.8
ÿ 15.5
1.63
8.80
ÿ 22.0
ÿ 11.8
ÿ 10.3
ÿ 1.39
ÿ 2.82
Stress-Strain
Hardness Retention, (%)
* The compounds were aged at 100 8C for 7 days.
Acknowledgment
Table 9. Heat aging* studies
Sample #
Cure system
Tensile Strength Retention, (%)
Strain at Break Retention, (%)
10 % Modulus Retention, (%)
200 % Modulus Retention, (%)
300 % Modulus Retention, (%)
Hardness Retention, (%)
EPDM1
EPDM/SBR1
High level
EPDM2
EPDM/SBR2
Low level
Stress-Strain
4.22
39.58
16.1
60.1
ÿ 38.0
ÿ 153
ÿ 7.21
18.9
ÿ 50.0
41.85
62.5
ÿ 212
Hardness
ÿ 7.14
ÿ 30.5
ÿ 4.17
ÿ 29.6
Table 10. Comparison of compound properties
Mooney (1 ‡ 4min) 100 8C
Hardness ªShore Aº
Temperature, 8C
Vol. Loss, mm3
23 8C, %
70 8C, %
Delta T, 8C
Permanent Set, %
EPDM1
EPDM/SBR1
High level
Compound Mooney
57.3
53.9
Hardness
70
72
Brittleness Temperature
> ÿ60*
> ÿ60*
DIN Abrasion
79.9
79.9
Zwick Rebound
54.1
47.5
62.3
54.3
Goodrich Flexometer
47.5
59.7
2.1
2.8
The authors wish to acknowledge the support and
encouragement given for this work by the Ameripol
Synpol Corporation.
References
* The compounds were aged at 140 8C for 7 days.
Sample #
Cure system
10 %, 20 %, and 9 % respectively. No significant change in hardness, brittleness
temperature, heat aging, and ozone resistance properties were observed.
The blend compounds, however,
showed slight reduction in tensile
strength, elongation and tear strength
compared to the EPDM compounds.
Also, the high temperature (140 8C)
heat aging properties of these blends
was also not comparable to the EPDM
compounds.
EPDM2
EPDM/SBR2
Low level
65.4
56.8
72
71
> ÿ60*
> ÿ60*
80.1
86
52.3
56.1
48.3
50.9
70.8
6.6
80.2
6.3
*At that temperature (ÿ60 8C), zero specimen was failed. No further tested.
[1] I.I. Ostromyslenski, J. Russ. Phys. Chem. Soc.,
47 (1915) 1467.
[2] J. Brooke Gardener, Rubber Chem. Technol. 41
(1968) 1312; 42 (1969) 1058; 43 (1970) 370.
[3] A. Robinson, J. Moore and L. Amberg, Rubber
World, 144, No.4 (1961) 86.
[4] P. Wei and J. Rehner, Jr., Rubber Chem. Technol., 35 (1962) 133.
[5] F.-X. Gullaumond, Rubber Chem. Technol., 49
(1976) 105.
[6] O. Olabisi, L.M. Robeson and M.T. Shaw, ªPolymer-Polymer Miscibility,º Academic Press Inc.,
New York (1979) Chp 2.
[7] C.M. Roland, Rubber Chem. Technol. 62 (1989)
456.
[8] J. Zhao, G.N. Ghebremeskel and J. Peasey,
Rubber World, 219, No. 3 (1998) 37.
[9] M.E. Woods and J.A. Davidson, Rubber Chem.
Technol., 49 (1976) 112.
[10] G.J. Lake and P.B. Lindley, J. Appl. Polym. Sci. 9
(1965) 1233.
[11] R.J. Chang and A.N. Gent, J. Polym. Sci.,
Polym. Phys. Ed., 19 (1981) 1619.
[12] L. Yanyo, Int. J. Tract. 39 (1989) 103.
[13] A.N. Gent and S.-M. Lai, J. Polym. Sci., Part B:
Polym. Phys. 32 (1994) 1543
[14] A.N. Gent and S.-M. Lai, C. Nah and C. Wong,
Rubber Chem. Technol. 67 (1994) 649.
[15] E. Southern, in Elastomers: Criteria for Engineering Design, C. Hepburn and R.J.W. Reynolds,
Eds., Applied Science Publishers, London
(1979) 273
[16] A.G. Thomas, J. Polym. Sci., 31 (1958) 467.
[17] L.D. Loan, Rubber Chem. Technol., 40 (1967)
149.
The authors
not significantly affected by the presence
of the SBR.
Summary
The purpose of this study was to find a
suitable cure system for replacing up to
30 parts of EPDM in some products by
the lower cost emulsion-SBR without deterioration in the mechanical and physical
properties of the end-products. The re-
228
sults of our study show that the addition
of a small amount of sulfur as a coagent
to the peroxide cure system in the EPDM/
SBR compounds have a remarkable positive influence on all the mechanical
properties.
When 30 parts of the EPDM was replaced by the lower cost emulsion SBR
cured with the peroxide and sulfur coagent, compression set, 100 % modulus
and 300 % modulus were improved by
All the authors are employees of the Ameripol Synpol
Corporation, Research & Development Department.
Dr. Zhao is a Materials Research Scientist, Dr. Ghebremeskel is the manger of the Materials and Analylical
Division, and J. Peasely is a technican in the Materials
Group.
Corresponding author
Junling Zhao
R + D Ameriol Synpol Corporation
P. O. Box 667
1215 Main Strat
Port Neches USA ± Texas 77651
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001