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Study on mechanical properties of nano-Fe3O4 reinforced nitrile butadiene rubber
Materials and Design 31 (2010) 1023–1028
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Study on mechanical properties of nano-Fe3O4 reinforced nitrile butadiene rubber Qilei Wang *, Fengyu Yang, Qian Yang, Junhui Chen, Hongyan Guan Research Institute of Magnetic Physics and Magnetic Technology, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
Article history: Received 1 June 2009 Accepted 22 July 2009 Available online 25 July 2009
a b s t r a c t In this study, we used nano-Fe3O4 particles as modified filler to prepare NBR/nano-Fe3O4 composites. With adding different mass fraction of nano-Fe3O4, mechanical properties such as tensile strength, modulus at 300% elongation, shore A hardness, surface microstructure, magnetic character and tribological properties such as friction and wear performance of the NBR/nano-Fe3O4 composites were investigated. The anti-wear and friction-reducing properties were also discussed. The results indicated that there was little change on mechanical properties, but the tribological properties were greatly improved, which laid both theoretical and applied foundation for the design of high-performance lubricating composites used for sealing. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nitrile butadiene rubber (NBR) products, such as rotary shaft seals, tires, and conveyor belts, are mainly used under dynamic sliding conditions. Decreasing the friction coefficient of NBR effectively is significant to the reduction of friction force and the lessening of wear. At present, there are two major areas in the anti-wear and friction-reducing research of rubber: one is to add particles or fiber into rubber to enhance wear resistance [1–6]; the other involves creating high-performance and wear-resistant rubber by using modern surface technology. Currently, the most widely used manner is to precipitate polytetrafluoroethylene (PTFE) film on the surface of polymer or macromolecular composites to reduce friction coefficient [7,8]. However, in practice, due to the low intensity and poor surface adhesion of PTFE film, wear resistance of the composites is poor. Study on the performance of rubber–metal pair has been an important area in the tribology research field. There has been many interrelated researches on determining the way to decrease friction coefficient and improve mechanical properties of rubber composites. Jin and his team have studied the changes of mechanical properties after the addition of calcium carbonate in the butadiene rubber [9]; Turki et al. analyzed the microstructure, physical and mechanical properties of mortar–rubber aggregates mixtures [10]. Feng et al. studied the tribological properties of nanoMnZnFe2O4 magnetic particles which was an additive in base oil [11]. However, data about the effect of magnetic nano-filler on mechanical properties and tribological properties of rubber is still rare. In this research, we selected the nano-Fe3O4 particles which have high magnetic saturation intensity as filler added in the
* Corresponding author. Tel.: +86 931 2976776. E-mail address: [email protected] (Q. Wang). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.07.038
NBR. After compounding and rolling, we got the NBR/nano-Fe3O4 composites with excellent magnetic properties and low relative density that are suitable to be processed into products with high precision and complex shape. Then a preliminary analysis on mechanical properties and tribological properties was carried out. The anti-wear properties and the repairing function of the wear surface were characterized using a universal test machine (UTM) and a 2206-surface roughness tester.
2. Experimental details 2.1. Sample preparation The NBR/nano-Fe3O4 composites were prepared by adding nano-Fe3O4 in the oil-resistant and corrosion-resistant NBR [12,13]. Compared with conventional permanent magnet rubber, thermal stabilities and magnetic properties of the NBR/nanoFe3O4 composites are better because of its high corrosion-resistance to oil such as gasoline, light oil, and to conventional chemical solvents. In the petroleum and chemical industry, there are high requirements of sealing elements for resistance to oil, acid–bases corrosion, and high temperature. Consequently, the better peculiar performance of the NBR/nano-Fe3O4 composites will bring itself wider potential application. We selected nano-Fe3O4 particles with a diameter less than 50 nm because of the excellent performance, and also because of they would be distributed equably during compounding. High stretch elongation and high elastic properties are often required for rubber products. In order to ensure the mechanical properties of the NBR/nano-Fe3O4 composites were close to the properties of industrial NBR, mass fraction of the nano-Fe3O4 was controlled at 8%, 10%, 12%, 14% and 16% during preparation. Raw materials and batch formula are summarized in Table 1.
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Table 1 Primary materials and batch formula. Raw material
Soot carbon N300 N41 gum Stearine DM accelerant Sulfur HomogenizingM40 Nano-Fe3O4
Sample (mass fraction, %) 1#
2#
3#
4#
5#
6#
25.9 69.7 0.70 0.70 1.0 2 0.0
23.6 64.0 0.7 0.7 1.0 2 8.0
22.8 62.8 0.7 0.7 1.0 2 10.0
21.4 62.2 0.7 0.7 1.0 2 12.0
20.5 61.1 0.7 0.7 1.0 2 14.0
19.6 60.0 0.7 0.7 1.0 2 16.0
The NBR/nano-Fe3O4 composites were prepared by dry compounding. First, raw materials were weighted according to the formula, and the N41-NBR was mixed to a certain extent. Then nano-Fe3O4 particles were added into the mixer according to mass fraction of 8%, 10%, 12%, 14% and 16%, respectively. After uniform mixing, the materials were rolled into film in X (S) K-160 smelting machine, and then the samples were stacked into the required thickness. Finally, the samples were vulcanized in a Y33-50A plate vulcanizing press at 145 °C, 10 MPa for 30 min. After vulcanization, the samples were magnetized with a U5-10 magnetizing machine for 20 min. The intensity of the magnetic field should be higher than 1.2 104GS. 2.2. Performance tests The test was conducted on the universal test machine (UTM). UTM is a transverse plane test machine which mainly tests surface-contact and analyzes tribological properties of materials. It is specially suitable for testing friction and wear properties of material, assessing anti-wear and friction-reducing properties of material under condition of oil lubrication or oil-free lubrication, and evaluating tribological properties and comprehensive performance of the material according to the change of test parameters and wear degree on the sample surface. The test was carried out in Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. The test-viewer software was used to monitor and test results. Before the test, dust on the surface of the specimens was washed and cleaned away. Then the specimens were wiped with dry cloth and were dried in drying oven. The environmental temperature was 15 °C, and the relative humidity was (60 ± 5)%. Reciprocating frequency of the tester (UTM) was 10 Hz, travel route 10 mm, time 6 min, and the pressure was 2 N. Surface microstructure of the composites was observed by a JSM-5600LV low-vacuum scanning electronic microscope, while the disposition of nano-Fe3O4 was measured by an energy dispersive X-ray spectrometer. The surface microstructure of the NBR/ nano-Fe3O4 composites was observed after the metal spraying process. Tensile stretch was measured after the specimens were vulcanizated with a DXLL-10000 electronic tensile tester according to GB/T528-92. Tensile stretch and maximum elongation were obtained at room temperature. Wear depth was measured on a 2206-surface roughness tester. Shore A hardness was characterized using a XY-I rubber durometer.Size of test samples:
Fig. 1. Test samples.
3. Results and discussion 3.1. Mechanical performance of the composites Fig. 2 shows the change curve of modulus at 300% elongation and tensile strength with different mass fraction of nano-Fe3O4. We can see from Fig. 2 that modulus at 300% elongation and tensile strength of the NBR/nano-Fe3O4 composites are slightly lower than that of directly mixed NBR. By increasing the mass fraction of nano-Fe3O4, modulus at 300% elongation and tensile strength gradually reduce and tend to ease. However, when the mass fraction is 12%, they reach a stable level. The reason for the decline of modulus at 300% elongation and tensile strength after adding nanoFe3O4 could be a large area of phase interface existing between nano-Fe3O4 particles and the rubber matrix [14,15]. Also the small binding force between the two phases and the increasing internal defects could be other reasons. As seen in Fig. 3, by increasing the mass fraction of nano-Fe3O4, maximum elongation of the NBR/nano-Fe3O4 composites declines gradually, and the elongation at break reaches the minimum when the mass fraction comes up to 10%. The main reason leading to smaller maximum elongation may be the strong restrictions on crystal layer and their congeries of nano-Fe3O4 on the polymer chains. As shown in Fig. 4, the hardness of the NBR/nano-Fe3O4 composites is larger than that of NBR. Shore A hardness of NBR is 63°, while shore A hardness of the NBR/nano-Fe3O4 composites is 68° when the mass fraction of nano-Fe3O4 is 16%. The reason for
(a) Sample size of modulus at 300% elongation, tensile strength: 6 mm 2 mm (middle). (b) Sample size of tribological property: £44 6 mm. (c) Sample size of shore A hardness: £24.5 8 mm. (d) Sample size of magnetic property: 116 mm 108 mm 1 mm. Test samples are shown in Fig. 1.
Fig. 2. Change curve of effect on tensile-strength and modulus at 300% elongation with increasing the mass fraction of nano-Fe3O4.
Q. Wang et al. / Materials and Design 31 (2010) 1023–1028
Fig. 3. Change curve of effect on maximum elongation with increasing the mass fraction of nano-Fe3O4.
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Fig. 5. Magnetic hysteresis loops of NBR/nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 8%, 10%, 12% and 14%.
Fig. 6. Changing curve of saturation magnetization strength (Bs), residual magnetic flux density (Br) and coercive force (Hc) with different mass fraction of nano-Fe3O4 particles. Fig. 4. Change curve of effect on shore A hardness with increasing the mass fraction of nano-Fe3O4.
the increase of hardness, on one hand, is that the hardness of nanoFe3O4 is bigger than that of NBR which deservedly increased the hardness of the composites [14]; and on the other hand, nanoFe3O4 particles filled the cavity of NBR, which increased the hardness as well. Fig. 5 shows the magnetic hysteresis loops of NBR/nano-Fe3O4 composites. Clearly, by increasing the mass fraction of nanoFe3O4, magnetic properties of the NBR/nano-Fe3O4 composites monotonely increase, saturation magnetization strength of the composites increase constantly. The composites show typical characteristics of paramagnetism. Due to internal defects in rubber materials, wall energy is reduced and the effect of magnetization is influenced, so the magnetic density needed during the magnetization process is relatively larger. With the magnetic intensity increasing, saturation magnetization strength of the NBR/nanoFe3O4 composites increases too. When the magnetic strength increased to 10,000 Oe, the value of saturation magnetization strength tends basically to steady. Fig. 6 shows the changing curve of saturation magnetization strength (Bs), residual magnetic flux density (Br), and coercive force (Hc) with different mass fraction of nano-Fe3O4 particles. As in 4, Hc of the NBR/nano-Fe3O4 composites is small. When the mass
fraction of nano-Fe3O4 is 12%, the value of Hc is 9.8 emu/g which is relatively smaller. Bs and Br keep good linear relationship with the mass fraction of nano-Fe3O4. By increasing the mass fraction of nano-Fe3O4, Bs and Br continuously become larger. This is because magnetic properties of the NBR/nano-Fe3O4 composites do not depend on the way nano-Fe3O4 particles scatter, also do not depend on physical processes between particles and substrate or particle and particles, but only depend on the mass fraction of nanoFe3O4 [18]. Meanwhile, nano-Fe3O4 particles are ferrimagnetics which have typical characteristic of high frequency, therefore, hysteresis losses are lesser. These all lay a foundation for further application of NBR/nano-Fe3O4 composites. By comparing it with the ordinary sintered magnet, magnetic properties of the NBR/nanoFe3O4 composites are superior. This is because the temperature during the preparation of the NBR/nano-Fe3O4 composites is lower than the sintering temperature of the ordinary sintered magnet [8,18], so that the influence of high-temperature demagnetization on the magnetic properties is insignificant. 3.2. Surface microstructure Surface microstructure of ordinary NBR is shown in Fig. 7a, while surface microstructure of the NBR/nano-Fe3O4 composites is shown in Fig. 7b. Through analysis and comparison, surface of
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Fig. 7. SEM pictures of surface microstructure of NBR filled with nano-Fe3O4: (a) 0% and (b) 10% (500).
Fig. 8. The back scattered image of microscopic surface of MNBR with nano-Fe3O4 10%. Fig. 9. Change curve of effect on friction coefficient with increasing the mass fraction of nano-Fe3O4 at 2 N pressure. Table 2 The content distribution of Fe of areas a, b, and c. Element
Line
Weight (%)
Cnts (s)
Atomic (%)
Fe (a area) Fe (b area) Fe (c area)
Ka Ka Ka
6.40 7.18 8.45
16.13 16.13 13.98
5.71 5.71 6.45
ordinary NBR was rough with obvious surface defects and more pits, while the surface of the NBR/nano-Fe3O4 composites tends to be smooth with less surface defects and pits because the nano-Fe3O4 particles filled cavities of NBR [16]. The back scattered image of microscopic surface of the NBR/nano-Fe3O4 composites is shown in Fig. 8. As in Fig. 8, we selected areas a, b and c to analyze the distribution of nano-Fe3O4. The results are shown in Table 2. Table 2 gives the content distribution of Fe. We can see from Table 2 that the percentage composition, atom percentage composition of Fe, and counting rate of X-ray are basically in line with the addition of nano-Fe3O4. There are minute differences between different regions owing to the inhomogenous mixing. However, in terms of overall effect, the mixing is relatively uniform. 3.3. Friction and wear properties Fig. 9 shows the changing curve of friction coefficient with increasing the mass fraction of nano-Fe3O4 at 2 N pressure. Clearly, the addition of nano-Fe3O4 obviously reduced the friction coefficient of NBR. Compared with unreinforced sample, the sample reinforced had a smaller friction coefficient. Under dry friction condition, with increasing the mass fraction of nano-Fe3O4, the friction
coefficient decreased first, increased when the mass fraction reaches 10%, and then decreased again at 12%. After that, with increasing the mass fraction of nano-Fe3O4, the friction coefficient increased gradually. The addition of nano-Fe3O4 enhanced the bonding force between the transfer film and the grinding surface and contributed to form a complete and solid friction transfer film [16], so that the friction always occur between the transfer film and the rubber matrix, while the friction coefficient maintained at a relatively low level. On the other hand, increased hardness enhanced the resistance to outer pressure, and made it difficult for loosed abrasive to penetrate into the rubber matrix, thus effectively reduced number and depth of penetrated abrasive on the surface of the composites, which reduced abrasive wear, and made it easy to form a stable transfer film on the grinding surface. 3.4. Analysis of wear on the surface The addition of nano-Fe3O4 was in accordance with the self-repair mechanism. The nano-Fe3O4 can repair the wear surface by padding micro valley and small part of defects on the friction surface, which makes the friction surface relatively flat. Fig. 10 shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites. No matter in steady state or unsteady state, basic feature of line contact abrasive wear is the ridge-like wear patterns on the wear surface with microscopic wear debris. In addition a small amount of nano-Fe3O4 has a significant effect on resistance to friction and wear, for it can improve the formation of transfer film and reduce friction factors.
Q. Wang et al. / Materials and Design 31 (2010) 1023–1028
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macro-hardness of NBR and the poor ability of anti-plow and antiadhesion, as well as cut and squeeze brought by the rough peak of dual face. Fig. 10b and c shows the surface topography profile curve of the NBR/nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 8% and 10%, respectively. There are fatigue flake, traces of adhesion, and furrows on the wear surface. The wear track is shallower than that in Fig. 10a. When the mass fraction of nano-Fe3O4 is 8% and 10%, the wear depth is 19 lm and 11 lm, respectively. This explains that when the mass fraction of nano-Fe3O4 is low, macrohardness and anti-plowing capacity will be improved due to the effect of hard phase. In the process of friction and wear, nano-Fe3O4 embed in the furrows under the action of pressure, thus they can be affected by the process of repairation. Fig. 10d shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 12%. Clearly, the sample surface is smooth, with no furrows on, and the wear track is weeny. This indicates that the tribological properties attain the optimum. Fig. 10e shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 16%. There are no significant furrows on the wear surface, but there are signs of shallow fatigue flake. Meanwhile, the sample surface is smooth and the wear rate is small, so the main wear form is adhesive wear. The addition of nano-Fe3O4 improved the macro-hardness and the anti-plowing ability of NBR because the effect of hard phase [16,17]. On the other hand, there was still absorption of the metal surface although nano-Fe3O4 did not show magnetic properties in the absence of the magnetic field [18]. Moreover, due to the increased adhesion between the transfer film and the wear surface, the strength to form adsorption film on the friction surface was enhanced and the wear performance was improved. With different mass fraction of nano-Fe3O4, wear forms were also different. With a smaller mass fraction, wear forms were mainly plowing and adhesive wear. With increasing the mass fraction, wear forms changed gradually into adhesive flake and fatigue flake.
4. Conclusions
Fig. 10. Surface topography profile curve of MNBR with different mass fraction of nano-Fe3O4.
Fig. 10a shows the surface topography profile curve of NBR. There are clear plastic deformations, plowing traces as well as adhesive flake on the wear surface. The width of wear track is 1830 lm, and the wear depth is 57 lm. This is due to the low-level
(1) Through experimental results we can see that with a smaller mass fraction of nano-Fe3O4, tensile strength and elongation at break of the NBR/nano-Fe3O4 composites are slightly weakened, but the hardness is increased. That is, a smaller mass fraction of nano-Fe3O4 can bring a little effect on the mechanical properties of NBR. (2) Clearly, by increasing the mass fraction of nano-Fe3O4, magnetic properties of the NBR/nano-Fe3O4 composites monotonely increase, saturation magnetization strength of the composites increase constantly. The magnetic properties of the NBR/nano-Fe3O4 composites are superior. (3) The NBR/nano-Fe3O4 composites show good anti-wear and friction-reducing properties. By increasing the mass fraction of nano-Fe3O4, friction coefficient and wear extent are significantly reduced. By contrast, the surface microstructure of the NBR/nano-Fe3O4 composites is smoother and there are lesser defects. (4) The best friction performance appears when the mass fraction of nano-Fe3O4 is 12%. The nano-Fe3O4 is conducive to forming physical adsorption film and self-repairing film on the friction surface. It has the effect of improving the resistance to wear and friction. Clearly, the addition of nanoFe3O4 particles can improve friction and wear performance of NBR, and is helpful for its application in sealing.
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Acknowledgments We thank the Natural Science Foundation of Gansu Province, China (No.: 3ZS051-A25-031), as well as Scientific and Technological Exchanges Project (No.: H20070021) of Wenzhou Science and Technology Bureau, China for their financial support. References [1] Huzs et al. Study on antiwear and reducing friction additive of nanometer ferric oxide. Tribol Int 1998;31:355–60. [2] Rattanasom N, Saowapark T, Deeprasertkul C. Reinforcement of natural rubber with silica/carbon black hybrid filler. Polym Test 2007;26:369–77. [3] Pongdhorn Sae-oui, Chakrit Sirisinha, Uthai Thepsuwan. Dependence of mechanical and aging properties of chloroprene rubber on silica and ethylene thiourea loadings. Eur Polym J 2007;43:185–93. [4] Xinchun Guan, Xufeng Dong, Jinping Ou. Magnetostrictive effect of magnetorheological elastomer. J Magn Magn Mater 2008;320:158–63. [5] Meng-Heng Yeh, Weng-Sing Hwang, Lin-Ri Cheng. Microstructure and mechanical properties of neoprene–montmorillonite nanocomposites. Appl Surf Sci 2007;253:4777–81. [6] Findik F, Yilmaz R, Köksal T. Investigation of mechanical and physical properties of several industrial rubbers. Mater Des 2004;25:269–76. [7] Chen Guoding, Xu Hua, et al. Friction analysis of elastomer-metal pair based on rough contact theory, vol. 36. Xi An Jiaotong. University; 2002. p. 322–4. [8] Makled MH, Matsui T, Tsuda H, et al. Magnetic and dynamic mechanical properties of barium ferrite–natural rubber composites. J Mater Proc Technol 2005;160:229–33.
[9] Fan-Long Jin, Soo-Jin Park. Thermo-mechanical behaviors of butadiene rubber reinforced with nano-sized calcium carbonate. Mater Sci Eng A 2008;478:406–8. [10] Turki M, Bretagne E, Rouis MJ, et al. Microstructure, physical and mechanical properties of mortar–rubber aggregates mixtures. Constr Build Mater 2009;23:2715–22. [11] Feng Xuejun, Yang Zhiyi. Tribological properties of nano-MnZnFe2O4 magnetic particles as an additive in base oil. Lubric Eng 2007;32:122–4. [12] Li Yingni. The magnetic rubber mixing process and performance. World Rubber Indus 2005;32:26–31. [13] Wei Zhang, Xinxing Zhang, Mei Liang. Mechanochemical preparation of surface-acetylated cellulose powder to enhance mechanical properties of cellulose-filler-reinforced NR vulcanizates. Compos Sci Technol 2008;68:2479–84. [14] Zhang Changdong, Wang Li, Song Guojun, et al. Structure and properties of nitrile rubber/polyvinyl chloride/organic montmorillonite nanocomposites prepared by mechanical blending. China Synth Rubber Indus 2009;32:127–30. [15] Wang Yiqing, Zhang Huifeng, Wu Youping, et al. Structure and properties of rectorite/nitrile rubber nanocomposites. China Synth Rubber Indus 2005;28:187–90. [16] Zhu M, Zhang ZZ, Wang K, et al. Study on the friction and wear properties of polyimide composites filled with polyfluortetraethylene and MoS2. Tribology 2004;24:522–6. [17] Degrange J-M, Thomine M, Kapsa Ph, et al. Influence of viscoelasticity on the tribological behaviour of carbon black filled nitrile rubber (NBR) for lip seal application. Wear 2005;259:684–92. [18] Minbo Tian. Magnetic material. Beijing: Tsinghua University Press; 2001.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Study on mechanical properties of nano-Fe3O4 reinforced nitrile butadiene rubber Qilei Wang *, Fengyu Yang, Qian Yang, Junhui Chen, Hongyan Guan Research Institute of Magnetic Physics and Magnetic Technology, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
Article history: Received 1 June 2009 Accepted 22 July 2009 Available online 25 July 2009
a b s t r a c t In this study, we used nano-Fe3O4 particles as modified filler to prepare NBR/nano-Fe3O4 composites. With adding different mass fraction of nano-Fe3O4, mechanical properties such as tensile strength, modulus at 300% elongation, shore A hardness, surface microstructure, magnetic character and tribological properties such as friction and wear performance of the NBR/nano-Fe3O4 composites were investigated. The anti-wear and friction-reducing properties were also discussed. The results indicated that there was little change on mechanical properties, but the tribological properties were greatly improved, which laid both theoretical and applied foundation for the design of high-performance lubricating composites used for sealing. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nitrile butadiene rubber (NBR) products, such as rotary shaft seals, tires, and conveyor belts, are mainly used under dynamic sliding conditions. Decreasing the friction coefficient of NBR effectively is significant to the reduction of friction force and the lessening of wear. At present, there are two major areas in the anti-wear and friction-reducing research of rubber: one is to add particles or fiber into rubber to enhance wear resistance [1–6]; the other involves creating high-performance and wear-resistant rubber by using modern surface technology. Currently, the most widely used manner is to precipitate polytetrafluoroethylene (PTFE) film on the surface of polymer or macromolecular composites to reduce friction coefficient [7,8]. However, in practice, due to the low intensity and poor surface adhesion of PTFE film, wear resistance of the composites is poor. Study on the performance of rubber–metal pair has been an important area in the tribology research field. There has been many interrelated researches on determining the way to decrease friction coefficient and improve mechanical properties of rubber composites. Jin and his team have studied the changes of mechanical properties after the addition of calcium carbonate in the butadiene rubber [9]; Turki et al. analyzed the microstructure, physical and mechanical properties of mortar–rubber aggregates mixtures [10]. Feng et al. studied the tribological properties of nanoMnZnFe2O4 magnetic particles which was an additive in base oil [11]. However, data about the effect of magnetic nano-filler on mechanical properties and tribological properties of rubber is still rare. In this research, we selected the nano-Fe3O4 particles which have high magnetic saturation intensity as filler added in the
* Corresponding author. Tel.: +86 931 2976776. E-mail address: [email protected] (Q. Wang). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.07.038
NBR. After compounding and rolling, we got the NBR/nano-Fe3O4 composites with excellent magnetic properties and low relative density that are suitable to be processed into products with high precision and complex shape. Then a preliminary analysis on mechanical properties and tribological properties was carried out. The anti-wear properties and the repairing function of the wear surface were characterized using a universal test machine (UTM) and a 2206-surface roughness tester.
2. Experimental details 2.1. Sample preparation The NBR/nano-Fe3O4 composites were prepared by adding nano-Fe3O4 in the oil-resistant and corrosion-resistant NBR [12,13]. Compared with conventional permanent magnet rubber, thermal stabilities and magnetic properties of the NBR/nanoFe3O4 composites are better because of its high corrosion-resistance to oil such as gasoline, light oil, and to conventional chemical solvents. In the petroleum and chemical industry, there are high requirements of sealing elements for resistance to oil, acid–bases corrosion, and high temperature. Consequently, the better peculiar performance of the NBR/nano-Fe3O4 composites will bring itself wider potential application. We selected nano-Fe3O4 particles with a diameter less than 50 nm because of the excellent performance, and also because of they would be distributed equably during compounding. High stretch elongation and high elastic properties are often required for rubber products. In order to ensure the mechanical properties of the NBR/nano-Fe3O4 composites were close to the properties of industrial NBR, mass fraction of the nano-Fe3O4 was controlled at 8%, 10%, 12%, 14% and 16% during preparation. Raw materials and batch formula are summarized in Table 1.
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Q. Wang et al. / Materials and Design 31 (2010) 1023–1028
Table 1 Primary materials and batch formula. Raw material
Soot carbon N300 N41 gum Stearine DM accelerant Sulfur HomogenizingM40 Nano-Fe3O4
Sample (mass fraction, %) 1#
2#
3#
4#
5#
6#
25.9 69.7 0.70 0.70 1.0 2 0.0
23.6 64.0 0.7 0.7 1.0 2 8.0
22.8 62.8 0.7 0.7 1.0 2 10.0
21.4 62.2 0.7 0.7 1.0 2 12.0
20.5 61.1 0.7 0.7 1.0 2 14.0
19.6 60.0 0.7 0.7 1.0 2 16.0
The NBR/nano-Fe3O4 composites were prepared by dry compounding. First, raw materials were weighted according to the formula, and the N41-NBR was mixed to a certain extent. Then nano-Fe3O4 particles were added into the mixer according to mass fraction of 8%, 10%, 12%, 14% and 16%, respectively. After uniform mixing, the materials were rolled into film in X (S) K-160 smelting machine, and then the samples were stacked into the required thickness. Finally, the samples were vulcanized in a Y33-50A plate vulcanizing press at 145 °C, 10 MPa for 30 min. After vulcanization, the samples were magnetized with a U5-10 magnetizing machine for 20 min. The intensity of the magnetic field should be higher than 1.2 104GS. 2.2. Performance tests The test was conducted on the universal test machine (UTM). UTM is a transverse plane test machine which mainly tests surface-contact and analyzes tribological properties of materials. It is specially suitable for testing friction and wear properties of material, assessing anti-wear and friction-reducing properties of material under condition of oil lubrication or oil-free lubrication, and evaluating tribological properties and comprehensive performance of the material according to the change of test parameters and wear degree on the sample surface. The test was carried out in Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. The test-viewer software was used to monitor and test results. Before the test, dust on the surface of the specimens was washed and cleaned away. Then the specimens were wiped with dry cloth and were dried in drying oven. The environmental temperature was 15 °C, and the relative humidity was (60 ± 5)%. Reciprocating frequency of the tester (UTM) was 10 Hz, travel route 10 mm, time 6 min, and the pressure was 2 N. Surface microstructure of the composites was observed by a JSM-5600LV low-vacuum scanning electronic microscope, while the disposition of nano-Fe3O4 was measured by an energy dispersive X-ray spectrometer. The surface microstructure of the NBR/ nano-Fe3O4 composites was observed after the metal spraying process. Tensile stretch was measured after the specimens were vulcanizated with a DXLL-10000 electronic tensile tester according to GB/T528-92. Tensile stretch and maximum elongation were obtained at room temperature. Wear depth was measured on a 2206-surface roughness tester. Shore A hardness was characterized using a XY-I rubber durometer.Size of test samples:
Fig. 1. Test samples.
3. Results and discussion 3.1. Mechanical performance of the composites Fig. 2 shows the change curve of modulus at 300% elongation and tensile strength with different mass fraction of nano-Fe3O4. We can see from Fig. 2 that modulus at 300% elongation and tensile strength of the NBR/nano-Fe3O4 composites are slightly lower than that of directly mixed NBR. By increasing the mass fraction of nano-Fe3O4, modulus at 300% elongation and tensile strength gradually reduce and tend to ease. However, when the mass fraction is 12%, they reach a stable level. The reason for the decline of modulus at 300% elongation and tensile strength after adding nanoFe3O4 could be a large area of phase interface existing between nano-Fe3O4 particles and the rubber matrix [14,15]. Also the small binding force between the two phases and the increasing internal defects could be other reasons. As seen in Fig. 3, by increasing the mass fraction of nano-Fe3O4, maximum elongation of the NBR/nano-Fe3O4 composites declines gradually, and the elongation at break reaches the minimum when the mass fraction comes up to 10%. The main reason leading to smaller maximum elongation may be the strong restrictions on crystal layer and their congeries of nano-Fe3O4 on the polymer chains. As shown in Fig. 4, the hardness of the NBR/nano-Fe3O4 composites is larger than that of NBR. Shore A hardness of NBR is 63°, while shore A hardness of the NBR/nano-Fe3O4 composites is 68° when the mass fraction of nano-Fe3O4 is 16%. The reason for
(a) Sample size of modulus at 300% elongation, tensile strength: 6 mm 2 mm (middle). (b) Sample size of tribological property: £44 6 mm. (c) Sample size of shore A hardness: £24.5 8 mm. (d) Sample size of magnetic property: 116 mm 108 mm 1 mm. Test samples are shown in Fig. 1.
Fig. 2. Change curve of effect on tensile-strength and modulus at 300% elongation with increasing the mass fraction of nano-Fe3O4.
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Fig. 3. Change curve of effect on maximum elongation with increasing the mass fraction of nano-Fe3O4.
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Fig. 5. Magnetic hysteresis loops of NBR/nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 8%, 10%, 12% and 14%.
Fig. 6. Changing curve of saturation magnetization strength (Bs), residual magnetic flux density (Br) and coercive force (Hc) with different mass fraction of nano-Fe3O4 particles. Fig. 4. Change curve of effect on shore A hardness with increasing the mass fraction of nano-Fe3O4.
the increase of hardness, on one hand, is that the hardness of nanoFe3O4 is bigger than that of NBR which deservedly increased the hardness of the composites [14]; and on the other hand, nanoFe3O4 particles filled the cavity of NBR, which increased the hardness as well. Fig. 5 shows the magnetic hysteresis loops of NBR/nano-Fe3O4 composites. Clearly, by increasing the mass fraction of nanoFe3O4, magnetic properties of the NBR/nano-Fe3O4 composites monotonely increase, saturation magnetization strength of the composites increase constantly. The composites show typical characteristics of paramagnetism. Due to internal defects in rubber materials, wall energy is reduced and the effect of magnetization is influenced, so the magnetic density needed during the magnetization process is relatively larger. With the magnetic intensity increasing, saturation magnetization strength of the NBR/nanoFe3O4 composites increases too. When the magnetic strength increased to 10,000 Oe, the value of saturation magnetization strength tends basically to steady. Fig. 6 shows the changing curve of saturation magnetization strength (Bs), residual magnetic flux density (Br), and coercive force (Hc) with different mass fraction of nano-Fe3O4 particles. As in 4, Hc of the NBR/nano-Fe3O4 composites is small. When the mass
fraction of nano-Fe3O4 is 12%, the value of Hc is 9.8 emu/g which is relatively smaller. Bs and Br keep good linear relationship with the mass fraction of nano-Fe3O4. By increasing the mass fraction of nano-Fe3O4, Bs and Br continuously become larger. This is because magnetic properties of the NBR/nano-Fe3O4 composites do not depend on the way nano-Fe3O4 particles scatter, also do not depend on physical processes between particles and substrate or particle and particles, but only depend on the mass fraction of nanoFe3O4 [18]. Meanwhile, nano-Fe3O4 particles are ferrimagnetics which have typical characteristic of high frequency, therefore, hysteresis losses are lesser. These all lay a foundation for further application of NBR/nano-Fe3O4 composites. By comparing it with the ordinary sintered magnet, magnetic properties of the NBR/nanoFe3O4 composites are superior. This is because the temperature during the preparation of the NBR/nano-Fe3O4 composites is lower than the sintering temperature of the ordinary sintered magnet [8,18], so that the influence of high-temperature demagnetization on the magnetic properties is insignificant. 3.2. Surface microstructure Surface microstructure of ordinary NBR is shown in Fig. 7a, while surface microstructure of the NBR/nano-Fe3O4 composites is shown in Fig. 7b. Through analysis and comparison, surface of
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Fig. 7. SEM pictures of surface microstructure of NBR filled with nano-Fe3O4: (a) 0% and (b) 10% (500).
Fig. 8. The back scattered image of microscopic surface of MNBR with nano-Fe3O4 10%. Fig. 9. Change curve of effect on friction coefficient with increasing the mass fraction of nano-Fe3O4 at 2 N pressure. Table 2 The content distribution of Fe of areas a, b, and c. Element
Line
Weight (%)
Cnts (s)
Atomic (%)
Fe (a area) Fe (b area) Fe (c area)
Ka Ka Ka
6.40 7.18 8.45
16.13 16.13 13.98
5.71 5.71 6.45
ordinary NBR was rough with obvious surface defects and more pits, while the surface of the NBR/nano-Fe3O4 composites tends to be smooth with less surface defects and pits because the nano-Fe3O4 particles filled cavities of NBR [16]. The back scattered image of microscopic surface of the NBR/nano-Fe3O4 composites is shown in Fig. 8. As in Fig. 8, we selected areas a, b and c to analyze the distribution of nano-Fe3O4. The results are shown in Table 2. Table 2 gives the content distribution of Fe. We can see from Table 2 that the percentage composition, atom percentage composition of Fe, and counting rate of X-ray are basically in line with the addition of nano-Fe3O4. There are minute differences between different regions owing to the inhomogenous mixing. However, in terms of overall effect, the mixing is relatively uniform. 3.3. Friction and wear properties Fig. 9 shows the changing curve of friction coefficient with increasing the mass fraction of nano-Fe3O4 at 2 N pressure. Clearly, the addition of nano-Fe3O4 obviously reduced the friction coefficient of NBR. Compared with unreinforced sample, the sample reinforced had a smaller friction coefficient. Under dry friction condition, with increasing the mass fraction of nano-Fe3O4, the friction
coefficient decreased first, increased when the mass fraction reaches 10%, and then decreased again at 12%. After that, with increasing the mass fraction of nano-Fe3O4, the friction coefficient increased gradually. The addition of nano-Fe3O4 enhanced the bonding force between the transfer film and the grinding surface and contributed to form a complete and solid friction transfer film [16], so that the friction always occur between the transfer film and the rubber matrix, while the friction coefficient maintained at a relatively low level. On the other hand, increased hardness enhanced the resistance to outer pressure, and made it difficult for loosed abrasive to penetrate into the rubber matrix, thus effectively reduced number and depth of penetrated abrasive on the surface of the composites, which reduced abrasive wear, and made it easy to form a stable transfer film on the grinding surface. 3.4. Analysis of wear on the surface The addition of nano-Fe3O4 was in accordance with the self-repair mechanism. The nano-Fe3O4 can repair the wear surface by padding micro valley and small part of defects on the friction surface, which makes the friction surface relatively flat. Fig. 10 shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites. No matter in steady state or unsteady state, basic feature of line contact abrasive wear is the ridge-like wear patterns on the wear surface with microscopic wear debris. In addition a small amount of nano-Fe3O4 has a significant effect on resistance to friction and wear, for it can improve the formation of transfer film and reduce friction factors.
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macro-hardness of NBR and the poor ability of anti-plow and antiadhesion, as well as cut and squeeze brought by the rough peak of dual face. Fig. 10b and c shows the surface topography profile curve of the NBR/nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 8% and 10%, respectively. There are fatigue flake, traces of adhesion, and furrows on the wear surface. The wear track is shallower than that in Fig. 10a. When the mass fraction of nano-Fe3O4 is 8% and 10%, the wear depth is 19 lm and 11 lm, respectively. This explains that when the mass fraction of nano-Fe3O4 is low, macrohardness and anti-plowing capacity will be improved due to the effect of hard phase. In the process of friction and wear, nano-Fe3O4 embed in the furrows under the action of pressure, thus they can be affected by the process of repairation. Fig. 10d shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 12%. Clearly, the sample surface is smooth, with no furrows on, and the wear track is weeny. This indicates that the tribological properties attain the optimum. Fig. 10e shows the surface topography profile curve of the NBR/ nano-Fe3O4 composites when the mass fraction of nano-Fe3O4 is 16%. There are no significant furrows on the wear surface, but there are signs of shallow fatigue flake. Meanwhile, the sample surface is smooth and the wear rate is small, so the main wear form is adhesive wear. The addition of nano-Fe3O4 improved the macro-hardness and the anti-plowing ability of NBR because the effect of hard phase [16,17]. On the other hand, there was still absorption of the metal surface although nano-Fe3O4 did not show magnetic properties in the absence of the magnetic field [18]. Moreover, due to the increased adhesion between the transfer film and the wear surface, the strength to form adsorption film on the friction surface was enhanced and the wear performance was improved. With different mass fraction of nano-Fe3O4, wear forms were also different. With a smaller mass fraction, wear forms were mainly plowing and adhesive wear. With increasing the mass fraction, wear forms changed gradually into adhesive flake and fatigue flake.
4. Conclusions
Fig. 10. Surface topography profile curve of MNBR with different mass fraction of nano-Fe3O4.
Fig. 10a shows the surface topography profile curve of NBR. There are clear plastic deformations, plowing traces as well as adhesive flake on the wear surface. The width of wear track is 1830 lm, and the wear depth is 57 lm. This is due to the low-level
(1) Through experimental results we can see that with a smaller mass fraction of nano-Fe3O4, tensile strength and elongation at break of the NBR/nano-Fe3O4 composites are slightly weakened, but the hardness is increased. That is, a smaller mass fraction of nano-Fe3O4 can bring a little effect on the mechanical properties of NBR. (2) Clearly, by increasing the mass fraction of nano-Fe3O4, magnetic properties of the NBR/nano-Fe3O4 composites monotonely increase, saturation magnetization strength of the composites increase constantly. The magnetic properties of the NBR/nano-Fe3O4 composites are superior. (3) The NBR/nano-Fe3O4 composites show good anti-wear and friction-reducing properties. By increasing the mass fraction of nano-Fe3O4, friction coefficient and wear extent are significantly reduced. By contrast, the surface microstructure of the NBR/nano-Fe3O4 composites is smoother and there are lesser defects. (4) The best friction performance appears when the mass fraction of nano-Fe3O4 is 12%. The nano-Fe3O4 is conducive to forming physical adsorption film and self-repairing film on the friction surface. It has the effect of improving the resistance to wear and friction. Clearly, the addition of nanoFe3O4 particles can improve friction and wear performance of NBR, and is helpful for its application in sealing.
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