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Evaluation of antioxidants in rapeseed oils for railway application
ARTICLE IN PRESS Tribology International 42 (2009) 987–994
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Evaluation of antioxidants in rapeseed oils for railway application Akihito Suzuki , Ratu Ulfiati 1, Masabumi Masuko Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan
a r t i c l e in fo
abstract
Article history: Received 7 October 2005 Received in revised form 28 January 2009 Accepted 3 February 2009 Available online 12 February 2009
In order to evaluate the applicability of biodegradable base-oil lubricants for railway applications, the influence of UV(ultraviolet)-B light (280–320 nm) on oxidation stability and the tribological performance of rapeseed oil (RO) have been investigated. Antioxidant free RO was oxidized immediately under UV-B irradiation at moderate temperature. Three sterically hindered phenolic-type antioxidants, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), octadecyl-3-(3,5-di-tert-butyl4-hydroxyphenyl)propionate, and 2-6-di-tert-butyl-4-methyl phenol were tested in order to increase the oxidation stability of RO. Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) was the most effective, prolonging the oxidation induction time from 3 h of RO to 40 h. The tribological performance of RO could be improved by the addition of metallic detergent (Ca–alkylsalicylate–CaCO3) and was stable against the UV-B irradiation. & 2009 Elsevier Ltd. All rights reserved.
Keywords: UV-induced degradation Biodegradable oil Additives
1. Introduction With the recent increased interest in global environmental problems, new technology regarding environmental protection has been developed and applied in all industries. There are three classifications of elemental technologies related to environmental protection for lubricants. The first involves the establishment of a recycling-based society by advocating the recycling of used oil. The second involves reductions in health and environmental risk based on the use of low-toxicity chemicals. The third effort involves reductions in the environmental load by the use of biodegradable lubricants. Measures to avoid the use of toxic chemicals have been taken for many years. Lubricants used in closed systems have come to be collected and recycled. Lubricants, however, can enter the environment through either normal use or leakage and spillage. Furthermore, in the case of an open lubrication system, the collection of lubricants is difficult and lubricants can easily enter the environment. As such, environmentally harmless lubricants have been developed. Biodegradable lubricants have a minimum impact on the environment and are suitable for various kinds of applications such as outboard twocycle engine oils, chain saw oils, wire rope lubricants, hydraulic oils for forest and agricultural equipment, etc. [1]. One of the promising usages of biodegradable lubricants is for railways. Oils or greases are applied to railways to maintain an adequate friction
Corresponding author. Tel./fax: +81 3 5734 2628.
E-mail address: [email protected] (A. Suzuki). Present address: Research and Development Center for Oil and Gas Technology, ‘‘LEMIGAS’’, Jakarta Selatan 12230, Indonesia. 1
0301-679X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2009.02.001
coefficient, to avoid wear damage, or to reduce vibration and noise. These lubricants are ‘‘once-through’’ lubricants and are never recovered, but are defused into the ground. The most commonly used base stocks for lubricants are mineral oils, which are produced from petroleum crude by various processing steps. Mineral oil-based lubricants have been used in all kind of applications such as automotive engines, transmissions, industrial gears, etc. From an environmental point of view, however, mineral oil-based lubricants have poor biodegradability. Furthermore, mineral oil-based lubricants contain many kinds of additives such as antioxidant agents, anti-wear agents, detergents and dispersants, anti-forms, extreme pressure agents, friction modifiers, and viscosity index improvers. Some of the additives are known to be toxic or harmful to the environment [2]. Natural oils and fats viz. vegetable oils and animal fats are known to have superior biodegradability. These oils, however, are known to have very poor oxidation stability. The oxidation of lubricating oils on the railways occurs primarily due to the sunlight effect. Oxidation products are acidic and can give rise to corrosive attack. Furthermore, oxidation of oil leads to an increase in viscosity, and polymerization of oxidation products leads to the formation of varnish and sludge deposits [3]. Improvements in oxidation stability are of great importance for the application of vegetable oils to railways lubricants. In addition to oxidation stability, tribological characteristics play an important role in car dynamic behavior because the forces generated between wheel and rail depends on the friction characteristics. A high friction coefficient is required for climbing a slope or braking near a station. On the other hand, a high friction coefficient is undesirable for tight curves, as it leads to an increase in squeak noise and rail corrugation. An inadequate friction
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coefficient leads to various troubles such as skid at braking, over-run at the station, and wear of wheel and rail [4]. Therefore, improvements in the tribological performance are of great importance. The focus of the present study was on obtaining biodegradable base-oil lubricants for railway applications that have high oxidation stability, a constant friction coefficient and viscosity with time, and a low wear rate. 2. Experimental
SQA:
SQE: O AO-A:
C
CH2CH2COCH2
HO
4 2.1. Lubricant The base oil used in this study was commercially available rapeseed oil (RO). It was used as received. The viscosity of the original RO was 79.9 mPa s at 20 1C; peroxide value, 29.2 ppm; total acid number, 0.16 mg KOH/g. In this study, squalane (SQA) was used as a reference sample, representative of mineral oil (saturated hydrocarbon). Furthermore, because RO has an unsaturated carbon bond, squalene (SQE) was also used as representative of an unsaturated compound. The oxidation of lubricating oils on railways primarily occurs due to the effects of sunlight, with temperature effects being relatively insignificant. The most frequently used low-temperature antioxidants of the free radical-acceptor type are electron donor compounds such as phenolic and aromatic amine materials. In the present study, three kinds of sterically hindered phenolictype antioxidants, [pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4hydroxyphenyl)propionate)] (AO-A), [octadecyl-3-(3,5-di-tertbutyl-4-hydroxyphenyl)propionate] (AO-B), and [2-6-di-tert-butyl4-methyl phenol] (DBMP), were used. AO-A is approved by US Food and Drug Administration (FDA) as an ingredient for use in lubricants with incidental food contact for use in and around food processing areas. DBMP is widely used as an antioxidant for fats and oils or in packaging material for fat-containing foods. The ROs containing 0.95 mass% of AO-A, 1 mass% of AO-B, and 1 mass% of DBMP were prepared and tested. Although increase of molecular weight improves thermal stability, the solubility of the antioxidant in oil decreases. Because the solubility of AO-A to RO was low, and the sample of the concentration of 1 mass% could not be made at the room temperature, the solution of 0.95 mass% was used for AO-A. In addition to the oxidation stability, tribological characteristics play an important role in car dynamic behavior. Spraying sand for locomotives is sometimes used to increase the friction coefficient [4]. In the present study, the effects of solid particles on tribological behavior were evaluated. Detergent and dispersant additive was used to improve the tribological performance of the RO. The additive used in this study was calcium alkylsalicylate– CaCO3 (DD). A RO containing 6 mass% Ca was prepared. The molecular structures of the chemicals are given in Fig. 1. Formulations of the prepared samples are shown in Table 1. 2.2. Sample preparation Radiation from the sun reaching the outer layers of the air around the earth shows a continuous energy spectrum in the wavelength region between 0.7 and 3000 nm. Wavelengths above 780 nm constitute the infrared (IR) region. On passing through the atmosphere, a portion of the long-wavelength IR rays is absorbed by water vapor and carbon dioxide. Therefore, only the shortwavelength portion of the IR rays reaches the surface of the earth. Wavelengths in the range of 100–380 nm constitute the ultraviolet (UV) spectral region. The short-wavelength UV radiation below 175 nm is absorbed by oxygen in the layers above 100 km. The radiation between 175 and 280 nm is absorbed by the ozone layer of the stratosphere. The radiation between 280 and 380 nm
O AO-B:
HO
CH2CH2COC18H37
DBMP:
HO
CH3
DD:
R
COO Ca.CaCO3 OH Fig. 1. Molecular structures of materials used.
Table 1 Formulations of samples. Sample
Base oil
RO RO+AO-A RO+AO-B RO+DBMP RO+DD SQA SQE
Rapeseed Rapeseed Rapeseed Rapeseed Rapeseed Squalane Squalene
oil oil oil oil oil
Additive
Additive conc. (mass%)
None AO-A AO-B DBMP DD None None
0 0.95 1 1 6 (mass% of Ca) 0 0
reaches the surface of the earth [5,6]. This portion of the UV rays initiates the degradation of materials. In this study, two UV-B tubes (Philips TL UV-B 20W/12) with wavelength in the range of 270–400 nm (maximum radiation at 315 nm) were used to prepare the degradation sample. A fixed amount of sample (350 mg) was poured into a steel pan (Fig. 2a) and then irradiated with UV-B light in a lighting box (Fig. 2b) with various oxidation times from 0 to 48 h at a constant temperature of 40 1C. The distance between light tubes and steep pan was about 20 cm and the intensity of the UV-B light on the steel pan was 19 W/m2. For a comparison with UV degradation, oxidation in the air was also carried out at high temperature (120 1C).
2.3. Analysis of lubricant degradation The lubricant degradation characteristics were evaluated by peroxide value (PV) [ASTM D-3703], total acid number (TAN) [ASTM D-664], molecular weight distribution, and viscosity change in this study. PV was determined by titration of liberated iodine (I2), which produced by the reaction of hydroperoxide in the sample oil and
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Table 2 Operation conditions of HPLC. Column
Shodex KF-805+KF-802.5
Column temperature Sample concentration Injection volume Eluent Flow rate of eluent Detector
30 1C 0.5 mass% 0.1 ml Tetrahydrofuran 1.0 ml/min UV (254 nm) Refractive index
18mm
neutralize free acids in 1 g of the sample, i.e. mg KOH/g. The sample oil was weighted accurately and dissolved with 75 mL of mixture solvent of toluene/2-propanol/water (100:99:1 v/v/v). The solution was titrated potentiometrically with 0.1 mol/L 2propanol solution of KOH at room temperature. Molecular weight distribution was determined using highperformance liquid chromatography (HPLC). The operation conditions of HPLC are shown in Table 2. Viscosity measurements were conducted using a rheometer with cone-plate geometry (diameter 25.0 mm, cone angle 2.0041) at 20 1C.
UV-B light 2.4. Friction and wear test
heating plate
Fig. 2. Oil degradation apparatus using UV light: (a) oil pan and (b) UV rays lighting box.
potassium iodide (KI), with sodium thiosulphate (Na2S2O3). In this study, sample oil was weighted accurately in a flask and added 30 mL of solvent (5% acetic acid in 2-propanol), 2 mL of toluene, 2 mL of KI saturated aqueous solution, and one piece of dry ice. The mixture was refluxed under nitrogen flow for 15 min at 70 1C. After the resulting solution was cooled to room temperature, the solution was poured into a titration vessel with magnetic stirrer bar and one piece of dry ice. The flask was rinsed with 15 mL of distilled water twice, and the rinsing water also poured into the titration vessel. The solution in the titration vessel was titrated potentiometrically with 0.01 mol/L Na2S2O3 solution. In this study, PV was expressed as the number of milligrams of peroxide oxygen per 1 kg of sample oil, i.e. ppm. TAN is a measure for total amount of both weak and strong organic acids present in the lubricant. This is expressed as the amount of potassium hydroxide (KOH) in mg required to
Tribological performance was evaluated using a highfrequency reciprocating tribometer with a ball-on-disk configuration. The ball was made of SUJ2 bearing steel 8.73 mm in diameter, and the disk was a bearing roller 10 mm in diameter made of SUJ2 bearing steel. The end surface of the roller was used for the friction and wear test. The disk specimens were polished with 3000 grit abrasive paper before the test. Both disk and ball specimens were cleaned with toluene in an ultrasonic bath, and then cleaned with an UV–ozone cleaner to remove the organic compounds remaining on the surface of the specimens. The test conditions were as follows: axial load, 19.6 N; stroke length, 3.0 mm; frequency, 30 Hz; temperature, room temperature. Total time of friction was 3 h (total sliding distance of 1944 m). Tribotest was stopped at 30 min, 1 h, 2 h, and 3 h, and wear scar diameter formed on the ball specimen was measured using an optical microscope. Wear rate was determined from the slope of wear volume as a function of sliding distance. Since the friction coefficient depends on the sliding velocity of the specimen and varies throughout the cycle, the mean friction coefficient in the range within 1.1 mm from the center position of reciprocation was adopted in this study. However, friction coefficient varied with time unstable in the early stage of the friction test, it became stable with the progress of the test. Sufficiently stable friction coefficients were used for the results in this study. In order to check the reliability of the test, tribotest was performed using SQA repeatedly. Friction coefficient and wear rate of SQA at room temperature were 0.10870.006 and 1.1 10 11 mm3/mm, respectively. Repeatability of the wear rate was within 25%.
3. Results and discussion 3.1. Base oil performance 3.1.1. Peroxide value The PV of sample oils after the irradiation with UV-B light, and the oxidation at 120 1C are shown in Fig. 3. RO gave an immediate increase in PV for both oxidation methods. As with RO, the PV of
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10000
Peroxide value, ppm
8000
R'OOH + •O
R + R'OO•
HO
RO RO+AO-A RO+AO-B RO+DBMP SQA SQE
R
R •O
6000
R
O
•
4000 R O
2000
•
R + R'OO•
O
OOR'
Fig. 4. Mechanisms of antioxidant action of hindered phenols.
0
10
20 30 Oxidation time, h
40
50 Table 3 Effectiveness of antioxidants for UV-B light oxidation.
Peroxide value, ppm
8000
6000
RO RO+AO-A RO+AO-B RO+DBMP SQA
4000
2000
0
10 Oxidation time, h
20
Fig. 3. Change in peroxide value against oxidation time: (a) UV-B light irradiation and (b) high temperature oxidation.
SQE under UV-B irradiation increased in a short time. UV-B light will significantly affect the oxidation of unsaturated compounds, which explains the poor oxidation stability of RO. The PV of SQA under UV-B irradiation did not significantly change, but it immediately increased under high-temperature conditions. These results indicate that saturated hydrocarbon is stable under UV-B irradiation at moderate temperature, but can be easily oxidized at high temperatures. The oxidation induction time of RO under UV-B irradiation was approximately 3 h. This time increased to 40 h with the addition of AO-A, 15 h with AO-B, and 29 h with DBMP. On the other hand, the oxidation induction time of RO for the high-temperature oxidation was approximately 1 h, and that increased to 11.3 h with the addition of AO-A, to 6.8 h with AO-B, and to 2.1 h with DBMP. DBMP is effective for UV-B light oxidation, but ineffective for high-temperature oxidation. According to thermogravimetric analysis by Gray et al., DBMP showed a mass loss of 5% at 90 1C and a mass loss of 95% at 145 1C [7]. Because of high volatility of DBHT, the sample containing DBMP lost its antioxidant activity during high-temperature oxidation test. On the other hand, the molecular weight of AO-A and AO-B is higher than that of DBMP, and these additives show low volatility [7,8]. Therefore, AO-A and AO-B showed excellent antioxidant activity for the hightemperature oxidation test.
Antioxidant
AO-A
AO-B
DBMP
Molar concentration (mmol/kg) Increase in oxidation induction time (h) Phenolic OH concentration (mmol/kg) Increase in oxidation induction time per phenolic OH concentration (h/(mmol/kg))
8.08 37 32.32 1.15
18.87 12 18.87 0.64
45.45 26 45.45 0.57
The most effective antioxidant for RO oxidation is AO-A, when oxidation induction times are compared with the same mass concentration. The additives used in this study were sterically hindered phenols, and it is considered that free radicals formed in the oil are stabilized by the various resonance structures of hindered phenols (Fig. 4). First, phenolic oxidation inhibitor traps a peroxy radical and becomes a phenoxy radical, then this phenoxy radical traps a different peroxy radical. In theory, one hindered phenolic group can inactivate two peroxy radicals. The three kinds of antioxidants used in this study have different molecular weights and different numbers of phenolic OH groups per molecule. Therefore, the effects of antioxidants for UV-B light oxidation are compared based on the increase in oxidation induction time per phenolic OH concentration (Table 3). Accordingly, AO-A is still the most effective among the three antioxidants and makes the oxidation induction time of RO twice longer than that of DBMP when compared with the same phenolic OH concentration. It is thought that the stability of generated phenoxy radicals affects the peroxy radical trapping capability. Unstable phenoxy radicals can easily make a coupling reaction and radical trapping capability decreases. High steric hindrance of AO-A might stabilize the generated phenoxy radical more, and hence AO-A showed the most excellent antioxidant performance. Steric hindrance of AO-A enhances not only its thermal stability but also its antioxidant activity. 3.1.2. Total acid number The results of the TAN measurements of samples after irradiation with UV-B light and the oxidation at 120 1C are shown in Fig. 5. There is no significant difference between the additivefree RO and RO with the antioxidant for UV-B irradiation samples. For the oxidation with UV-B light irradiation, the change in TAN was less sensitive than that in PV. On the other hand, a clear difference could be observed in TAN with the high-temperature oxidation. The TAN of RO and RO+DBMP increased sharply after 3 h, RO+AO-B increased after 7 h, but RO+AO-A gradually increased. AO-A is the most effective, while DBMP is ineffective for high-temperature oxidation. It is considered that the difference in
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991
3 RO+AO-A RO+AO-B
Intensity (arb. unit)
Total acid number, mg KOH / g
RO
RO+DBMP
2
UV-B 0h 12 h
1
24 h 36 h 48 h
0
10
20 30 Oxidation time, h
40
Intensity (arb. unit)
6
Total acid number, mg KOH / g
RO
5
102
50
RO+AO-A RO+AO-B RO+DBMP
4
104
105
120 °C 0h 3h 5h
3
7h
2
102
1
0
103
103 104 Molecular weight (g/mol)
105
Fig. 6. Molecular weight distribution of RO: (a) UV-B light irradiation and (b) high temperature oxidation.
5
10
15
Oxidation time, h Fig. 5. Change in total acid number against oxidation time: (a) UV-B light irradiation and (b) high temperature oxidation.
antioxidant properties under high temperature is related to the volatility of additives. 3.1.3. Molecular weight distribution Molecular weight distribution measurements were limited for RO and RO+AO-A. HPLC chromatograms of RO under UV-B light irradiation and high-temperature oxidation are shown in Fig. 6. Chromatograms show the growth in high-molecular weight materials with oxidation time. In the chromatograms, peaks considered to correspond to the dimers and trimers of RO have been observed. However, low-molecular weight compounds have not been detected. The changes in chromatograms with oxidation time were similar for both oxidation methods. HPLC chromatograms of RO+AO-A are shown in Fig. 7. The tendencies of the molecular weight distribution were not different from those of RO, but the formation of oxidation products was inhibited by the addition of AO-A. 3.1.4. Viscosity The viscosity of oxidized samples with UV-B light was measured. RO, RO+AO-A, SQA, and SQE were tested. The changes
in viscosity with oxidation time are shown in Fig. 8. RO showed an increase in viscosity from the beginning of UV-B irradiation. In contrast, the viscosity of RO+AO-A did not change for 36 h after the start of the irradiation. The formation of oxidation products was inhibited by the addition of AO-A. Based on the results of the molecular weight distribution measurements, the increase in viscosity of RO is caused by the formation of high-molecular weight materials such as dimers and trimers. The viscosity of SQA did not change within the period of this oxidation test, but that of SQE increased from the beginning of UV-B light irradiation.
3.2. Tribological performance of RO The changes in the friction coefficient and wear rate against the oxidation time are shown in Fig. 9. The friction coefficient of RO decreased at the beginning of UV-B irradiation, then increased slightly with oxidation time. The same behavior was observed for SQE. For the RO+AO-A, a tendency toward a decrease in the friction coefficient was observed until 36 h. The friction coefficient of SQA did not change with oxidation time. The wear rate of RO increased with oxidation time, with an especially significant increase being observed after 24 h. On the other hand, the wear rate of RO+AO-A decreased until 24 h. The wear rate of SQE decreased sharply at 12 h of oxidation, then increased slightly until 36 h, and increased rapidly at 48 h. SQA did not show a significant change in the wear rate.
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Intensity (arb. unit)
0.12
Friction coefficient
0.11 UV-B 12 h 24 h 36 h 48 h
102
103
104
RO RO+AO-A SQA SQE
0.10
0.09
105
0.08 Intensity (arb. unit)
0
40
50
20 30 Oxidation time, h
40
50
1.0
3h
7h
105
Fig. 7. Molecular weight distribution of RO+AO-A: (a) UV-B light irradiation and (b) high temperature oxidation.
Wear rate, mm3/mm
0.8 5h
103 104 Molecular weight (g/mol)
20 30 Oxidation time, h
[x10-10]
120 °C
102
10
RO RO+AO-A SQA SQE
0.6
0.4
0.2
Viscosity, mPa·s
104
0 RO RO+AO-A SQA SQE
103
10
Fig. 9. Change in friction coefficient and wear rate against oxidation time.
depend on PV, and enhancement of the oxidation stability is important.
102 3.3. Improvements in tribological performance
101 0
10
20 30 Oxidation time, h
40
50
Fig. 8. Viscosity change against degradation time with UV-B light.
Since the oils tested here have different oxidation stabilities for UV-B light, the degree of degradation of oils is different when compared with the same irradiation time with UV-B light. The results of the PV and TAN measurements suggest that the change in PV is more sensitive than that in TAN. Therefore, all the data in Fig. 10 are summarized as the relationship between PV and the friction coefficient or wear rate. Overall, the friction coefficient and wear rate initially decrease with increasing PV. However, the greater the increase in PV, the greater the friction coefficient and wear rate. Tribological characteristics
As shown in Fig. 9, the friction coefficient of RO is lower than that of SQA and SQE. The friction coefficient of the grease used by The Japan Railways Group is 0.116, and this value is comparable to that of SQA or SQE. On the other hand, the friction coefficient of RO is 0.091, and this is too low for railway application. The low friction coefficient of rapeseed-based oil compared with mineral oil lubricant has been reported [9]. A higher friction coefficient is required for climbing a slope or braking near a station. In order to increase the friction coefficient between wheel and rail, spraying sand is sometimes used for locomotives. In this study, detergent and dispersant additive, which contains small solid particles of CaCO3, was used to improve the tribological performance. The changes in the friction coefficient and wear rate against the oxidation time for RO+DD are shown in Fig. 11. The friction coefficient of RO increased from 0.091 to 0.111 with the addition of DD, and this value is comparable to that of JR grease. This increase in the friction coefficient might have been caused by the solid particles of CaCO3. Furthermore, the friction coefficient of RO+DD was found to be stable with
ARTICLE IN PRESS A. Suzuki et al. / Tribology International 42 (2009) 987–994
0.12
0.11
Friction coefficient
Friction coefficient
0.12
RO RO+AO-A SQA SQE
0.10
0.09
0.08 0
2000
4000 6000 Peroxide value, ppm
8000
0.10
0.09
0.08
10000
0
10
20 30 Oxidation time, h
40
50
20 30 Oxidation time, h
40
50
[x10-10]
1.0
1.0 RO RO+AO-A SQA SQE
0.6
0.4
0.2
0
RO RO+DD JR Grease
0.8 Wear rate, mm3/mm
Wear rate, mm3/mm
RO RO+DD JR Grease
0.11
[x10-10]
0.8
993
0.6
0.4
0.2
2000
4000 6000 Peroxide value, ppm
8000
10000 0
10
Fig. 10. Change in friction coefficient and wear rate against peroxide value. Fig. 11. Enhancement of tribological performance by addition of DD.
4. Conclusions
104 RO RO+DD
Viscosity, mPa·s
oxidation time. On the other hand, the wear rate of RO+DD is higher than that of RO before UV-B irradiation, but lower than that of JR grease. In response to irradiation with UV-B light, the wear rate of RO+DD decreased and became almost stable with oxidation time. The viscosity changes of RO+DD in response to irradiation with UV-B light are shown in Fig. 12. With the addition of the DD, the viscosity of the lubricant became higher than that of the original RO. However, the viscosity did not change significantly with oxidation time in contrast to the viscosity of RO, which gradually increased with oxidation time. The friction coefficient, wear rate, and viscosity of RO+DD did not change significantly after irradiation with UV-B light. It can be concluded that DD has a double function as an antioxidant and an anti-wear property for RO.
103
102
101
0
10
20 30 Oxidation time, h
40
50
Fig. 12. Effect of addition of DD on viscosity of RO.
In order to examine the possibility of using RO as a lubricating oil for railroad tracks, the oxidation stability and tribological characteristics were studied. The degradation characteristics of RO and the effects of antioxidant against UV-B light irradiation were investigated by means of measurements of peroxide value, total acid number, molecular weight distribution, and the change in viscosity. The RO was easily oxidized by irradiation of UV-B light
under moderate temperature, and the main degradation products were polymerization materials such as dimers and trimers. Antioxidant AO-A was found to be the most effective for UVassisted-degradation of RO.
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[6] Pospı´sˇil J, Klemchuk PP. Oxidation inhibition in organic materials, vol. II. Boca Raton, FL: CRC Press; 1990. p. 32–3. [7] Gray RL, Lee RE, Sanders BM. Low volatility antioxidants for scorch protection of polyurethane foams. J Vinyl Additive Technol 1996;2:265–9. [8] Vlase T, Doca N, Vlase G, Bolcu C, Borcan F. Kinetics of non-isothermal decomposition of three IRGANOX-type antioxidants. J Thermal Anal Calorimetry 2008;92:15–8. [9] Arnsek A, Vizintin J. Scuffing load capacity of rapeseed-based oil. Lubrication Eng 1999;55(8):11–8.
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Evaluation of antioxidants in rapeseed oils for railway application Akihito Suzuki , Ratu Ulfiati 1, Masabumi Masuko Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan
a r t i c l e in fo
abstract
Article history: Received 7 October 2005 Received in revised form 28 January 2009 Accepted 3 February 2009 Available online 12 February 2009
In order to evaluate the applicability of biodegradable base-oil lubricants for railway applications, the influence of UV(ultraviolet)-B light (280–320 nm) on oxidation stability and the tribological performance of rapeseed oil (RO) have been investigated. Antioxidant free RO was oxidized immediately under UV-B irradiation at moderate temperature. Three sterically hindered phenolic-type antioxidants, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), octadecyl-3-(3,5-di-tert-butyl4-hydroxyphenyl)propionate, and 2-6-di-tert-butyl-4-methyl phenol were tested in order to increase the oxidation stability of RO. Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) was the most effective, prolonging the oxidation induction time from 3 h of RO to 40 h. The tribological performance of RO could be improved by the addition of metallic detergent (Ca–alkylsalicylate–CaCO3) and was stable against the UV-B irradiation. & 2009 Elsevier Ltd. All rights reserved.
Keywords: UV-induced degradation Biodegradable oil Additives
1. Introduction With the recent increased interest in global environmental problems, new technology regarding environmental protection has been developed and applied in all industries. There are three classifications of elemental technologies related to environmental protection for lubricants. The first involves the establishment of a recycling-based society by advocating the recycling of used oil. The second involves reductions in health and environmental risk based on the use of low-toxicity chemicals. The third effort involves reductions in the environmental load by the use of biodegradable lubricants. Measures to avoid the use of toxic chemicals have been taken for many years. Lubricants used in closed systems have come to be collected and recycled. Lubricants, however, can enter the environment through either normal use or leakage and spillage. Furthermore, in the case of an open lubrication system, the collection of lubricants is difficult and lubricants can easily enter the environment. As such, environmentally harmless lubricants have been developed. Biodegradable lubricants have a minimum impact on the environment and are suitable for various kinds of applications such as outboard twocycle engine oils, chain saw oils, wire rope lubricants, hydraulic oils for forest and agricultural equipment, etc. [1]. One of the promising usages of biodegradable lubricants is for railways. Oils or greases are applied to railways to maintain an adequate friction
Corresponding author. Tel./fax: +81 3 5734 2628.
E-mail address: [email protected] (A. Suzuki). Present address: Research and Development Center for Oil and Gas Technology, ‘‘LEMIGAS’’, Jakarta Selatan 12230, Indonesia. 1
0301-679X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2009.02.001
coefficient, to avoid wear damage, or to reduce vibration and noise. These lubricants are ‘‘once-through’’ lubricants and are never recovered, but are defused into the ground. The most commonly used base stocks for lubricants are mineral oils, which are produced from petroleum crude by various processing steps. Mineral oil-based lubricants have been used in all kind of applications such as automotive engines, transmissions, industrial gears, etc. From an environmental point of view, however, mineral oil-based lubricants have poor biodegradability. Furthermore, mineral oil-based lubricants contain many kinds of additives such as antioxidant agents, anti-wear agents, detergents and dispersants, anti-forms, extreme pressure agents, friction modifiers, and viscosity index improvers. Some of the additives are known to be toxic or harmful to the environment [2]. Natural oils and fats viz. vegetable oils and animal fats are known to have superior biodegradability. These oils, however, are known to have very poor oxidation stability. The oxidation of lubricating oils on the railways occurs primarily due to the sunlight effect. Oxidation products are acidic and can give rise to corrosive attack. Furthermore, oxidation of oil leads to an increase in viscosity, and polymerization of oxidation products leads to the formation of varnish and sludge deposits [3]. Improvements in oxidation stability are of great importance for the application of vegetable oils to railways lubricants. In addition to oxidation stability, tribological characteristics play an important role in car dynamic behavior because the forces generated between wheel and rail depends on the friction characteristics. A high friction coefficient is required for climbing a slope or braking near a station. On the other hand, a high friction coefficient is undesirable for tight curves, as it leads to an increase in squeak noise and rail corrugation. An inadequate friction
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coefficient leads to various troubles such as skid at braking, over-run at the station, and wear of wheel and rail [4]. Therefore, improvements in the tribological performance are of great importance. The focus of the present study was on obtaining biodegradable base-oil lubricants for railway applications that have high oxidation stability, a constant friction coefficient and viscosity with time, and a low wear rate. 2. Experimental
SQA:
SQE: O AO-A:
C
CH2CH2COCH2
HO
4 2.1. Lubricant The base oil used in this study was commercially available rapeseed oil (RO). It was used as received. The viscosity of the original RO was 79.9 mPa s at 20 1C; peroxide value, 29.2 ppm; total acid number, 0.16 mg KOH/g. In this study, squalane (SQA) was used as a reference sample, representative of mineral oil (saturated hydrocarbon). Furthermore, because RO has an unsaturated carbon bond, squalene (SQE) was also used as representative of an unsaturated compound. The oxidation of lubricating oils on railways primarily occurs due to the effects of sunlight, with temperature effects being relatively insignificant. The most frequently used low-temperature antioxidants of the free radical-acceptor type are electron donor compounds such as phenolic and aromatic amine materials. In the present study, three kinds of sterically hindered phenolictype antioxidants, [pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4hydroxyphenyl)propionate)] (AO-A), [octadecyl-3-(3,5-di-tertbutyl-4-hydroxyphenyl)propionate] (AO-B), and [2-6-di-tert-butyl4-methyl phenol] (DBMP), were used. AO-A is approved by US Food and Drug Administration (FDA) as an ingredient for use in lubricants with incidental food contact for use in and around food processing areas. DBMP is widely used as an antioxidant for fats and oils or in packaging material for fat-containing foods. The ROs containing 0.95 mass% of AO-A, 1 mass% of AO-B, and 1 mass% of DBMP were prepared and tested. Although increase of molecular weight improves thermal stability, the solubility of the antioxidant in oil decreases. Because the solubility of AO-A to RO was low, and the sample of the concentration of 1 mass% could not be made at the room temperature, the solution of 0.95 mass% was used for AO-A. In addition to the oxidation stability, tribological characteristics play an important role in car dynamic behavior. Spraying sand for locomotives is sometimes used to increase the friction coefficient [4]. In the present study, the effects of solid particles on tribological behavior were evaluated. Detergent and dispersant additive was used to improve the tribological performance of the RO. The additive used in this study was calcium alkylsalicylate– CaCO3 (DD). A RO containing 6 mass% Ca was prepared. The molecular structures of the chemicals are given in Fig. 1. Formulations of the prepared samples are shown in Table 1. 2.2. Sample preparation Radiation from the sun reaching the outer layers of the air around the earth shows a continuous energy spectrum in the wavelength region between 0.7 and 3000 nm. Wavelengths above 780 nm constitute the infrared (IR) region. On passing through the atmosphere, a portion of the long-wavelength IR rays is absorbed by water vapor and carbon dioxide. Therefore, only the shortwavelength portion of the IR rays reaches the surface of the earth. Wavelengths in the range of 100–380 nm constitute the ultraviolet (UV) spectral region. The short-wavelength UV radiation below 175 nm is absorbed by oxygen in the layers above 100 km. The radiation between 175 and 280 nm is absorbed by the ozone layer of the stratosphere. The radiation between 280 and 380 nm
O AO-B:
HO
CH2CH2COC18H37
DBMP:
HO
CH3
DD:
R
COO Ca.CaCO3 OH Fig. 1. Molecular structures of materials used.
Table 1 Formulations of samples. Sample
Base oil
RO RO+AO-A RO+AO-B RO+DBMP RO+DD SQA SQE
Rapeseed Rapeseed Rapeseed Rapeseed Rapeseed Squalane Squalene
oil oil oil oil oil
Additive
Additive conc. (mass%)
None AO-A AO-B DBMP DD None None
0 0.95 1 1 6 (mass% of Ca) 0 0
reaches the surface of the earth [5,6]. This portion of the UV rays initiates the degradation of materials. In this study, two UV-B tubes (Philips TL UV-B 20W/12) with wavelength in the range of 270–400 nm (maximum radiation at 315 nm) were used to prepare the degradation sample. A fixed amount of sample (350 mg) was poured into a steel pan (Fig. 2a) and then irradiated with UV-B light in a lighting box (Fig. 2b) with various oxidation times from 0 to 48 h at a constant temperature of 40 1C. The distance between light tubes and steep pan was about 20 cm and the intensity of the UV-B light on the steel pan was 19 W/m2. For a comparison with UV degradation, oxidation in the air was also carried out at high temperature (120 1C).
2.3. Analysis of lubricant degradation The lubricant degradation characteristics were evaluated by peroxide value (PV) [ASTM D-3703], total acid number (TAN) [ASTM D-664], molecular weight distribution, and viscosity change in this study. PV was determined by titration of liberated iodine (I2), which produced by the reaction of hydroperoxide in the sample oil and
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Table 2 Operation conditions of HPLC. Column
Shodex KF-805+KF-802.5
Column temperature Sample concentration Injection volume Eluent Flow rate of eluent Detector
30 1C 0.5 mass% 0.1 ml Tetrahydrofuran 1.0 ml/min UV (254 nm) Refractive index
18mm
neutralize free acids in 1 g of the sample, i.e. mg KOH/g. The sample oil was weighted accurately and dissolved with 75 mL of mixture solvent of toluene/2-propanol/water (100:99:1 v/v/v). The solution was titrated potentiometrically with 0.1 mol/L 2propanol solution of KOH at room temperature. Molecular weight distribution was determined using highperformance liquid chromatography (HPLC). The operation conditions of HPLC are shown in Table 2. Viscosity measurements were conducted using a rheometer with cone-plate geometry (diameter 25.0 mm, cone angle 2.0041) at 20 1C.
UV-B light 2.4. Friction and wear test
heating plate
Fig. 2. Oil degradation apparatus using UV light: (a) oil pan and (b) UV rays lighting box.
potassium iodide (KI), with sodium thiosulphate (Na2S2O3). In this study, sample oil was weighted accurately in a flask and added 30 mL of solvent (5% acetic acid in 2-propanol), 2 mL of toluene, 2 mL of KI saturated aqueous solution, and one piece of dry ice. The mixture was refluxed under nitrogen flow for 15 min at 70 1C. After the resulting solution was cooled to room temperature, the solution was poured into a titration vessel with magnetic stirrer bar and one piece of dry ice. The flask was rinsed with 15 mL of distilled water twice, and the rinsing water also poured into the titration vessel. The solution in the titration vessel was titrated potentiometrically with 0.01 mol/L Na2S2O3 solution. In this study, PV was expressed as the number of milligrams of peroxide oxygen per 1 kg of sample oil, i.e. ppm. TAN is a measure for total amount of both weak and strong organic acids present in the lubricant. This is expressed as the amount of potassium hydroxide (KOH) in mg required to
Tribological performance was evaluated using a highfrequency reciprocating tribometer with a ball-on-disk configuration. The ball was made of SUJ2 bearing steel 8.73 mm in diameter, and the disk was a bearing roller 10 mm in diameter made of SUJ2 bearing steel. The end surface of the roller was used for the friction and wear test. The disk specimens were polished with 3000 grit abrasive paper before the test. Both disk and ball specimens were cleaned with toluene in an ultrasonic bath, and then cleaned with an UV–ozone cleaner to remove the organic compounds remaining on the surface of the specimens. The test conditions were as follows: axial load, 19.6 N; stroke length, 3.0 mm; frequency, 30 Hz; temperature, room temperature. Total time of friction was 3 h (total sliding distance of 1944 m). Tribotest was stopped at 30 min, 1 h, 2 h, and 3 h, and wear scar diameter formed on the ball specimen was measured using an optical microscope. Wear rate was determined from the slope of wear volume as a function of sliding distance. Since the friction coefficient depends on the sliding velocity of the specimen and varies throughout the cycle, the mean friction coefficient in the range within 1.1 mm from the center position of reciprocation was adopted in this study. However, friction coefficient varied with time unstable in the early stage of the friction test, it became stable with the progress of the test. Sufficiently stable friction coefficients were used for the results in this study. In order to check the reliability of the test, tribotest was performed using SQA repeatedly. Friction coefficient and wear rate of SQA at room temperature were 0.10870.006 and 1.1 10 11 mm3/mm, respectively. Repeatability of the wear rate was within 25%.
3. Results and discussion 3.1. Base oil performance 3.1.1. Peroxide value The PV of sample oils after the irradiation with UV-B light, and the oxidation at 120 1C are shown in Fig. 3. RO gave an immediate increase in PV for both oxidation methods. As with RO, the PV of
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10000
Peroxide value, ppm
8000
R'OOH + •O
R + R'OO•
HO
RO RO+AO-A RO+AO-B RO+DBMP SQA SQE
R
R •O
6000
R
O
•
4000 R O
2000
•
R + R'OO•
O
OOR'
Fig. 4. Mechanisms of antioxidant action of hindered phenols.
0
10
20 30 Oxidation time, h
40
50 Table 3 Effectiveness of antioxidants for UV-B light oxidation.
Peroxide value, ppm
8000
6000
RO RO+AO-A RO+AO-B RO+DBMP SQA
4000
2000
0
10 Oxidation time, h
20
Fig. 3. Change in peroxide value against oxidation time: (a) UV-B light irradiation and (b) high temperature oxidation.
SQE under UV-B irradiation increased in a short time. UV-B light will significantly affect the oxidation of unsaturated compounds, which explains the poor oxidation stability of RO. The PV of SQA under UV-B irradiation did not significantly change, but it immediately increased under high-temperature conditions. These results indicate that saturated hydrocarbon is stable under UV-B irradiation at moderate temperature, but can be easily oxidized at high temperatures. The oxidation induction time of RO under UV-B irradiation was approximately 3 h. This time increased to 40 h with the addition of AO-A, 15 h with AO-B, and 29 h with DBMP. On the other hand, the oxidation induction time of RO for the high-temperature oxidation was approximately 1 h, and that increased to 11.3 h with the addition of AO-A, to 6.8 h with AO-B, and to 2.1 h with DBMP. DBMP is effective for UV-B light oxidation, but ineffective for high-temperature oxidation. According to thermogravimetric analysis by Gray et al., DBMP showed a mass loss of 5% at 90 1C and a mass loss of 95% at 145 1C [7]. Because of high volatility of DBHT, the sample containing DBMP lost its antioxidant activity during high-temperature oxidation test. On the other hand, the molecular weight of AO-A and AO-B is higher than that of DBMP, and these additives show low volatility [7,8]. Therefore, AO-A and AO-B showed excellent antioxidant activity for the hightemperature oxidation test.
Antioxidant
AO-A
AO-B
DBMP
Molar concentration (mmol/kg) Increase in oxidation induction time (h) Phenolic OH concentration (mmol/kg) Increase in oxidation induction time per phenolic OH concentration (h/(mmol/kg))
8.08 37 32.32 1.15
18.87 12 18.87 0.64
45.45 26 45.45 0.57
The most effective antioxidant for RO oxidation is AO-A, when oxidation induction times are compared with the same mass concentration. The additives used in this study were sterically hindered phenols, and it is considered that free radicals formed in the oil are stabilized by the various resonance structures of hindered phenols (Fig. 4). First, phenolic oxidation inhibitor traps a peroxy radical and becomes a phenoxy radical, then this phenoxy radical traps a different peroxy radical. In theory, one hindered phenolic group can inactivate two peroxy radicals. The three kinds of antioxidants used in this study have different molecular weights and different numbers of phenolic OH groups per molecule. Therefore, the effects of antioxidants for UV-B light oxidation are compared based on the increase in oxidation induction time per phenolic OH concentration (Table 3). Accordingly, AO-A is still the most effective among the three antioxidants and makes the oxidation induction time of RO twice longer than that of DBMP when compared with the same phenolic OH concentration. It is thought that the stability of generated phenoxy radicals affects the peroxy radical trapping capability. Unstable phenoxy radicals can easily make a coupling reaction and radical trapping capability decreases. High steric hindrance of AO-A might stabilize the generated phenoxy radical more, and hence AO-A showed the most excellent antioxidant performance. Steric hindrance of AO-A enhances not only its thermal stability but also its antioxidant activity. 3.1.2. Total acid number The results of the TAN measurements of samples after irradiation with UV-B light and the oxidation at 120 1C are shown in Fig. 5. There is no significant difference between the additivefree RO and RO with the antioxidant for UV-B irradiation samples. For the oxidation with UV-B light irradiation, the change in TAN was less sensitive than that in PV. On the other hand, a clear difference could be observed in TAN with the high-temperature oxidation. The TAN of RO and RO+DBMP increased sharply after 3 h, RO+AO-B increased after 7 h, but RO+AO-A gradually increased. AO-A is the most effective, while DBMP is ineffective for high-temperature oxidation. It is considered that the difference in
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3 RO+AO-A RO+AO-B
Intensity (arb. unit)
Total acid number, mg KOH / g
RO
RO+DBMP
2
UV-B 0h 12 h
1
24 h 36 h 48 h
0
10
20 30 Oxidation time, h
40
Intensity (arb. unit)
6
Total acid number, mg KOH / g
RO
5
102
50
RO+AO-A RO+AO-B RO+DBMP
4
104
105
120 °C 0h 3h 5h
3
7h
2
102
1
0
103
103 104 Molecular weight (g/mol)
105
Fig. 6. Molecular weight distribution of RO: (a) UV-B light irradiation and (b) high temperature oxidation.
5
10
15
Oxidation time, h Fig. 5. Change in total acid number against oxidation time: (a) UV-B light irradiation and (b) high temperature oxidation.
antioxidant properties under high temperature is related to the volatility of additives. 3.1.3. Molecular weight distribution Molecular weight distribution measurements were limited for RO and RO+AO-A. HPLC chromatograms of RO under UV-B light irradiation and high-temperature oxidation are shown in Fig. 6. Chromatograms show the growth in high-molecular weight materials with oxidation time. In the chromatograms, peaks considered to correspond to the dimers and trimers of RO have been observed. However, low-molecular weight compounds have not been detected. The changes in chromatograms with oxidation time were similar for both oxidation methods. HPLC chromatograms of RO+AO-A are shown in Fig. 7. The tendencies of the molecular weight distribution were not different from those of RO, but the formation of oxidation products was inhibited by the addition of AO-A. 3.1.4. Viscosity The viscosity of oxidized samples with UV-B light was measured. RO, RO+AO-A, SQA, and SQE were tested. The changes
in viscosity with oxidation time are shown in Fig. 8. RO showed an increase in viscosity from the beginning of UV-B irradiation. In contrast, the viscosity of RO+AO-A did not change for 36 h after the start of the irradiation. The formation of oxidation products was inhibited by the addition of AO-A. Based on the results of the molecular weight distribution measurements, the increase in viscosity of RO is caused by the formation of high-molecular weight materials such as dimers and trimers. The viscosity of SQA did not change within the period of this oxidation test, but that of SQE increased from the beginning of UV-B light irradiation.
3.2. Tribological performance of RO The changes in the friction coefficient and wear rate against the oxidation time are shown in Fig. 9. The friction coefficient of RO decreased at the beginning of UV-B irradiation, then increased slightly with oxidation time. The same behavior was observed for SQE. For the RO+AO-A, a tendency toward a decrease in the friction coefficient was observed until 36 h. The friction coefficient of SQA did not change with oxidation time. The wear rate of RO increased with oxidation time, with an especially significant increase being observed after 24 h. On the other hand, the wear rate of RO+AO-A decreased until 24 h. The wear rate of SQE decreased sharply at 12 h of oxidation, then increased slightly until 36 h, and increased rapidly at 48 h. SQA did not show a significant change in the wear rate.
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Intensity (arb. unit)
0.12
Friction coefficient
0.11 UV-B 12 h 24 h 36 h 48 h
102
103
104
RO RO+AO-A SQA SQE
0.10
0.09
105
0.08 Intensity (arb. unit)
0
40
50
20 30 Oxidation time, h
40
50
1.0
3h
7h
105
Fig. 7. Molecular weight distribution of RO+AO-A: (a) UV-B light irradiation and (b) high temperature oxidation.
Wear rate, mm3/mm
0.8 5h
103 104 Molecular weight (g/mol)
20 30 Oxidation time, h
[x10-10]
120 °C
102
10
RO RO+AO-A SQA SQE
0.6
0.4
0.2
Viscosity, mPa·s
104
0 RO RO+AO-A SQA SQE
103
10
Fig. 9. Change in friction coefficient and wear rate against oxidation time.
depend on PV, and enhancement of the oxidation stability is important.
102 3.3. Improvements in tribological performance
101 0
10
20 30 Oxidation time, h
40
50
Fig. 8. Viscosity change against degradation time with UV-B light.
Since the oils tested here have different oxidation stabilities for UV-B light, the degree of degradation of oils is different when compared with the same irradiation time with UV-B light. The results of the PV and TAN measurements suggest that the change in PV is more sensitive than that in TAN. Therefore, all the data in Fig. 10 are summarized as the relationship between PV and the friction coefficient or wear rate. Overall, the friction coefficient and wear rate initially decrease with increasing PV. However, the greater the increase in PV, the greater the friction coefficient and wear rate. Tribological characteristics
As shown in Fig. 9, the friction coefficient of RO is lower than that of SQA and SQE. The friction coefficient of the grease used by The Japan Railways Group is 0.116, and this value is comparable to that of SQA or SQE. On the other hand, the friction coefficient of RO is 0.091, and this is too low for railway application. The low friction coefficient of rapeseed-based oil compared with mineral oil lubricant has been reported [9]. A higher friction coefficient is required for climbing a slope or braking near a station. In order to increase the friction coefficient between wheel and rail, spraying sand is sometimes used for locomotives. In this study, detergent and dispersant additive, which contains small solid particles of CaCO3, was used to improve the tribological performance. The changes in the friction coefficient and wear rate against the oxidation time for RO+DD are shown in Fig. 11. The friction coefficient of RO increased from 0.091 to 0.111 with the addition of DD, and this value is comparable to that of JR grease. This increase in the friction coefficient might have been caused by the solid particles of CaCO3. Furthermore, the friction coefficient of RO+DD was found to be stable with
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0.12
0.11
Friction coefficient
Friction coefficient
0.12
RO RO+AO-A SQA SQE
0.10
0.09
0.08 0
2000
4000 6000 Peroxide value, ppm
8000
0.10
0.09
0.08
10000
0
10
20 30 Oxidation time, h
40
50
20 30 Oxidation time, h
40
50
[x10-10]
1.0
1.0 RO RO+AO-A SQA SQE
0.6
0.4
0.2
0
RO RO+DD JR Grease
0.8 Wear rate, mm3/mm
Wear rate, mm3/mm
RO RO+DD JR Grease
0.11
[x10-10]
0.8
993
0.6
0.4
0.2
2000
4000 6000 Peroxide value, ppm
8000
10000 0
10
Fig. 10. Change in friction coefficient and wear rate against peroxide value. Fig. 11. Enhancement of tribological performance by addition of DD.
4. Conclusions
104 RO RO+DD
Viscosity, mPa·s
oxidation time. On the other hand, the wear rate of RO+DD is higher than that of RO before UV-B irradiation, but lower than that of JR grease. In response to irradiation with UV-B light, the wear rate of RO+DD decreased and became almost stable with oxidation time. The viscosity changes of RO+DD in response to irradiation with UV-B light are shown in Fig. 12. With the addition of the DD, the viscosity of the lubricant became higher than that of the original RO. However, the viscosity did not change significantly with oxidation time in contrast to the viscosity of RO, which gradually increased with oxidation time. The friction coefficient, wear rate, and viscosity of RO+DD did not change significantly after irradiation with UV-B light. It can be concluded that DD has a double function as an antioxidant and an anti-wear property for RO.
103
102
101
0
10
20 30 Oxidation time, h
40
50
Fig. 12. Effect of addition of DD on viscosity of RO.
In order to examine the possibility of using RO as a lubricating oil for railroad tracks, the oxidation stability and tribological characteristics were studied. The degradation characteristics of RO and the effects of antioxidant against UV-B light irradiation were investigated by means of measurements of peroxide value, total acid number, molecular weight distribution, and the change in viscosity. The RO was easily oxidized by irradiation of UV-B light
under moderate temperature, and the main degradation products were polymerization materials such as dimers and trimers. Antioxidant AO-A was found to be the most effective for UVassisted-degradation of RO.
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[6] Pospı´sˇil J, Klemchuk PP. Oxidation inhibition in organic materials, vol. II. Boca Raton, FL: CRC Press; 1990. p. 32–3. [7] Gray RL, Lee RE, Sanders BM. Low volatility antioxidants for scorch protection of polyurethane foams. J Vinyl Additive Technol 1996;2:265–9. [8] Vlase T, Doca N, Vlase G, Bolcu C, Borcan F. Kinetics of non-isothermal decomposition of three IRGANOX-type antioxidants. J Thermal Anal Calorimetry 2008;92:15–8. [9] Arnsek A, Vizintin J. Scuffing load capacity of rapeseed-based oil. Lubrication Eng 1999;55(8):11–8.