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The influence of CB loading on thermal aging resistance of SBR and NBR rubber compounds under different aging temperature
Materials and Design 30 (2009) 791–795
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
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
Article history: Received 19 April 2008 Accepted 27 May 2008 Available online 4 June 2008
a b s t r a c t Aging temperatures play important role in changing the mechanical behavior of rubber, so thermal aging test under different temperatures was carried out to investigate the effect of aging temperatures on the tension, compression and hardness properties of styrene butadiene rubber (SBR) and nitrile butadiene rubber (NBR) compounds filled with different carbon black (CB) loading. The obtained results of five different compositions for SBR and NBR with 0, 20, 30, 50 and 70 phr of CB were compared. The dependences of the mechanical properties on aging temperature and CB loading were found. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Particle filled elastomeric composites have become attractive owing to their low cost and widespread industrial applications [1]. Most usage of elastomers would be impossible without the reinforcing character of certain fillers, such as carbon blacks which have favourably modify properties such as stiffness, tensile strength, heat distortion, mouldability and other important properties. Most of the rubber used in industry subjected to thermal aging which result from the exposure of the rubber to a high temperatures then reduce this temperature that lead to drastically changes in the rubber properties (thermal degradation). Elastomers degrade in a wide variety of environments and service conditions; this degradation limits the service lifetime of many elastomers [2–4]. For instance, in oxygen containing environments, the mechanical strength of rubbers can be greatly affected by oxidation, especially at relatively high temperatures [5]. In order to determine the resistance of a vulcanizate to oxidation, the accelerated aging tests are typically used. Most investigations on the aging of elastomers have been focused on the changes in the chemical nature of the elastomer during extended exposure to heat, oxygen, ozone, and various other environments. During thermal aging, main-chain scission, crosslink formation and crosslink breakage can occur, leading to severe changes in the mechanical properties of elastomers [6–8]. Oxidative degradation is generally considered to be the most serious problem in the use of rubber at high temperature.
* Corresponding author. Tel.: +20 882372217. E-mail address: [email protected] (A. Mostafa). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.05.065
The thermal aging is clarified on the most desired properties needed in rubber industry, tension, compression and hardness properties, so the effect of fillers on these properties has been done by several investigators. Mandal et al. [9] studied the influence of carbon black filler loading on the hardness characteristics of two types of carboxylated acrylonitrile butadiene rubber (XNBR) vulcanizates having different crosslinking reaction systems, in both crosslinking systems the increase in hardness degree is linear. Nunes et al. [10] expanded the work of Mandal and studying the influence of two types of precipitated silica filler loading on the hardness of polyurethane elastomer. As expected, hardness increases as silica concentration increases. Pandey [11] shows that the poor mechanical strength of the gum vulcanizate can be improved by adding various types of fillers (carbon black, silica and mica), these improvement due to the filler–matrix interaction. Badawy [12] studied the effect of variation of high abrasion furnace black (HAF) content on the elastic behavior of butyl rubber (IIR) vulcanizates. Young’s modulus and tensile elongation at break of these composites have been studied as a function of both HAF content and working temperature. Badawy shows that the relative Young’s modulus (modulus for the composite divided by that for the unfilled butyl rubber) depends on the weight percentage of the filler at different temperature. Where, as the filler content increases, Young’s modulus increases. Today the most sophisticated investigation techniques are used to characterize reinforcing fillers and to understand the very origin of rubber–filler interactions. Since interactions between the various 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 in the uncured state of the compound, and the reinforcement obtained after vulcanization. This research will be concerned
792
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
Table 1 Composition of carbon black filled SBR and NBR systems Ingredients Phra
Formula no. S1
S0
S2
S3
S4
N0
N1
N2
N3
N4
SBR-1502 100 100 100 100 100 – – – – – NBR – – – – – 100 100 100 100 100 ZnO 5 5 5 5 5 5 5 5 5 5 Stearic acid 2 2 2 2 2 2 2 2 2 2 Processing oil 10 10 10 10 10 10 10 10 10 10 Carbon black 0 20 30 50 70 0 20 30 50 70 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 Sulfur 2 2 2 2 2 2 2 2 2 2 a
primarily with the filler loading and its effects on thermal aging properties of the vulcanizates. Many researchers have extensively studied the thermal aging behavior of filled rubber, but the effect of carbon black loading on aging resistance of SBR and NBR rubber compounds are seldom treated [13–15]. Therefore the aim of this paper is to determine the effect of CB loading on the ability of filled elastomer to withstand the effect of thermal aging under different aging temperatures to detect its behavior under deterioration environment which rubber usually subjected. The thermal aging is clarified on the most desired properties needed in rubber industry, tension, compression
Parts per hundred of rubber by weight.
1.6
25ºC
Stress, MPa
1
70ºC
0.8
100ºC 125ºC
0.6
Stress, MPa
1.2
1.4
25ºC
1.2
70ºC 100ºC
1
125ºC
0.8 0.6 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
(a) Gum
500
Strain%
(a) Gum
6
25ºC
2.5
5
70ºC
25ºC
Stress, MPa
Stress, MPa
2
70ºC 1.5
100ºC 125ºC
1
4
100ºC 125ºC
3 2 1
0.5
0 0
0 0
50
100
150
200
250
300
350
400
50
100
150
200
250
300
350
400
Strain%
450
Strain%
(b) NBR with 20 phr CB
(b) SBR with 20 phr CB 18
14
14
100ºC
8
125ºC
6
Stress, MPa
Stress, MPa
70ºC 10
25ºC 70ºC
16
25ºC 12
100ºC 125ºC
12 10 8 6
4
4
2
2 0
0
0
50
100
150 200 Strain%
250
300
350
(c) SBR with 70 phr CB Fig. 1. The stress–strain curves for SBR vulcanizates filled with different CB loading under different aging temperatures.
0
50
100
150
200
250
300
350
Strain%
(c) NBR with 70 phr CB Fig. 2. The stress–strain curves for NBR vulcanizates filled with different CB loading under different aging temperatures.
793
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
and hardness properties. The results are compared for SRR and NBR rubber compounds.
the specimens shall be in the form of a rigid cylinder whose length 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 carbon black according to the recipe shown in Table 1. Ingredients of the rubber compounds were mixed on a two-roll laboratory mill of 80 mm diameter, 300 mm length, the speed of slow roll being 24 rpm and the gear ratio 1.4. The ingredients were added according to ASTM D15 [16]. For each type of rubber compounds, the vulcanization process was performed by compression molding process at 160 °C for 25 min under a pressure of approximately 400 kN/m2 from an electrical resistance heating press. To determine the deterioration of the physical properties of SBR and NBR vulcanizates after aging in oven, a circulated air oven (Thermolyne-oven series 9000) is used. Place the specimens for aging in the oven after it has been preheated to the operating temperature. Then the oven temperature adjusted to the operating temperature (70 or 100 or 125 °C). At the termination of the aging interval (48 h), remove the specimens from the oven, cool to room temperature on a flat surface and allow them to rest not less than 16 h or more than 96 h before determination of the physical properties. A tension test on dumbbell-shaped specimens is carried out according to ASTM D412 standard [17]. For compression (ASTM D 695 standard) [18] and hardness (ASTM D 676 standard) [19] tests
There have been several methods developed to monitor the aging condition of rubbers. Tensile strength and elongation at break testing are two parameters often found to be the most direct and useful indicators of the remaining mechanical properties [20,21]. The results of thermal aging test are represented in Figs. 1–7. Figs. 1 and 2 show the stress–strain curves for SBR and NBR vulcanizates filled with different CB loading at room temperature 25 °C and with different aging temperature at 70, 100 and 125 °C. From these figures it is clear that at the beginning of the tension test for small strain the stress–strain curves are come close for each type of filled compounds at different aging temperatures, while as strain increase these curves diverse from each other. Also, the shapes of stress–strain curves were not change due to the thermal aging. Figs. 3 and 4 show the effect of CB loading on the 300% modulus and the elongation at break under different aging temperatures for SBR and NBR vulcanizates, respectively. It can be found that due to thermal aging both the 300% modulus and elongation at break decreases. This is due to the oxidative degradation developed very rapidly leading to this marked decrease [22–25] This phenomenon is more pronounced as the aging temperature increases. It is evident from these figures that gum vulcanizates
500
12
450
300% Modulus, MPa
Elongation at Break %
25°C 10
70°C 100°C
8
125°C 6 4 2
350 300 250 200
25°C
150
70°C
100
100°C
50
125°C
0
0 0
10
20
30
40
50
60
70
0
80
10
20
30
40
50
60
Carbon Black Loading, phr
Carbon Black Loading, phr
(a) SBR vulcanizates
(a) SBR vulcanizates
70
80
400
18 16
25°C
14
70°C
350
Elongation at Break %
300% Modulus, Mpa
400
100°C
12
125°C
10 8 6 4
300 250 200
25°C
150
70°C
100
100°C 50
2
125°C
0
0 0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
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 under different aging temperatures.
70
80
Fig. 4. Variation of the elongation at break versus CB content for filled vulcanizates under different aging temperatures.
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
has more resistance (300% modulus) towards aging as compared with the loaded samples. This detectable resistance was decreased with increasing CB loading, which may be attributed to the fact that CB accelerates the oxygen uptake of sulfur-cured rubber and the reaction is accompanied by a rapid degradation of rubber. So, CB work as a catalyst for the direct oxidation leading to their deactivation. This leads to the decrease in stability with increasing CB content. It can be found that NBR filled compounds offer more resistance to thermal degradation compared with SBR filled compounds due to the high rubber–filler interaction and the presence of nitrile group. Fig. 5 shows the variations of hardness versus aging temperatures for SBR and NBR with different CB loading. It can first be observed that the measurements are generally very consistent demonstrating that the use of the hardness test is both sensitive and repeatable enough to detect the thermal degradation. It is clear that for each type of filled compounds the hardness value increases as aging temperatures increases. This is due to the high crosslinks formation and the oxidizing skin which results from oxygen uptake at the surface of the specimen [26]. So, the increase in aging temperature results in increasing the hardness of both types of vulcanizates. This increase in hardness as aging temperature increase is nearly representing a linear relationship. At a given CB loading, hardness of NBR filled vulcanizates is clearly greater than those in SBR vulcanizates due to the high rubber–filler interaction and the presence of the nitrile group. Figs. 6 and 7 represent the results of the compressive stress– strain tests for SBR and NBR vulcanizates filled with different CB loading under different aging temperatures 25, 70, 100 and 125 °C upto 25% strain. It is clear that at aging temperature
125 °C the compressive strength is lower than the compressive strength at lower aging temperature (100 and 70 °C) for each filled compounds, but the difference is not vast as in the case of tension test; this is due to the formation of high oxidation process which results in chain scission. 4. Conclusions From the current investigation of thermal aging behavior of SBR and NBR compounds filled with different CB loading the following conclusions were derived from the experimental results 1.6 1.4
25ºC
1.2
Stress, MPa
794
70ºC 100ºC 125ºC
1 0.8 0.6 0.4 0.2 0 0
5
10
15
20
25
30
Strain%
(a) Gum 1.8
1.4
50 40
S0 30
Stress, MPa
60
Hardness, Shore A
25ºC 70ºC 100ºC 125ºC
1.6
70
S1
10
1 0.8 0.6 0.4
S2
20
1.2
S3
0.2
S4
0 0
0 0
25
50
75
100
125
5
10
150
15
20
25
30
Strain%
Temperature, °C
(b) SBR with 20 phr CB
(a) SBR filled vulcanizates 4 80
3.5
25ºC 70ºC 100ºC 125ºC
3
60 50 40
N0
30
N1
20
N2
10
Stress, MPa
Hardness, Shore A
70
2.5 2 1.5 1
N3
0.5
N4
0
0
0 0
25
50
75
100
125
150
Temperature, °C
5
10
15
20
25
30
Strain%
(c)SBR with 70 phr CB
(b) NBR filled vulcanizates Fig. 5. Effect of aging temperatures on the hardness of filled vulcanizates.
Fig. 6. The compressive stress–strain curves for SBR vulcanizates filled with different CB loading under different aging temperatures.
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
1.8
1.4
25ºC 70ºC 100ºC
1.2
125ºC
Stress, MPa
1.6
1 0.8 0.6 0.4 0.2 0 0
5
10
15
20
25
30
Strain%
(a) Gum
795
attributed to the fact that CB accelerates the oxygen uptake of sulfur-cured rubber and the reaction is accompanied by a rapid degradation of rubber. (3) NBR filled compounds offer more resistance to thermal degradation compared with SBR filled compounds due to the high rubber–filler interaction and the presence of nitrile group. (4) For both types of vulcanizates, as CB loading increases the hardness increases as a result to increase in crosslinking which make the vulcanizates more rigid. At a given CB loading, hardness of NBR filled vulcanizates is clearly greater than those in SBR vulcanizates due to the high rubber–filler interaction and the presence of the nitrile group. (5) The increase in aging temperature results in decrease the compressive strength of both types of vulcanizates. This is due to the formation of high oxidation process which results in chain scission. References
2.5
25ºC 70ºC 100ºC 125ºC
Stress, MPa
2
1.5
1
0.5
0 0
5
10
15
20
25
30
Strain%
(b) NBR with 20 phr CB 10 9
25ºC 70ºC 100ºC 125ºC
8
Stress, MPa
7 6 5 4 3 2 1 0 0
5
10
15
20
25
30
Strain%
(c) NBR with 70 phr CB Fig. 7. The compressive stress–strain curves for NBR vulcanizates filled with different CB loading under different aging temperatures.
(1) Due to the thermal aging both the 300% modulus and the elongation at break for SBR and NBR filled vulcanizates decrease. This is due to the oxidative degradation developed very rapidly leading to this marked decrease. This phenomenon is more pronounced with the increase of aging temperature. (2) The gum vulcanizates have more resistance (300% modulus) towards aging as compared with the CB loading vulcanizates. This detectable resistance was decreased with increasing CB loading and aging temperature. Which may be
[1] Pukanszky B. Influence of interface interaction on the ultimate tensile properties of polymer composites. Composites 1990;21:255–62. [2] Fan R, Zhang Y, Huang C. Effect of crosslink structures on dynamic mechanical properties of natural rubber vulcanizates under different aging conditions. J Appl Polym Sci 2001;81:710–8. [3] Deuri AS, Bhowmick AK. Effect of ageing on critical cut length and morphology of fracture surface in tensile rupture of natural rubber. J Mater Sci 1987;22:4299–306. [4] Budrugeac P. Accelerated thermal aging of NBR and other materials under air or oxygen pressures and degradation of polymers. Die Angew Makromol Chem 1997;247:19–30. [5] Gillen KT, Clough RL, Wise J. Prediction of elastomer lifetimes from accelerated thermal-aging experiments. Adv Chem Ser 1996;249:557–76. [6] Ha-Anh T, Vu-Khanh T. Prediction of mechanical properties of polychloroprene during thermo-oxidative aging. Polym Test 2005;24:775–880. [7] Bystritskaya EV, Pomerantsev AL, Rodionova OY. Evolutionary design of experiment for accelerated aging tests. Polym Test 2000;19:221–9. [8] Hamed GR, Zhao J. Tensile behavior after oxidative aging of gum and blackfilled vulcanizates of SBR and NR. Rubber Chem Technol 1999;72:721–30. [9] Mandal UK, Aggarwal S. Studies on rubber–filler interaction in carboxylated nitrile rubber through microhardness measurement. Polym Test 2001;20:305–11. [10] Nunes RCR, Fonseca JLC, Pereira MR. Polymer–filler interactions and mechanical properties of a polyurethane elastomer. Polym Test 2000;19:93–103. [11] Pandey KN, Setua DK, Mathur GN. Fracture topography of rubber surfaces: an SEM study. Polym Test 2003;22:353–9. [12] Badawy MM. Effect of HAF-carbon black on the elastic behavior of butyl rubber vulcanizates. Polym Test 2000;19:341–8. [13] Traian Z. New assessment in thermal degradation of polymers. Polym Test 2001;20:3–6. [14] Clelina M, Gillen KT, Assink RA. Accelerated aging and lifetime prediction: review of non-Arrhenius behavior due to two competing processes. Polym Degrad Stab 2005;90:395–404. [15] Jitendra K, Raghunatha K, Singh RP. An overview on the degradability of polymer nanocomposits. Polym Degrad Stab 2005;88:234–50. [16] ASTM D 15. Standard test method for rubber property-Sample preparation for physical testing of rubber products. Philadelphia: annual book of ASTM standards; 1958. [17] ASTM D 412. Standard test method for tension testing of vulcanized rubber. Philadelphia: annual book of ASTM standards; 1958. [18] ASTM D 695. Standard test method for compression properties of rigid plastics. Philadelphia: annual book of ASTM standards; 1958. [19] ASTM D 676. Standard test method for indentation of rubber by means of a durometer. Philadelphia: annual book of ASTM standards; 1958. [20] Sun Y, Luo S, Watkins K, Wong CP. Electrical approach to monitor the thermal oxidation aging of carbon black filled ethylene propylene rubber. Polym Degrad Stab 2004;86:209–15. [21] Shi XQ, Wang ZP, Pang HLJ, Zhang XR. Investigation of effect of temperature and strain rate on mechanical properties of underfill material by use of microtensile specimens. Polym Test 2002;21:725–33. [22] Budrugeac P. Accelerated thermal aging of Nitrile-butadiene rubber under air pressure. Polym Degrad Stab 1995;47:129–32. [23] George M. Accelerated thermal aging of an EVA compound. Polym Degrad Stab 1995;50:29–32. [24] Oldfield D, Symes T. Long term natural aging of silicone elastomers. Polym Test 1996;15:115–28. [25] Zaharescu T, Jipa S, Podina C. Thermal behavior of ethylene–propalyne elastomer. Polym Test 1998;17:99–106. [26] Brown RP, Soulagnet G. Microhardness profiles on aged rubber compounds. Polym Test 2001;20:295–303.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
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
Article history: Received 19 April 2008 Accepted 27 May 2008 Available online 4 June 2008
a b s t r a c t Aging temperatures play important role in changing the mechanical behavior of rubber, so thermal aging test under different temperatures was carried out to investigate the effect of aging temperatures on the tension, compression and hardness properties of styrene butadiene rubber (SBR) and nitrile butadiene rubber (NBR) compounds filled with different carbon black (CB) loading. The obtained results of five different compositions for SBR and NBR with 0, 20, 30, 50 and 70 phr of CB were compared. The dependences of the mechanical properties on aging temperature and CB loading were found. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Particle filled elastomeric composites have become attractive owing to their low cost and widespread industrial applications [1]. Most usage of elastomers would be impossible without the reinforcing character of certain fillers, such as carbon blacks which have favourably modify properties such as stiffness, tensile strength, heat distortion, mouldability and other important properties. Most of the rubber used in industry subjected to thermal aging which result from the exposure of the rubber to a high temperatures then reduce this temperature that lead to drastically changes in the rubber properties (thermal degradation). Elastomers degrade in a wide variety of environments and service conditions; this degradation limits the service lifetime of many elastomers [2–4]. For instance, in oxygen containing environments, the mechanical strength of rubbers can be greatly affected by oxidation, especially at relatively high temperatures [5]. In order to determine the resistance of a vulcanizate to oxidation, the accelerated aging tests are typically used. Most investigations on the aging of elastomers have been focused on the changes in the chemical nature of the elastomer during extended exposure to heat, oxygen, ozone, and various other environments. During thermal aging, main-chain scission, crosslink formation and crosslink breakage can occur, leading to severe changes in the mechanical properties of elastomers [6–8]. Oxidative degradation is generally considered to be the most serious problem in the use of rubber at high temperature.
* Corresponding author. Tel.: +20 882372217. E-mail address: [email protected] (A. Mostafa). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.05.065
The thermal aging is clarified on the most desired properties needed in rubber industry, tension, compression and hardness properties, so the effect of fillers on these properties has been done by several investigators. Mandal et al. [9] studied the influence of carbon black filler loading on the hardness characteristics of two types of carboxylated acrylonitrile butadiene rubber (XNBR) vulcanizates having different crosslinking reaction systems, in both crosslinking systems the increase in hardness degree is linear. Nunes et al. [10] expanded the work of Mandal and studying the influence of two types of precipitated silica filler loading on the hardness of polyurethane elastomer. As expected, hardness increases as silica concentration increases. Pandey [11] shows that the poor mechanical strength of the gum vulcanizate can be improved by adding various types of fillers (carbon black, silica and mica), these improvement due to the filler–matrix interaction. Badawy [12] studied the effect of variation of high abrasion furnace black (HAF) content on the elastic behavior of butyl rubber (IIR) vulcanizates. Young’s modulus and tensile elongation at break of these composites have been studied as a function of both HAF content and working temperature. Badawy shows that the relative Young’s modulus (modulus for the composite divided by that for the unfilled butyl rubber) depends on the weight percentage of the filler at different temperature. Where, as the filler content increases, Young’s modulus increases. Today the most sophisticated investigation techniques are used to characterize reinforcing fillers and to understand the very origin of rubber–filler interactions. Since interactions between the various 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 in the uncured state of the compound, and the reinforcement obtained after vulcanization. This research will be concerned
792
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
Table 1 Composition of carbon black filled SBR and NBR systems Ingredients Phra
Formula no. S1
S0
S2
S3
S4
N0
N1
N2
N3
N4
SBR-1502 100 100 100 100 100 – – – – – NBR – – – – – 100 100 100 100 100 ZnO 5 5 5 5 5 5 5 5 5 5 Stearic acid 2 2 2 2 2 2 2 2 2 2 Processing oil 10 10 10 10 10 10 10 10 10 10 Carbon black 0 20 30 50 70 0 20 30 50 70 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 Sulfur 2 2 2 2 2 2 2 2 2 2 a
primarily with the filler loading and its effects on thermal aging properties of the vulcanizates. Many researchers have extensively studied the thermal aging behavior of filled rubber, but the effect of carbon black loading on aging resistance of SBR and NBR rubber compounds are seldom treated [13–15]. Therefore the aim of this paper is to determine the effect of CB loading on the ability of filled elastomer to withstand the effect of thermal aging under different aging temperatures to detect its behavior under deterioration environment which rubber usually subjected. The thermal aging is clarified on the most desired properties needed in rubber industry, tension, compression
Parts per hundred of rubber by weight.
1.6
25ºC
Stress, MPa
1
70ºC
0.8
100ºC 125ºC
0.6
Stress, MPa
1.2
1.4
25ºC
1.2
70ºC 100ºC
1
125ºC
0.8 0.6 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
(a) Gum
500
Strain%
(a) Gum
6
25ºC
2.5
5
70ºC
25ºC
Stress, MPa
Stress, MPa
2
70ºC 1.5
100ºC 125ºC
1
4
100ºC 125ºC
3 2 1
0.5
0 0
0 0
50
100
150
200
250
300
350
400
50
100
150
200
250
300
350
400
Strain%
450
Strain%
(b) NBR with 20 phr CB
(b) SBR with 20 phr CB 18
14
14
100ºC
8
125ºC
6
Stress, MPa
Stress, MPa
70ºC 10
25ºC 70ºC
16
25ºC 12
100ºC 125ºC
12 10 8 6
4
4
2
2 0
0
0
50
100
150 200 Strain%
250
300
350
(c) SBR with 70 phr CB Fig. 1. The stress–strain curves for SBR vulcanizates filled with different CB loading under different aging temperatures.
0
50
100
150
200
250
300
350
Strain%
(c) NBR with 70 phr CB Fig. 2. The stress–strain curves for NBR vulcanizates filled with different CB loading under different aging temperatures.
793
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
and hardness properties. The results are compared for SRR and NBR rubber compounds.
the specimens shall be in the form of a rigid cylinder whose length 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 carbon black according to the recipe shown in Table 1. Ingredients of the rubber compounds were mixed on a two-roll laboratory mill of 80 mm diameter, 300 mm length, the speed of slow roll being 24 rpm and the gear ratio 1.4. The ingredients were added according to ASTM D15 [16]. For each type of rubber compounds, the vulcanization process was performed by compression molding process at 160 °C for 25 min under a pressure of approximately 400 kN/m2 from an electrical resistance heating press. To determine the deterioration of the physical properties of SBR and NBR vulcanizates after aging in oven, a circulated air oven (Thermolyne-oven series 9000) is used. Place the specimens for aging in the oven after it has been preheated to the operating temperature. Then the oven temperature adjusted to the operating temperature (70 or 100 or 125 °C). At the termination of the aging interval (48 h), remove the specimens from the oven, cool to room temperature on a flat surface and allow them to rest not less than 16 h or more than 96 h before determination of the physical properties. A tension test on dumbbell-shaped specimens is carried out according to ASTM D412 standard [17]. For compression (ASTM D 695 standard) [18] and hardness (ASTM D 676 standard) [19] tests
There have been several methods developed to monitor the aging condition of rubbers. Tensile strength and elongation at break testing are two parameters often found to be the most direct and useful indicators of the remaining mechanical properties [20,21]. The results of thermal aging test are represented in Figs. 1–7. Figs. 1 and 2 show the stress–strain curves for SBR and NBR vulcanizates filled with different CB loading at room temperature 25 °C and with different aging temperature at 70, 100 and 125 °C. From these figures it is clear that at the beginning of the tension test for small strain the stress–strain curves are come close for each type of filled compounds at different aging temperatures, while as strain increase these curves diverse from each other. Also, the shapes of stress–strain curves were not change due to the thermal aging. Figs. 3 and 4 show the effect of CB loading on the 300% modulus and the elongation at break under different aging temperatures for SBR and NBR vulcanizates, respectively. It can be found that due to thermal aging both the 300% modulus and elongation at break decreases. This is due to the oxidative degradation developed very rapidly leading to this marked decrease [22–25] This phenomenon is more pronounced as the aging temperature increases. It is evident from these figures that gum vulcanizates
500
12
450
300% Modulus, MPa
Elongation at Break %
25°C 10
70°C 100°C
8
125°C 6 4 2
350 300 250 200
25°C
150
70°C
100
100°C
50
125°C
0
0 0
10
20
30
40
50
60
70
0
80
10
20
30
40
50
60
Carbon Black Loading, phr
Carbon Black Loading, phr
(a) SBR vulcanizates
(a) SBR vulcanizates
70
80
400
18 16
25°C
14
70°C
350
Elongation at Break %
300% Modulus, Mpa
400
100°C
12
125°C
10 8 6 4
300 250 200
25°C
150
70°C
100
100°C 50
2
125°C
0
0 0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
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 under different aging temperatures.
70
80
Fig. 4. Variation of the elongation at break versus CB content for filled vulcanizates under different aging temperatures.
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
has more resistance (300% modulus) towards aging as compared with the loaded samples. This detectable resistance was decreased with increasing CB loading, which may be attributed to the fact that CB accelerates the oxygen uptake of sulfur-cured rubber and the reaction is accompanied by a rapid degradation of rubber. So, CB work as a catalyst for the direct oxidation leading to their deactivation. This leads to the decrease in stability with increasing CB content. It can be found that NBR filled compounds offer more resistance to thermal degradation compared with SBR filled compounds due to the high rubber–filler interaction and the presence of nitrile group. Fig. 5 shows the variations of hardness versus aging temperatures for SBR and NBR with different CB loading. It can first be observed that the measurements are generally very consistent demonstrating that the use of the hardness test is both sensitive and repeatable enough to detect the thermal degradation. It is clear that for each type of filled compounds the hardness value increases as aging temperatures increases. This is due to the high crosslinks formation and the oxidizing skin which results from oxygen uptake at the surface of the specimen [26]. So, the increase in aging temperature results in increasing the hardness of both types of vulcanizates. This increase in hardness as aging temperature increase is nearly representing a linear relationship. At a given CB loading, hardness of NBR filled vulcanizates is clearly greater than those in SBR vulcanizates due to the high rubber–filler interaction and the presence of the nitrile group. Figs. 6 and 7 represent the results of the compressive stress– strain tests for SBR and NBR vulcanizates filled with different CB loading under different aging temperatures 25, 70, 100 and 125 °C upto 25% strain. It is clear that at aging temperature
125 °C the compressive strength is lower than the compressive strength at lower aging temperature (100 and 70 °C) for each filled compounds, but the difference is not vast as in the case of tension test; this is due to the formation of high oxidation process which results in chain scission. 4. Conclusions From the current investigation of thermal aging behavior of SBR and NBR compounds filled with different CB loading the following conclusions were derived from the experimental results 1.6 1.4
25ºC
1.2
Stress, MPa
794
70ºC 100ºC 125ºC
1 0.8 0.6 0.4 0.2 0 0
5
10
15
20
25
30
Strain%
(a) Gum 1.8
1.4
50 40
S0 30
Stress, MPa
60
Hardness, Shore A
25ºC 70ºC 100ºC 125ºC
1.6
70
S1
10
1 0.8 0.6 0.4
S2
20
1.2
S3
0.2
S4
0 0
0 0
25
50
75
100
125
5
10
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15
20
25
30
Strain%
Temperature, °C
(b) SBR with 20 phr CB
(a) SBR filled vulcanizates 4 80
3.5
25ºC 70ºC 100ºC 125ºC
3
60 50 40
N0
30
N1
20
N2
10
Stress, MPa
Hardness, Shore A
70
2.5 2 1.5 1
N3
0.5
N4
0
0
0 0
25
50
75
100
125
150
Temperature, °C
5
10
15
20
25
30
Strain%
(c)SBR with 70 phr CB
(b) NBR filled vulcanizates Fig. 5. Effect of aging temperatures on the hardness of filled vulcanizates.
Fig. 6. The compressive stress–strain curves for SBR vulcanizates filled with different CB loading under different aging temperatures.
A. Mostafa et al. / Materials and Design 30 (2009) 791–795
1.8
1.4
25ºC 70ºC 100ºC
1.2
125ºC
Stress, MPa
1.6
1 0.8 0.6 0.4 0.2 0 0
5
10
15
20
25
30
Strain%
(a) Gum
795
attributed to the fact that CB accelerates the oxygen uptake of sulfur-cured rubber and the reaction is accompanied by a rapid degradation of rubber. (3) NBR filled compounds offer more resistance to thermal degradation compared with SBR filled compounds due to the high rubber–filler interaction and the presence of nitrile group. (4) For both types of vulcanizates, as CB loading increases the hardness increases as a result to increase in crosslinking which make the vulcanizates more rigid. At a given CB loading, hardness of NBR filled vulcanizates is clearly greater than those in SBR vulcanizates due to the high rubber–filler interaction and the presence of the nitrile group. (5) The increase in aging temperature results in decrease the compressive strength of both types of vulcanizates. This is due to the formation of high oxidation process which results in chain scission. References
2.5
25ºC 70ºC 100ºC 125ºC
Stress, MPa
2
1.5
1
0.5
0 0
5
10
15
20
25
30
Strain%
(b) NBR with 20 phr CB 10 9
25ºC 70ºC 100ºC 125ºC
8
Stress, MPa
7 6 5 4 3 2 1 0 0
5
10
15
20
25
30
Strain%
(c) NBR with 70 phr CB Fig. 7. The compressive stress–strain curves for NBR vulcanizates filled with different CB loading under different aging temperatures.
(1) Due to the thermal aging both the 300% modulus and the elongation at break for SBR and NBR filled vulcanizates decrease. This is due to the oxidative degradation developed very rapidly leading to this marked decrease. This phenomenon is more pronounced with the increase of aging temperature. (2) The gum vulcanizates have more resistance (300% modulus) towards aging as compared with the CB loading vulcanizates. This detectable resistance was decreased with increasing CB loading and aging temperature. Which may be
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