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Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water
ARTICLE IN PRESS Journal of Materials Science & Technology ■■ (2015) ■■–■■
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Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water Xiaoren Lv *, Shuyuan Song, Huiming Wang, Shijie Wang School of Mechanical Engineering, Shenyang University of Technology, Shenyang 110870, China
A R T I C L E
I N F O
Article history: Received 24 November 2014 Received in revised form 18 January 2015 Accepted 2 February 2015 Available online Key words: CO2 NBR rubber Water Swelling Wear
The swelling and tribological behaviors of nitrile-butadience (NBR) rubbers with three acrylonitrile contents (N18, N26 and N41) in water with different CO2 gas flows are investigated by immersion and wear experiments. The results show that the bubbles of CO2 into water severely destroy the cross-linking network of rubber and form defects on the surface, such as cracks, holes and lamellar perks. These defects lead to an increase in the static and dynamic swelling increment. The dynamic swelling increment is almost three or four times larger than the static swelling increment. The hardness and wear resistance of rubbers in water with CO2 gas remarkably decreases in contrast to that in water, and they gradually decrease with an increase in the gas flow in water. The bubbles of CO2 decrease the steady frictional coefficient of rubber in water due to the presence of the gas in water lubricant film. The steady frictional coefficient in water with different CO2 gas flow basically remains 0.1. N41 with high acrylonitrile content shows better swelling and wear resistances than N18 and N26 because of its dense molecular network. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction In recent years, rubber materials as friction elements have been widely applied in oil machineries, such as seals, rubber stators, rubber thrust bearing etc[1]. During the service life, rubber materials not only are long-term immersed in the liquid or gas medium, but also periodically experience the extrusion and shear from the metal pairs[2–4]. Therefore, the swelling and wear resistance of rubber are the dominant factors in determining the service life of rubber/metal friction pairs, and studies on this subject have been conducted. Zeng et al.[5] investigated the corrosion properties of AFLAS rubber in the harsh environment for 7 days and found that the tensile properties and hardness of immersed rubber were decreased. Hu et al.[6] studied the thermal aging and scaling of NBR in alkaline solution. After the NBR was immersed in alkaline solution with the pH value of 11 and the temperature of 70 °C for 215 days, many cracks were observed on the surface and a lot of small sized scales existed near the cracks. Mofidi et al.[7] performed the tribological behavior of an elastomer aged in different oils. They concluded that aging the nitrile rubber in lubricant fluids would increase the abrasive wear in both dry and lubricated conditions. Chandrasekaran and Batchelor[8] studied the friction and wear of butyl rubber in the presence of lubricants in an X-ray
* Corresponding author. Ph.D.; Tel.: +86 24 25496678; Fax: +86 24 25496729. E-mail address: [email protected] (X. Lv).
microfocus instrument. The experimental results indicated that the presence of lubricant such as water or oil reduced the coefficient of friction but caused accelerated wear due to the chemical deterioration of rubber. Recently, various kinds of rubber modification and rubber composites have been developed to improve the swelling and wear resistance of rubber in liquid or gas medium[9–11]. Water flooding as the main secondary oil recovery technology is applied in most low yield and low efficient oil wells[12,13]. In order to further improve the oil recovery and simultaneously reduce CO2 emissions into the atmosphere, CO2 gas is also injected into the above wells[14,15]. Gas can infiltrate into the molecular network, influence the material properties, such as tensile, hardness and so on, and cause a fracture in the rubber[16–18]. At the same time, the bubbles of gas into liquid can change its viscosity and the composition of lubricating film, and further make the load-carrying ability and friction force of lubricating film change[19]. The friction and wear behaviors of rubber in water have been investigated[20–22], but the effect of CO2 gas on the swelling and tribological behaviors in water is still unknown. Till now there have been few works reported about the mechanism of CO2 gas. In this work, the nitrile-butadience (NBR) rubbers and deionized water are chosen as the experimental materials. The swelling and tribological behaviors of rubbers in water with different CO2 gas flow are investigated to reveal the mechanism of CO2. This research would provide a benchmark in experimental verifications and theoretical guidance for a selection of friction elements used in the oilfield.
http://dx.doi.org/10.1016/j.jmst.2015.09.014 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Please cite this article in press as: Xiaoren Lv, Shuyuan Song, Huiming Wang, Shijie Wang, Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.09.014
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2. Experimental The materials used in this work were three kinds of NBR rubbers with different acrylonitrile contents, 18 wt%, 26 wt% and 41 wt%, and were abbreviated as N18, N26 and N41, respectively. The main components of NBR rubbers and their weight fraction were listed as follow: masterbatch 100, carbon black 70, vulcanizing agent 3, activating agent 12, accelerating agent 4 and anti-aging agent 5. After the masterbatch was plasticated on the open roll mill, various components were successively added for mixing. Mixed compound was finally vulcanized using the press vulcanizer. The samples used for immersion and wear experiments were pressed into blocks with the size of 40 mm × 25 mm × 6 mm using the special mould. The hardness of the vulcanized samples was about 75 Shore A. The static immersion experiments of NBR rubbers in water (deionized water, industrial grade) with CO2 gas (industrial grade) were performed at an ambient temperature and pressure. The schematic representation of the immersion tester was shown in Fig. 1, in which the control valve was used to adjust the gas flow and the atomizer played a role in dispersing the gas into the liquid medium uniformly. The volume of liquid medium in the beaker was 1 L, and the flow of CO2 gas was set to 0, 0.5, 2, 10, 20, 40 and 80 mL min−1, respectively. After being immersed in the water with CO2 gas for the set time, the samples were taken out and dried by the filter paper. The hardness of immersed samples was measured by the shore durometer with an accuracy of 0.1 Shore A. The weight and size of samples before and after immersion experiment were respectively measured by the electrical balance with an accuracy of 0.1 mg and the vernier caliper with an accuracy of 0.01 mm. The weight variation between immersed sample and un-immersed one was defined as the static swelling increment. The weight variation rate (Qw) of samples before and after the immersion experiment was calculated by Eq. (1).
Qw =
W2 − W1 × 100% W1
(1)
where W1 and W2 are the weight of samples before and after the immersion experiment (mg), respectively. The volume variation rate (Qv) of samples before and after the immersion experiment was provided by Eq. (2).
Qv =
V2 − V1 × 100% V1
(2)
Fig. 2. Schematic representation of wear tester.
where V1 and V2 are the volume of sample before and after the immersion experiment (mm3), respectively. The proportion of cross-linking chain, tail suspension chain and free chain in rubber molecular network after immersed in water with or without gas flow of 80 mL min−1 for 1 day is measured using the VTMR20-010V-T nuclear magnetic resonance (NMR) spectrometer. Test parameters are set as follows: resonance frequency 21.31 MHz, magnet strength 0.5 T, magnet temperature 35 °C and test temperature 90 °C. Variation rate (Qp) in the proportion of various chain before and after the bubbles of CO2 gas into water was calculated by Eq. (3).
Qp =
P2 − P1 × 100% P1
(3)
where P1 and P2 were the proportion of various chain in rubber molecular network after immersed in water and water with gas flow of 80 mL min−1 for 1 day (%), respectively. The tribological behaviors of NBR samples in water with different CO2 gas flows were investigated using a ring-on-block tester at ambient temperature and pressure. The schematic representation of wear tester is shown in Fig. 2. The ring with the size of Φ178 mm × 12 mm was made of 40Cr steel. Its hardness and surface roughness were about HRC 62 and Ra 0.1 μm, respectively. During the test, the ring was rotated on the rubber block at a load of 200 N and a rotating speed of 150 rpm for 2 h. The steady frictional coefficient was collected when the wear condition became stable. After the wear experiment, the worn samples were taken out and dried using the filter paper. Worn volume of samples was measured using the 3000 profile measuring microscopy system. In order to investigate the dynamic swelling behavior of the rubber during the wear process, the blotted worn samples were placed in the oven at a temperature of 100 °C for 1 h. The weight loss of worn samples before and after the drying treatment, i.e. dynamic swelling increment due to the infiltration of water generated from the extrusion of metal friction pair, was measured using the above electrical balance. The above result data were the average of 3 individual tests. The morphologies of immersed and worn surfaces were observed using a CAMBR IDGES-360 SEM microscope. Before taking morphology observation, the rubber samples were coated with a thin layer of carbon by sputtering. 3. Results and Discussion 3.1. Swelling behavior of NBR rubber in water with CO2 gas
Fig. 1. Schematic representation of immersion tester.
When rubber is used as the friction sealing material, the liquid or gas medium can readily infiltrate into the molecular network of
Please cite this article in press as: Xiaoren Lv, Shuyuan Song, Huiming Wang, Shijie Wang, Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.09.014
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Fig. 3. Qw (a) and Qv (b) of immersed NBR samples in the water with different gas flow of CO2 for 1 day.
rubber and increase its weight and volume[2]. Weight variation rate (Qw) and volume variation rate (Qv) (collectively known as swelling variation rate) of rubber samples after being immersed in water with different gas flows for 1 day are shown in Fig. 3. Qw and Qv of rubber samples increase up to the gas flow of approximately 40 mL min−1, following with a slow increase tendency. It can be concluded that the swelling of rubber in gas–liquid two-phase is larger than that in liquid one-phase. This is consistent with the observation of Zeng et al.[5] CO2 gas in water will not only infiltrate into the molecular network of rubber, but also provide more channels for the infiltration of water. These two effects of gas contribute to the increase in swelling variation rate of rubber in water. However, there is an extreme value in the swelling variation rate of rubber at ambient temperature and pressure due to the limited gap in the molecular network structures of rubber[18]. Therefore, when the flow of CO2 gas is more than 40 mL min−1, the swelling variation rate of rubber in water slowly increase. The volume expansion of rubber friction elements in water with different CO2 gas flow can ensure its sealing performance. It can be also seen from Fig. 3 that queuing sequence of the swelling variation rate of NBR samples with three kinds of acrylonitrile contents in the same experimental condition is N18 > N26 > N41. The increase in acrylonitrile content (i.e., CN groups) of the molecular chain increases the interaction forces between the molecular chains, resulting in the formation of dense molecular networks. The denser the molecular network is, the lower swelling variation rate of rubber is[23]. N41 rubber with high acrylonitrile content shows better swelling resistance to water with CO2 gas than N18 rubber with low acrylonitrile content. Molecular network of rubber is composed of cross-linking chain, tail suspension chain with one free end and free chain with two free ends. According to the scaling concept in polymer physics[24], the increase of CN polar groups in molecular chain decreases its internal rotation performance. Therefore, more molecular chains with high acrylonitrile content participates into the formation of crosslinking network, and N41 rubber owns the higher proportion of cross-linking chain and the lower proportion of tail suspension chain and free chain with comparison to that of N18 rubber. The variation rate in the proportion of various chains before and after the bubbles of CO2 gas into water is listed in Table 1. After CO2 gas is bubbled into water, the proportion of cross-linking chain decreased and the proportion of tail suspension chain and free chain increased. Our results suggest that the bubbles of gas into liquid lead to the decrease of cross-linking density of rubber. The variation rate
in the proportion of cross-linking gradually decreased with increasing acrylonitrile content in molecular chain, while the variation percentage in the proportion of tail suspension and free chain shows the opposite tendency. It can be further confirmed from Table 1 that the molecular network with high proportion of cross-linking chain can endure the swelling from the water with CO2 gas as shown in Fig. 3. Fig. 4 shows the surface micrographs of rubber samples after being immersed in water with different CO2 gas flows for 1 day. Almost no defects appear on the surface of sample immersed in water as shown in Fig. 4(a). After CO2 is bubbled into water, surface defects, such as cracks, holes and the lamellar perks, can be found on the surface of immersed sample because of the volume increase by the swelling. By contrast the morphologies in Fig. 4, the increase of CO2 gas flow in water further damages the sample surface. This is in agreement with the observation by Yamabe et al.[25] and Hu et al.[6]. But they did not further reveal the subsurface and internal defect. Therefore, in order to better show the effect of gas on the swelling behavior of rubber in water, the morphologies in the cross section of immersed sample are shown in Fig. 5. As for the surface of sample immersed in water, the morphology of the top surface is almost the same as that of the matrix. However, a thin swelling effect layer obviously appears on the surface of sample immersed in water with CO2, in which there are lots of cracks from the surface to the inner. The morphologies of surface and crosssection directly confirm that CO2 gas in water destroys the crosslinking network and forms the defects on the sample surface. Fig. 6 illustrates the hardness variation of NBR samples before and after the immersion experiment. With an increase in the CO2 gas flow in water, more and more gas and liquid mediums infiltrate into rubber, damage the cross-linking network and sample surface, and gradually decrease the hardness of NBR rubber. It is also worth noting that decline amplitude in the hardness of N41 after immersed in water with CO2 gas for 1 day is lower than that of N18.
Table 1 Variation rate (%) in the proportion of various chain after CO2 gas was bubbled into water
N18 N26 N41
Cross-linking chain
Tail suspension chain
Free chain
−10.89 −6.14 −4.84
2.42 1.15 1.08
2.99 1.95 2.88
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Fig. 4. Surface morphologies of immersed N18 samples in water with different gas flow of CO2 for 1day (a) in water, (b) in water with gas flow of 20 mL min−1 and (c) in water with gas flow of 40 mL min−1.
Fig. 5. Morphologies in the cross-section of immersed NBR samples in water with gas flow of CO2 for 1 day: (a) in water; (b) in water with gas flow of 20 mL min−1; (c) in water with gas flow of 40 mL min−1.
Fig. 6. Variation of hardness of NBR samples in water with different CO2 gas flow for 1 day.
3.2. Tribological behaviors of rubber samples in water with CO2 The friction and wear behaviors of materials are closely related to their component, structure, lubricating, and so on. Serbin et al.[20] reported that the type of rubber base is one of the main factors determining their wear resistance during two-body abrasion in a water medium. The bubbles of CO2 into water not only change the properties of NBR rubbers with different acrylonitrile content as described above, but also influence the friction and wear behaviors of rubbers in water. Fig. 7 shows the worn volume of NBR rubber samples in water with different CO2 gas flows. It can be seen from Fig. 7 that
Fig. 7. Worn volume of NBR in water with different CO2 gas flow.
the bubbles of CO2 gas into water remarkably increase the worn volume of rubbers in water. Worn volume of samples gradually increases with an increase in CO2 gas flow in water. The reason for this is that the increase of CO2 gas flow in water enlarges the damage to the rubber surface and leads to the decrease of the anti-shear ability. Worn volume of N41 in water with different CO2 gas flow is lower than that of N18, due to its higher hardness and lower surface defects. Fig. 8 shows the steady frictional coefficient of rubber samples in water with different CO2 gas flow. In the same experimental condition, the frictional coefficient of three kinds of samples is basically
Please cite this article in press as: Xiaoren Lv, Shuyuan Song, Huiming Wang, Shijie Wang, Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.09.014
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Fig. 8. Steady frictional coefficient of NBR in the water with different CO2 gas flow.
the same, which will be only related to the fluid lubricant film between the rubber block and metal wheel and have little to do with the properties of rubber samples. It can also be clearly seen from Fig. 8 that the frictional coefficient of rubber in water with CO2 gas is lower than that in water. The frictional coefficient of rubber slowly decreases with an increase in CO2 gas flows in water and basically remains 0.1. When CO2 gas is bubbled into water, the lubricant film will be composed of water and gas referred to the theory of oil– air lubrication[19]. It is well known that the frictional coefficient of gas lubricant film is lower than that of liquid lubricant film with comparison to the internal friction of these two media[26]. Therefore, the gradual decrease of frictional coefficient with the increase of CO2 gas flow in water can be attributed to the increase of gas proportion in water lubricant film. Fig. 9 gives the worn morphologies of three kinds of NBR rubber samples in water and water with CO2 gas flow of 20 mL min−1. In
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water lubrication, the number of ridge shaped wear pattern on the worn surface of rubber samples gradually reduces with an increase in the acrylonitrile content of molecular chain. The worn morphology of N41 is smooth and exhibits almost no ridge shaped wear pattern. Apart from the deep plough, worn morphologies of three kinds of rubber samples in water lubrication with CO2 gas all show the ridge shaped wear pattern, which are different from that in water lubrication. It can be confirmed from Fig. 9 that the wear loss of rubber sample increases after CO2 gas is bubbled into water, as shown in Fig. 7. In order to better understand the wear mechanism of rubber in water with CO2 gas, the formation process of wear pattern is revealed using the physical model created by Zhang [27]. Fig. 10 shows the formation process of wear pattern of rubber in water lubrication with or without CO2 gas. In water lubrication, two processes, i.e. surface tear (including the growth of crack, the formation of tongue) and the break of tongue end by the tensile stress, are periodically generated on the rubber surface under the repeated action of the normal and shear force from the metal friction pair. The rubber surface is gradually worn in the form of lamination spalling and forms the ridge shaped worn pattern with the sharp top[28]. In water lubrication with CO2 gas, cracks, holes and lamellar peeling generated by the bubbles of CO2 into water increase the number of crack sources. These crack sources not only increase the locations of surface tear, but also shorten the time for the formation and growth of surface tear and enlarge the depth of the crack growth. During the process of wear, more delamination peelings are easily generated on the rubber surface after CO2 is bubbled into water, resulting in the deep and coarse wear pattern.
3.3. Effect of friction on the swelling of rubber In fluid lubrication, the liquid medium is infiltrated into the rubber under the action of extrusion, causing the dynamic swelling of rubber. In the long-term use of the sealing material, the
Fig. 9. Worn morphologies of three kinds of NBR rubbers: (a) N18, water, (b) N26, water, (c) N41, water, (d) N18, water + CO2, (e) N26, water + CO2, (f) N41, water + CO2.
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Fig. 10. Formation process of wear pattern of rubber: (a) formation and growth of crack; (b) formation of tongue; (c) turnover of tongue; (d) break of tongue.
dynamic swelling will make the rubber age and further affect the tribological properties of materials[29,30]. Ch’ng[30] reported that the amount of swelling of rubber in solvent is affected by the presence of loading. It is essential to understand the interaction between the loading and CO2 gas in the swelling of rubber in water. Fig. 11 shows the dynamic and static swelling increment of three kinds of NBR samples in water with different CO2 gas flows for 2 h. Dynamic swelling increment of rubber samples gradually increases with an increase in CO2 gas flows in water. As CO2 gas flow increases, more gas will be extruded into rubber, destroy the cross-linking network, and generate the defects such as crack, holes and so on for the infiltration of water into rubber. This leads to the increase of dynamic swelling increment. The dynamic swelling increment of samples in the wear experiment is almost three or four times larger than the static swelling increment in the immersion experiment, as shown in Fig. 11. This indicated that the influence of extrusion in the wear process on the material properties is extreme. Compared with the swelling and tribological behaviors of three kinds of NBR rubber (N18, N26 and N41) in water with or without CO2 gas, it could be found that the rubber with relatively dense crosslinking network should be chosen as the friction sealing material. The dense cross-linking network could reduce the infiltration of liquid or gas medium, resulting in the decrease of destruction to the material surface and ultimately the improvement of wear resistance.
4. Conclusions In this study, the swelling and tribological behaviors of NBR rubbers with three acrylonitrile contents in water with different CO2 gas flows are investigated by immersion and wear experiments. From our work, the following conclusions can be drawn: (1) With an increase in CO2 gas flows in water, the weight variation rate and volume variation rate of swelled samples increase rapidly at first, and then tend to be stable. The bubbles of CO2 into water severely destroy the molecular network, decrease the proportion of internal cross-linking chain, and increase the proportion of tail suspension chain and free chain. This leads to the formation of cracks, holes and lamellar peeling on the sample surface. Hardness of swelled samples in water with CO2 gas decreases due to the infiltration of water and surface defects. (2) The wear loss and dynamic swelling increment of samples increase with an increase in CO2 gas flows in the water, while the frictional coefficient of samples shows the opposite tendency due to the presence of gas in water lubricant film. (3) The molecular network of rubber becomes denser with an increase in the acrylonitrile content of molecular chain. N41 shows the better swelling and wear resistances to the water with or without CO2 gas.
Fig. 11. Dynamic swelling increment (a) and static swelling increment (b) of NBR samples in water with different gas flow for 2 h.
Please cite this article in press as: Xiaoren Lv, Shuyuan Song, Huiming Wang, Shijie Wang, Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.09.014
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Acknowledgments This research was financially supported, in part, by the National Natural Science Foundation of China (No. 50878178), the Natural Science Foundation of Liaoning Province, China (No. 2013020039) and the Science and Technology Program of Shenyang Municipality, China (No. F13-077-2-00). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water Xiaoren Lv *, Shuyuan Song, Huiming Wang, Shijie Wang School of Mechanical Engineering, Shenyang University of Technology, Shenyang 110870, China
A R T I C L E
I N F O
Article history: Received 24 November 2014 Received in revised form 18 January 2015 Accepted 2 February 2015 Available online Key words: CO2 NBR rubber Water Swelling Wear
The swelling and tribological behaviors of nitrile-butadience (NBR) rubbers with three acrylonitrile contents (N18, N26 and N41) in water with different CO2 gas flows are investigated by immersion and wear experiments. The results show that the bubbles of CO2 into water severely destroy the cross-linking network of rubber and form defects on the surface, such as cracks, holes and lamellar perks. These defects lead to an increase in the static and dynamic swelling increment. The dynamic swelling increment is almost three or four times larger than the static swelling increment. The hardness and wear resistance of rubbers in water with CO2 gas remarkably decreases in contrast to that in water, and they gradually decrease with an increase in the gas flow in water. The bubbles of CO2 decrease the steady frictional coefficient of rubber in water due to the presence of the gas in water lubricant film. The steady frictional coefficient in water with different CO2 gas flow basically remains 0.1. N41 with high acrylonitrile content shows better swelling and wear resistances than N18 and N26 because of its dense molecular network. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction In recent years, rubber materials as friction elements have been widely applied in oil machineries, such as seals, rubber stators, rubber thrust bearing etc[1]. During the service life, rubber materials not only are long-term immersed in the liquid or gas medium, but also periodically experience the extrusion and shear from the metal pairs[2–4]. Therefore, the swelling and wear resistance of rubber are the dominant factors in determining the service life of rubber/metal friction pairs, and studies on this subject have been conducted. Zeng et al.[5] investigated the corrosion properties of AFLAS rubber in the harsh environment for 7 days and found that the tensile properties and hardness of immersed rubber were decreased. Hu et al.[6] studied the thermal aging and scaling of NBR in alkaline solution. After the NBR was immersed in alkaline solution with the pH value of 11 and the temperature of 70 °C for 215 days, many cracks were observed on the surface and a lot of small sized scales existed near the cracks. Mofidi et al.[7] performed the tribological behavior of an elastomer aged in different oils. They concluded that aging the nitrile rubber in lubricant fluids would increase the abrasive wear in both dry and lubricated conditions. Chandrasekaran and Batchelor[8] studied the friction and wear of butyl rubber in the presence of lubricants in an X-ray
* Corresponding author. Ph.D.; Tel.: +86 24 25496678; Fax: +86 24 25496729. E-mail address: [email protected] (X. Lv).
microfocus instrument. The experimental results indicated that the presence of lubricant such as water or oil reduced the coefficient of friction but caused accelerated wear due to the chemical deterioration of rubber. Recently, various kinds of rubber modification and rubber composites have been developed to improve the swelling and wear resistance of rubber in liquid or gas medium[9–11]. Water flooding as the main secondary oil recovery technology is applied in most low yield and low efficient oil wells[12,13]. In order to further improve the oil recovery and simultaneously reduce CO2 emissions into the atmosphere, CO2 gas is also injected into the above wells[14,15]. Gas can infiltrate into the molecular network, influence the material properties, such as tensile, hardness and so on, and cause a fracture in the rubber[16–18]. At the same time, the bubbles of gas into liquid can change its viscosity and the composition of lubricating film, and further make the load-carrying ability and friction force of lubricating film change[19]. The friction and wear behaviors of rubber in water have been investigated[20–22], but the effect of CO2 gas on the swelling and tribological behaviors in water is still unknown. Till now there have been few works reported about the mechanism of CO2 gas. In this work, the nitrile-butadience (NBR) rubbers and deionized water are chosen as the experimental materials. The swelling and tribological behaviors of rubbers in water with different CO2 gas flow are investigated to reveal the mechanism of CO2. This research would provide a benchmark in experimental verifications and theoretical guidance for a selection of friction elements used in the oilfield.
http://dx.doi.org/10.1016/j.jmst.2015.09.014 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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2. Experimental The materials used in this work were three kinds of NBR rubbers with different acrylonitrile contents, 18 wt%, 26 wt% and 41 wt%, and were abbreviated as N18, N26 and N41, respectively. The main components of NBR rubbers and their weight fraction were listed as follow: masterbatch 100, carbon black 70, vulcanizing agent 3, activating agent 12, accelerating agent 4 and anti-aging agent 5. After the masterbatch was plasticated on the open roll mill, various components were successively added for mixing. Mixed compound was finally vulcanized using the press vulcanizer. The samples used for immersion and wear experiments were pressed into blocks with the size of 40 mm × 25 mm × 6 mm using the special mould. The hardness of the vulcanized samples was about 75 Shore A. The static immersion experiments of NBR rubbers in water (deionized water, industrial grade) with CO2 gas (industrial grade) were performed at an ambient temperature and pressure. The schematic representation of the immersion tester was shown in Fig. 1, in which the control valve was used to adjust the gas flow and the atomizer played a role in dispersing the gas into the liquid medium uniformly. The volume of liquid medium in the beaker was 1 L, and the flow of CO2 gas was set to 0, 0.5, 2, 10, 20, 40 and 80 mL min−1, respectively. After being immersed in the water with CO2 gas for the set time, the samples were taken out and dried by the filter paper. The hardness of immersed samples was measured by the shore durometer with an accuracy of 0.1 Shore A. The weight and size of samples before and after immersion experiment were respectively measured by the electrical balance with an accuracy of 0.1 mg and the vernier caliper with an accuracy of 0.01 mm. The weight variation between immersed sample and un-immersed one was defined as the static swelling increment. The weight variation rate (Qw) of samples before and after the immersion experiment was calculated by Eq. (1).
Qw =
W2 − W1 × 100% W1
(1)
where W1 and W2 are the weight of samples before and after the immersion experiment (mg), respectively. The volume variation rate (Qv) of samples before and after the immersion experiment was provided by Eq. (2).
Qv =
V2 − V1 × 100% V1
(2)
Fig. 2. Schematic representation of wear tester.
where V1 and V2 are the volume of sample before and after the immersion experiment (mm3), respectively. The proportion of cross-linking chain, tail suspension chain and free chain in rubber molecular network after immersed in water with or without gas flow of 80 mL min−1 for 1 day is measured using the VTMR20-010V-T nuclear magnetic resonance (NMR) spectrometer. Test parameters are set as follows: resonance frequency 21.31 MHz, magnet strength 0.5 T, magnet temperature 35 °C and test temperature 90 °C. Variation rate (Qp) in the proportion of various chain before and after the bubbles of CO2 gas into water was calculated by Eq. (3).
Qp =
P2 − P1 × 100% P1
(3)
where P1 and P2 were the proportion of various chain in rubber molecular network after immersed in water and water with gas flow of 80 mL min−1 for 1 day (%), respectively. The tribological behaviors of NBR samples in water with different CO2 gas flows were investigated using a ring-on-block tester at ambient temperature and pressure. The schematic representation of wear tester is shown in Fig. 2. The ring with the size of Φ178 mm × 12 mm was made of 40Cr steel. Its hardness and surface roughness were about HRC 62 and Ra 0.1 μm, respectively. During the test, the ring was rotated on the rubber block at a load of 200 N and a rotating speed of 150 rpm for 2 h. The steady frictional coefficient was collected when the wear condition became stable. After the wear experiment, the worn samples were taken out and dried using the filter paper. Worn volume of samples was measured using the 3000 profile measuring microscopy system. In order to investigate the dynamic swelling behavior of the rubber during the wear process, the blotted worn samples were placed in the oven at a temperature of 100 °C for 1 h. The weight loss of worn samples before and after the drying treatment, i.e. dynamic swelling increment due to the infiltration of water generated from the extrusion of metal friction pair, was measured using the above electrical balance. The above result data were the average of 3 individual tests. The morphologies of immersed and worn surfaces were observed using a CAMBR IDGES-360 SEM microscope. Before taking morphology observation, the rubber samples were coated with a thin layer of carbon by sputtering. 3. Results and Discussion 3.1. Swelling behavior of NBR rubber in water with CO2 gas
Fig. 1. Schematic representation of immersion tester.
When rubber is used as the friction sealing material, the liquid or gas medium can readily infiltrate into the molecular network of
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Fig. 3. Qw (a) and Qv (b) of immersed NBR samples in the water with different gas flow of CO2 for 1 day.
rubber and increase its weight and volume[2]. Weight variation rate (Qw) and volume variation rate (Qv) (collectively known as swelling variation rate) of rubber samples after being immersed in water with different gas flows for 1 day are shown in Fig. 3. Qw and Qv of rubber samples increase up to the gas flow of approximately 40 mL min−1, following with a slow increase tendency. It can be concluded that the swelling of rubber in gas–liquid two-phase is larger than that in liquid one-phase. This is consistent with the observation of Zeng et al.[5] CO2 gas in water will not only infiltrate into the molecular network of rubber, but also provide more channels for the infiltration of water. These two effects of gas contribute to the increase in swelling variation rate of rubber in water. However, there is an extreme value in the swelling variation rate of rubber at ambient temperature and pressure due to the limited gap in the molecular network structures of rubber[18]. Therefore, when the flow of CO2 gas is more than 40 mL min−1, the swelling variation rate of rubber in water slowly increase. The volume expansion of rubber friction elements in water with different CO2 gas flow can ensure its sealing performance. It can be also seen from Fig. 3 that queuing sequence of the swelling variation rate of NBR samples with three kinds of acrylonitrile contents in the same experimental condition is N18 > N26 > N41. The increase in acrylonitrile content (i.e., CN groups) of the molecular chain increases the interaction forces between the molecular chains, resulting in the formation of dense molecular networks. The denser the molecular network is, the lower swelling variation rate of rubber is[23]. N41 rubber with high acrylonitrile content shows better swelling resistance to water with CO2 gas than N18 rubber with low acrylonitrile content. Molecular network of rubber is composed of cross-linking chain, tail suspension chain with one free end and free chain with two free ends. According to the scaling concept in polymer physics[24], the increase of CN polar groups in molecular chain decreases its internal rotation performance. Therefore, more molecular chains with high acrylonitrile content participates into the formation of crosslinking network, and N41 rubber owns the higher proportion of cross-linking chain and the lower proportion of tail suspension chain and free chain with comparison to that of N18 rubber. The variation rate in the proportion of various chains before and after the bubbles of CO2 gas into water is listed in Table 1. After CO2 gas is bubbled into water, the proportion of cross-linking chain decreased and the proportion of tail suspension chain and free chain increased. Our results suggest that the bubbles of gas into liquid lead to the decrease of cross-linking density of rubber. The variation rate
in the proportion of cross-linking gradually decreased with increasing acrylonitrile content in molecular chain, while the variation percentage in the proportion of tail suspension and free chain shows the opposite tendency. It can be further confirmed from Table 1 that the molecular network with high proportion of cross-linking chain can endure the swelling from the water with CO2 gas as shown in Fig. 3. Fig. 4 shows the surface micrographs of rubber samples after being immersed in water with different CO2 gas flows for 1 day. Almost no defects appear on the surface of sample immersed in water as shown in Fig. 4(a). After CO2 is bubbled into water, surface defects, such as cracks, holes and the lamellar perks, can be found on the surface of immersed sample because of the volume increase by the swelling. By contrast the morphologies in Fig. 4, the increase of CO2 gas flow in water further damages the sample surface. This is in agreement with the observation by Yamabe et al.[25] and Hu et al.[6]. But they did not further reveal the subsurface and internal defect. Therefore, in order to better show the effect of gas on the swelling behavior of rubber in water, the morphologies in the cross section of immersed sample are shown in Fig. 5. As for the surface of sample immersed in water, the morphology of the top surface is almost the same as that of the matrix. However, a thin swelling effect layer obviously appears on the surface of sample immersed in water with CO2, in which there are lots of cracks from the surface to the inner. The morphologies of surface and crosssection directly confirm that CO2 gas in water destroys the crosslinking network and forms the defects on the sample surface. Fig. 6 illustrates the hardness variation of NBR samples before and after the immersion experiment. With an increase in the CO2 gas flow in water, more and more gas and liquid mediums infiltrate into rubber, damage the cross-linking network and sample surface, and gradually decrease the hardness of NBR rubber. It is also worth noting that decline amplitude in the hardness of N41 after immersed in water with CO2 gas for 1 day is lower than that of N18.
Table 1 Variation rate (%) in the proportion of various chain after CO2 gas was bubbled into water
N18 N26 N41
Cross-linking chain
Tail suspension chain
Free chain
−10.89 −6.14 −4.84
2.42 1.15 1.08
2.99 1.95 2.88
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Fig. 4. Surface morphologies of immersed N18 samples in water with different gas flow of CO2 for 1day (a) in water, (b) in water with gas flow of 20 mL min−1 and (c) in water with gas flow of 40 mL min−1.
Fig. 5. Morphologies in the cross-section of immersed NBR samples in water with gas flow of CO2 for 1 day: (a) in water; (b) in water with gas flow of 20 mL min−1; (c) in water with gas flow of 40 mL min−1.
Fig. 6. Variation of hardness of NBR samples in water with different CO2 gas flow for 1 day.
3.2. Tribological behaviors of rubber samples in water with CO2 The friction and wear behaviors of materials are closely related to their component, structure, lubricating, and so on. Serbin et al.[20] reported that the type of rubber base is one of the main factors determining their wear resistance during two-body abrasion in a water medium. The bubbles of CO2 into water not only change the properties of NBR rubbers with different acrylonitrile content as described above, but also influence the friction and wear behaviors of rubbers in water. Fig. 7 shows the worn volume of NBR rubber samples in water with different CO2 gas flows. It can be seen from Fig. 7 that
Fig. 7. Worn volume of NBR in water with different CO2 gas flow.
the bubbles of CO2 gas into water remarkably increase the worn volume of rubbers in water. Worn volume of samples gradually increases with an increase in CO2 gas flow in water. The reason for this is that the increase of CO2 gas flow in water enlarges the damage to the rubber surface and leads to the decrease of the anti-shear ability. Worn volume of N41 in water with different CO2 gas flow is lower than that of N18, due to its higher hardness and lower surface defects. Fig. 8 shows the steady frictional coefficient of rubber samples in water with different CO2 gas flow. In the same experimental condition, the frictional coefficient of three kinds of samples is basically
Please cite this article in press as: Xiaoren Lv, Shuyuan Song, Huiming Wang, Shijie Wang, Effect of CO2 Gas on the Swelling and Tribological Behaviors of NBR Rubber in Water, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.09.014
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Fig. 8. Steady frictional coefficient of NBR in the water with different CO2 gas flow.
the same, which will be only related to the fluid lubricant film between the rubber block and metal wheel and have little to do with the properties of rubber samples. It can also be clearly seen from Fig. 8 that the frictional coefficient of rubber in water with CO2 gas is lower than that in water. The frictional coefficient of rubber slowly decreases with an increase in CO2 gas flows in water and basically remains 0.1. When CO2 gas is bubbled into water, the lubricant film will be composed of water and gas referred to the theory of oil– air lubrication[19]. It is well known that the frictional coefficient of gas lubricant film is lower than that of liquid lubricant film with comparison to the internal friction of these two media[26]. Therefore, the gradual decrease of frictional coefficient with the increase of CO2 gas flow in water can be attributed to the increase of gas proportion in water lubricant film. Fig. 9 gives the worn morphologies of three kinds of NBR rubber samples in water and water with CO2 gas flow of 20 mL min−1. In
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water lubrication, the number of ridge shaped wear pattern on the worn surface of rubber samples gradually reduces with an increase in the acrylonitrile content of molecular chain. The worn morphology of N41 is smooth and exhibits almost no ridge shaped wear pattern. Apart from the deep plough, worn morphologies of three kinds of rubber samples in water lubrication with CO2 gas all show the ridge shaped wear pattern, which are different from that in water lubrication. It can be confirmed from Fig. 9 that the wear loss of rubber sample increases after CO2 gas is bubbled into water, as shown in Fig. 7. In order to better understand the wear mechanism of rubber in water with CO2 gas, the formation process of wear pattern is revealed using the physical model created by Zhang [27]. Fig. 10 shows the formation process of wear pattern of rubber in water lubrication with or without CO2 gas. In water lubrication, two processes, i.e. surface tear (including the growth of crack, the formation of tongue) and the break of tongue end by the tensile stress, are periodically generated on the rubber surface under the repeated action of the normal and shear force from the metal friction pair. The rubber surface is gradually worn in the form of lamination spalling and forms the ridge shaped worn pattern with the sharp top[28]. In water lubrication with CO2 gas, cracks, holes and lamellar peeling generated by the bubbles of CO2 into water increase the number of crack sources. These crack sources not only increase the locations of surface tear, but also shorten the time for the formation and growth of surface tear and enlarge the depth of the crack growth. During the process of wear, more delamination peelings are easily generated on the rubber surface after CO2 is bubbled into water, resulting in the deep and coarse wear pattern.
3.3. Effect of friction on the swelling of rubber In fluid lubrication, the liquid medium is infiltrated into the rubber under the action of extrusion, causing the dynamic swelling of rubber. In the long-term use of the sealing material, the
Fig. 9. Worn morphologies of three kinds of NBR rubbers: (a) N18, water, (b) N26, water, (c) N41, water, (d) N18, water + CO2, (e) N26, water + CO2, (f) N41, water + CO2.
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Fig. 10. Formation process of wear pattern of rubber: (a) formation and growth of crack; (b) formation of tongue; (c) turnover of tongue; (d) break of tongue.
dynamic swelling will make the rubber age and further affect the tribological properties of materials[29,30]. Ch’ng[30] reported that the amount of swelling of rubber in solvent is affected by the presence of loading. It is essential to understand the interaction between the loading and CO2 gas in the swelling of rubber in water. Fig. 11 shows the dynamic and static swelling increment of three kinds of NBR samples in water with different CO2 gas flows for 2 h. Dynamic swelling increment of rubber samples gradually increases with an increase in CO2 gas flows in water. As CO2 gas flow increases, more gas will be extruded into rubber, destroy the cross-linking network, and generate the defects such as crack, holes and so on for the infiltration of water into rubber. This leads to the increase of dynamic swelling increment. The dynamic swelling increment of samples in the wear experiment is almost three or four times larger than the static swelling increment in the immersion experiment, as shown in Fig. 11. This indicated that the influence of extrusion in the wear process on the material properties is extreme. Compared with the swelling and tribological behaviors of three kinds of NBR rubber (N18, N26 and N41) in water with or without CO2 gas, it could be found that the rubber with relatively dense crosslinking network should be chosen as the friction sealing material. The dense cross-linking network could reduce the infiltration of liquid or gas medium, resulting in the decrease of destruction to the material surface and ultimately the improvement of wear resistance.
4. Conclusions In this study, the swelling and tribological behaviors of NBR rubbers with three acrylonitrile contents in water with different CO2 gas flows are investigated by immersion and wear experiments. From our work, the following conclusions can be drawn: (1) With an increase in CO2 gas flows in water, the weight variation rate and volume variation rate of swelled samples increase rapidly at first, and then tend to be stable. The bubbles of CO2 into water severely destroy the molecular network, decrease the proportion of internal cross-linking chain, and increase the proportion of tail suspension chain and free chain. This leads to the formation of cracks, holes and lamellar peeling on the sample surface. Hardness of swelled samples in water with CO2 gas decreases due to the infiltration of water and surface defects. (2) The wear loss and dynamic swelling increment of samples increase with an increase in CO2 gas flows in the water, while the frictional coefficient of samples shows the opposite tendency due to the presence of gas in water lubricant film. (3) The molecular network of rubber becomes denser with an increase in the acrylonitrile content of molecular chain. N41 shows the better swelling and wear resistances to the water with or without CO2 gas.
Fig. 11. Dynamic swelling increment (a) and static swelling increment (b) of NBR samples in water with different gas flow for 2 h.
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Acknowledgments This research was financially supported, in part, by the National Natural Science Foundation of China (No. 50878178), the Natural Science Foundation of Liaoning Province, China (No. 2013020039) and the Science and Technology Program of Shenyang Municipality, China (No. F13-077-2-00). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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