Materials and Design 30 (2009) 791–795

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Materials and Design

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Technical Report

The influence of CB loading on thermal aging resistance of SBR and NBR

rubber compounds under different aging temperature
A. Mostafa *, A. Abouel-Kasem, M.R. Bayoumi, M.G. El-Sebaie
Mechanical Engineering Department, Faculty of Engineering, Assiut University, Elgamaa Street Assiut University, Assiut 71516, Egypt

a r t i c l e i n f o a b s t r a c t

Article history: Aging temperatures play important role in changing the mechanical behavior of rubber, so thermal aging
Received 19 April 2008 test under different temperatures was carried out to investigate the effect of aging temperatures on the
Accepted 27 May 2008 tension, compression and hardness properties of styrene butadiene rubber (SBR) and nitrile butadiene
Available online 4 June 2008
rubber (NBR) compounds filled with different carbon black (CB) loading. The obtained results of five dif-
ferent compositions for SBR and NBR with 0, 20, 30, 50 and 70 phr of CB were compared. The dependenc-
es of the mechanical properties on aging temperature and CB loading were found.
Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The thermal aging is clarified on the most desired properties

needed in rubber industry, tension, compression and hardness
Particle filled elastomeric composites have become attractive properties, so the effect of fillers on these properties has been done
owing to their low cost and widespread industrial applications by several investigators. Mandal et al. [9] studied the influence of
[1]. Most usage of elastomers would be impossible without the carbon black filler loading on the hardness characteristics of two
reinforcing character of certain fillers, such as carbon blacks which types of carboxylated acrylonitrile butadiene rubber (XNBR) vulca-
have favourably modify properties such as stiffness, tensile nizates having different crosslinking reaction systems, in both
strength, heat distortion, mouldability and other important crosslinking systems the increase in hardness degree is linear.
properties. Nunes et al. [10] expanded the work of Mandal and studying the
Most of the rubber used in industry subjected to thermal aging influence of two types of precipitated silica filler loading on the
which result from the exposure of the rubber to a high tempera- hardness of polyurethane elastomer. As expected, hardness in-
tures then reduce this temperature that lead to drastically changes creases as silica concentration increases.
in the rubber properties (thermal degradation). Elastomers de- Pandey [11] shows that the poor mechanical strength of the
grade in a wide variety of environments and service conditions; gum vulcanizate can be improved by adding various types of fillers
this degradation limits the service lifetime of many elastomers (carbon black, silica and mica), these improvement due to the fil-
[2–4]. For instance, in oxygen containing environments, the ler–matrix interaction. Badawy [12] studied the effect of variation
mechanical strength of rubbers can be greatly affected by oxida- of high abrasion furnace black (HAF) content on the elastic behav-
tion, especially at relatively high temperatures [5]. In order to ior of butyl rubber (IIR) vulcanizates. Young’s modulus and tensile
determine the resistance of a vulcanizate to oxidation, the acceler- elongation at break of these composites have been studied as a
ated aging tests are typically used. Most investigations on the function of both HAF content and working temperature. Badawy
aging of elastomers have been focused on the changes in the chem- shows that the relative Young’s modulus (modulus for the compos-
ical nature of the elastomer during extended exposure to heat, oxy- ite divided by that for the unfilled butyl rubber) depends on the
gen, ozone, and various other environments. During thermal aging, weight percentage of the filler at different temperature. Where,
main-chain scission, crosslink formation and crosslink breakage as the filler content increases, Young’s modulus increases.
can occur, leading to severe changes in the mechanical properties Today the most sophisticated investigation techniques are used
of elastomers [6–8]. Oxidative degradation is generally considered to characterize reinforcing fillers and to understand the very origin
to be the most serious problem in the use of rubber at high of rubber–filler interactions. Since interactions between the vari-
temperature. ous compounding ingredients obviously take place in the early
time of material preparation, i.e. during mixing, it is quite logic
to expect some links between the properties induced by the filler
* Corresponding author. Tel.: +20 882372217. in the uncured state of the compound, and the reinforcement
E-mail address: ah_mostaffa@yahoo.com (A. Mostafa). obtained after vulcanization. This research will be concerned

0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.matdes.2008.05.065
792 A. Mostafa et al. / Materials and Design 30 (2009) 791–795

Table 1 primarily with the filler loading and its effects on thermal aging
Composition of carbon black filled SBR and NBR systems properties of the vulcanizates.
Ingredients Formula no. Many researchers have extensively studied the thermal aging
Phra behavior of filled rubber, but the effect of carbon black loading
S0 S1 S2 S3 S4 N0 N1 N2 N3 N4
on aging resistance of SBR and NBR rubber compounds are seldom
SBR-1502 100 100 100 100 100 – – – – –
NBR – – – – – 100 100 100 100 100
treated [13–15]. Therefore the aim of this paper is to determine the
ZnO 5 5 5 5 5 5 5 5 5 5 effect of CB loading on the ability of filled elastomer to withstand
Stearic acid 2 2 2 2 2 2 2 2 2 2 the effect of thermal aging under different aging temperatures to
Processing oil 10 10 10 10 10 10 10 10 10 10 detect its behavior under deterioration environment which rubber
Carbon black 0 20 30 50 70 0 20 30 50 70
usually subjected. The thermal aging is clarified on the most de-
MBTS 2 2 2 2 2 2 2 2 2 2
DPG 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 sired properties needed in rubber industry, tension, compression
Sulfur 2 2 2 2 2 2 2 2 2 2
a
Parts per hundred of rubber by weight.
1.6

1.4 25ºC
1.2
1.2 70ºC
25ºC

Stress, MPa
1 1 100ºC
0.8 125ºC
0.8 70ºC
Stress, MPa

0.6
0.6 100ºC
125ºC 0.4

0.4 0.2

0
0.2
0 50 100 150 200 250 300 350 400
Strain%
0
0 50 100 150 200 250 300 350 400 450 500 (a) Gum
Strain%
(a) Gum 6
25ºC
2.5
5
70ºC
25ºC
2 4
Stress, MPa

70ºC 100ºC
Stress, MPa

1.5 3 125ºC
100ºC
2
1 125ºC

1
0.5
0
0 50 100 150 200 250 300 350 400
0
0 50 100 150 200 250 300 350 400 450 Strain%
Strain% (b) NBR with 20 phr CB
(b) SBR with 20 phr CB
18
14
16 25ºC
25ºC
12 70ºC
14
70ºC 100ºC
10 12 125ºC
Stress, MPa

100ºC
Stress, MPa

10
8 125ºC
8
6
6
4 4

2 2
0
0 0 50 100 150 200 250 300 350
0 50 100 150 200 250 300 350
Strain% Strain%
(c) SBR with 70 phr CB (c) NBR with 70 phr CB
Fig. 1. The stress–strain curves for SBR vulcanizates filled with different CB loading Fig. 2. The stress–strain curves for NBR vulcanizates filled with different CB loading
under different aging temperatures. under different aging temperatures.
 A. Mostafa et al. / Materials and Design 30 (2009) 791–795 793

and hardness properties. The results are compared for SRR and NBR the specimens shall be in the form of a rigid cylinder whose length
rubber compounds. is twice its principal diameter, specimen size is 12 mm in diameter
by 25 mm in length.
2. Experimental
3. Results and discussion
The samples investigated in this study, were composed of NBR
and SBR-1502 compounded with different concentration of N550 There have been several methods developed to monitor the
carbon black according to the recipe shown in Table 1. Ingredients aging condition of rubbers. Tensile strength and elongation at
of the rubber compounds were mixed on a two-roll laboratory mill break testing are two parameters often found to be the most direct
of 80 mm diameter, 300 mm length, the speed of slow roll being and useful indicators of the remaining mechanical properties
24 rpm and the gear ratio 1.4. The ingredients were added accord- [20,21]. The results of thermal aging test are represented in Figs.
ing to ASTM D15 [16]. For each type of rubber compounds, the vul- 1–7. Figs. 1 and 2 show the stress–strain curves for SBR and NBR
canization process was performed by compression molding vulcanizates filled with different CB loading at room temperature
process at 160 °C for 25 min under a pressure of approximately 25 °C and with different aging temperature at 70, 100 and
400 kN/m2 from an electrical resistance heating press. 125 °C. From these figures it is clear that at the beginning of the
To determine the deterioration of the physical properties of SBR tension test for small strain the stress–strain curves are come close
and NBR vulcanizates after aging in oven, a circulated air oven for each type of filled compounds at different aging temperatures,
(Thermolyne-oven series 9000) is used. Place the specimens for while as strain increase these curves diverse from each other. Also,
aging in the oven after it has been preheated to the operating tem- the shapes of stress–strain curves were not change due to the ther-
perature. Then the oven temperature adjusted to the operating mal aging. Figs. 3 and 4 show the effect of CB loading on the 300%
temperature (70 or 100 or 125 °C). At the termination of the aging modulus and the elongation at break under different aging temper-
interval (48 h), remove the specimens from the oven, cool to room atures for SBR and NBR vulcanizates, respectively. It can be found
temperature on a flat surface and allow them to rest not less than that due to thermal aging both the 300% modulus and elongation
16 h or more than 96 h before determination of the physical prop- at break decreases. This is due to the oxidative degradation devel-
erties. A tension test on dumbbell-shaped specimens is carried out oped very rapidly leading to this marked decrease [22–25] This
according to ASTM D412 standard [17]. For compression (ASTM D phenomenon is more pronounced as the aging temperature in-
695 standard) [18] and hardness (ASTM D 676 standard) [19] tests creases. It is evident from these figures that gum vulcanizates

500
12
450
25°C
10 400
Elongation at Break %

70°C
300% Modulus, MPa

350
8 100°C
300
125°C
250
6
200 25°C
4 150 70°C
100 100°C
2 50 125°C
0
0 0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
Carbon Black Loading, phr Carbon Black Loading, phr

(a) SBR vulcanizates (a) SBR vulcanizates

18 400

16 25°C 350
70°C
Elongation at Break %

14
300% Modulus, Mpa

300
12
100°C
250
125°C
10
200
8
150 25°C
6
70°C
100
4 100°C
50
2 125°C
0 0
0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80
Carbon Black Loading, phr Carbon Black Loading, phr
(b) NBR vulcanizates (b) NBR vulcanizates
Fig. 3. Variation of the 300% modulus versus CB content for filled vulcanizates Fig. 4. Variation of the elongation at break versus CB content for filled vulcanizates
under different aging temperatures. under different aging temperatures.
794 A. Mostafa et al. / Materials and Design 30 (2009) 791–795

has more resistance (300% modulus) towards aging as compared 125 °C the compressive strength is lower than the compressive
with the loaded samples. This detectable resistance was decreased strength at lower aging temperature (100 and 70 °C) for each filled
with increasing CB loading, which may be attributed to the fact compounds, but the difference is not vast as in the case of tension
that CB accelerates the oxygen uptake of sulfur-cured rubber and test; this is due to the formation of high oxidation process which
the reaction is accompanied by a rapid degradation of rubber. So, results in chain scission.
CB work as a catalyst for the direct oxidation leading to their deac-
tivation. This leads to the decrease in stability with increasing CB 4. Conclusions
content. It can be found that NBR filled compounds offer more
resistance to thermal degradation compared with SBR filled com- From the current investigation of thermal aging behavior of SBR
pounds due to the high rubber–filler interaction and the presence and NBR compounds filled with different CB loading the following
of nitrile group. conclusions were derived from the experimental results
Fig. 5 shows the variations of hardness versus aging tempera-
tures for SBR and NBR with different CB loading. It can first be ob- 1.6
served that the measurements are generally very consistent
demonstrating that the use of the hardness test is both sensitive 1.4
25ºC
and repeatable enough to detect the thermal degradation. It is clear 1.2
that for each type of filled compounds the hardness value increases 70ºC

Stress, MPa
as aging temperatures increases. This is due to the high crosslinks 1 100ºC
125ºC
formation and the oxidizing skin which results from oxygen uptake 0.8
at the surface of the specimen [26]. So, the increase in aging tem-
perature results in increasing the hardness of both types of vulca- 0.6
nizates. This increase in hardness as aging temperature increase is 0.4
nearly representing a linear relationship. At a given CB loading,
hardness of NBR filled vulcanizates is clearly greater than those 0.2
in SBR vulcanizates due to the high rubber–filler interaction and 0
the presence of the nitrile group. 0 5 10 15 20 25 30
Figs. 6 and 7 represent the results of the compressive stress– Strain%
strain tests for SBR and NBR vulcanizates filled with different CB
loading under different aging temperatures 25, 70, 100 and (a) Gum
125 °C upto 25% strain. It is clear that at aging temperature
1.8

70 1.6 25ºC
70ºC
1.4 100ºC
60
125ºC
1.2
Hardness, Shore A

Stress, MPa

50
1
40 0.8
S0
30 0.6
S1
0.4
20 S2
S3 0.2
10
S4 0
0 0 5 10 15 20 25 30
0 25 50 75 100 125 150 Strain%
Temperature, °C (b) SBR with 20 phr CB
(a) SBR filled vulcanizates
4
80
3.5
25ºC
70
3 70ºC
100ºC
Hardness, Shore A

60
Stress, MPa

2.5 125ºC
50
2
40 N0 1.5
30 N1
1
20 N2
0.5
N3
10
N4 0
0 0 5 10 15 20 25 30
0 25 50 75 100 125 150 Strain%
Temperature, °C
(c)SBR with 70 phr CB
(b) NBR filled vulcanizates
Fig. 6. The compressive stress–strain curves for SBR vulcanizates filled with
Fig. 5. Effect of aging temperatures on the hardness of filled vulcanizates. different CB loading under different aging temperatures.
 A. Mostafa et al. / Materials and Design 30 (2009) 791–795 795

1.8 attributed to the fact that CB accelerates the oxygen uptake

25ºC of sulfur-cured rubber and the reaction is accompanied by a
1.6
70ºC rapid degradation of rubber.
1.4 100ºC (3) NBR filled compounds offer more resistance to thermal degra-
1.2 125ºC dation compared with SBR filled compounds due to the high
Stress, MPa

rubber–filler interaction and the presence of nitrile group.

1
(4) For both types of vulcanizates, as CB loading increases the
0.8 hardness increases as a result to increase in crosslinking
0.6 which make the vulcanizates more rigid. At a given CB load-
ing, hardness of NBR filled vulcanizates is clearly greater
0.4
than those in SBR vulcanizates due to the high rubber–filler
0.2 interaction and the presence of the nitrile group.
0 (5) The increase in aging temperature results in decrease the
0 5 10 15 20 25 30 compressive strength of both types of vulcanizates. This is
Strain% due to the formation of high oxidation process which results
(a) Gum in chain scission.

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