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Impact of polydimethylsiloxanes on physicochemical and tribological properties of naphthenic mineral oil (KN 4010)-based titanium complex grease
Chinese Journal of Chemical Engineering 27 (2019) 944–948
Contents lists available at ScienceDirect
Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Article
Impact of polydimethylsiloxanes on physicochemical and tribological properties of naphthenic mineral oil (KN 4010)-based titanium complex grease☆ Jitai Li, Chu Zhai, Hengbo Yin ⁎, Aili Wang, Lingqin Shen Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 15 June 2018 Received in revised form 21 August 2018 Accepted 3 September 2018 Available online 8 September 2018 Keywords: Complexes Surface Polymers Titanium complex grease Polydimethylsiloxane Tribological properties
a b s t r a c t Titanium complex greases were prepared by using naphthenic mineral oil and polydimethylsiloxane as the mixed base oil. The effect of polydimethylsiloxane molecular weight and polydimethylsiloxane content in mixed base oil on the physicochemical and tribological properties of titanium complex greases was investigated. As compared to the sole mineral oil-based titanium complex grease, the use of polydimethylsiloxane (H201-350) as a co-base oil increased the dropping point from 310 to 329 °C, decreased the oil separation from 3.7% to 2.3%, reduced the corrosion extent, and obviously improved the tribological properties. When the mixed oil-based titanium complex grease was used as a lubricant, lubricating films of polydimethylsiloxane were probably formed on the surfaces of friction pairs, giving good lubricating property. © 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.
1. Introduction Lubricating grease plays a significant role in the industrial production to reduce friction energy loss, protect machinery from wear, and prolong the operation life [1–3]. Since 1960s, Al-, Ca-, and Li-complex greases and polyurea greases were investigated and commercially used in the world [4–9], these greases with high dropping points (N230 °C) exhibit good lubrication performance for a long operation time. However, these commercial greases are able to leak, overflow or exhaust from machinery and equipment during the operation at high temperature and pressure, causing high friction and wear and subsequently making damage to the operating machinery and equipment [10]. With the development in the metallurgical, automobile, and aviation industries, increasing numbers of mechanical equipment have worked at high temperature and extreme pressure [11,12]. It is a great challenge to let lubricating greases work under these severe conditions. Kumar et al. [13–16] invented mineral oil-based titanium complex greases and disclosed their preparation methods and tribological properties. Titanium complex greases have several advantages, such as shearing reversibility, good at low and high working temperatures ranging from − 20 °C to 150 °C, anti-wear under extreme pressure, ☆ Supported by the National Natural Science Foundation of China (21506078, 21506082) and China Postdoctoral Science Foundation (2016M591786, 2016M601739). ⁎ Corresponding author. E-mail address: [email protected] (H. Yin).
high corrosion and oxidation resistances, and compatibility with other greases [14,15]. As the evaluation index of working temperature, the dropping point of mineral oil-based titanium complex grease was up to 294 °C, higher than those of Li- and Al-complex greases of 263 °C and 275 °C, respectively [14]. Additives were commonly used in the greases to improve their properties. Antioxidants, such as diphenylamine and phenol, were used to improve the oxidation resistance and extend lubricating life time [17]. Tackifiers, such as polyisobutene and emulsion, were used to improve the adhesion performances. Anti-friction and anti-wear agents, such as WS2, MoS2, nano-SiO2, and TiO2, were used to reduce the friction coefficient, protect rubbing surface, and improve load-bearing capacity [18–20]. However, base oils have great impact on the properties of complex greases. It was reported that vegetable oil-based titanium complex greases exhibited high dropping point around 330 °C, higher than mineral oil-based greases. Unfortunately, due to the poor thermo-oxidative stability of vegetable oil, the vegetable oil-based titanium complex greases cannot work at a high temperature for a long time [13,21,22]. Polydimethylsiloxanes, a series of synthetic oils from organosilicone industry, with superior thermal stability, high oxidation resistance, high waterproofness, and wide working temperature range have been widely used in medical treatment, electromechanical, and coating fields [23]. To the best of our knowledge, the use of polydimethylsiloxane as a lubricant has rarely been reported [24] and the use of polydimethylsiloxane as a co-base oil for the titanium complex grease has not been reported.
https://doi.org/10.1016/j.cjche.2018.09.002 1004-9541/© 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
In the present work, polydimethylsiloxanes with different molecular weights were mixed with naphthenic mineral oil (KN 4010) as mixed base oils for the preparation of titanium complex greases. The effect of polydimethylsiloxane molecular weight and polydimethylsiloxane content in the mixed base oil on the physicochemical and tribological properties of titanium complex greases was investigated. The presence of polydimethylsiloxane significantly improved the tribological properties of titanium complex greases. 2. Experimental 2.1. Materials Polydimethylsiloxanes (H201-100, H201-350, H201-500) were supplied by Jiangsu Hongda New Materials Co. Ltd. Naphthenic mineral oil (KN 4010) and 12-hydroxystearic acid were supplied by Wuxi Hanmei Advanced Materials Co., Ltd. The physical properties of polydimethylsiloxanes and KN 4010 are listed in Table 1. Titanium tetraisopropanolate was purchased from Yangzhou Lida Resin Co., Ltd. Benzoic acid was of reagent grade and was purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used through all the experiments. 2.2. Preparation of titanium complex grease The preparation process of titanium complex grease is illustrated as follows. Naphthenic mineral oil (KN 4010) and polydimethylsiloxane (0%–25 wt% in the mixed base oil) were used as a mixed base oil. The mixed base oil (125 g), 12-hydroxystearic acid (75.7 g), and benzoic acid (30.8 g) were mixed and heated to 90 °C in a 1 L three-necked round-bottom flask equipped with a mechanical stirrer and a condenser. Titanium tetraisopropanolate (143.2 g) was added into the mixture and the saponification reaction was carried out at 150 °C for 8 h. After the saponification reaction, the formed isopropanol was evaporated. And then, mixed base oil (125 g) was added and the temperature was increased to the refining temperature of 210 °C at a heating rate of 20 °C·h−1 and kept at 210 °C for 5 min. Nitrogen was used as shielding
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gas throughout the whole heating process. The as-prepared greases were dumped into a stainless mug and cooled down to the room temperature. A little amount of water (5 ml) was added to the as-prepared titanium complex greases when they were grinded to obtain the titanium complex grease samples. 2.3. Characterization of titanium complex grease Dropping points of greases were determined on an XH–141 dropping point tester according to the grease dropping point test method of GB/T 3498-2008 (similar to ASTM D2265). Penetrations of the greases were determined on an XH–140A grease penetration tester according to the standard of GB/T 269-1991 (similar to ASTM D217). Oil separation was tested using an FDH–4701 steel mesh tester according to the standard of SH/T 0324-1992 (similar to ASTM D6184). Evaporation loss and copper corrosion were characterized according to the standards of GB 7325-1987 and GB 7326-1987 (similar to ASTM D972 and ASTM D4048), respectively. Friction and wear behaviors (tribological properties) were examined on an MS–10JB four-ball tester according to the standard of SH/T 02042004 (similar to ASTM D2266) at a rotating speed of 1450 r·min−1 , a load of 392 N, and working temperature of 75 °C for 60 min. The test steel balls with the diameter of 12.7 mm and the hardness of 60 HRC were made of GCr15 bearing steel. The morphology of the worn surface was observed on a Zeiss Evo–18 scanning electron microscope. The surface elements (C, O, Si, Ti, Cr, and Fe) on the worn surface were measured on an Oxford Max–20 energy disperse spectroscopy at a current intensity of 300 Pa for 90 s. 3. Result and Discussion 3.1. Physicochemical properties of titanium complex greases The penetration degrees, dropping points, copper corrosions, oil separation contents, and oil evaporation contents of the titanium complex greases are listed in Table 2. For the dropping points, it was found that when polydimethylsiloxane (H201-100) with the smallest average
Table 1 Physical properties of mineral oil and polydimethylsiloxanes Reagents
Brands
Viscosity, 20 °C/mm2 ·s−1
Flashing point/°C
Molecular weight
Refractive index
Density/g·ml−1
Mineral oil
KN 4010 H201–100 H201–350 H201–500
335 100 350 500
220 N300 N300 N300
– 6250 13,680 17,520
1.492 1.401 1.403 1.404
0.905 0.969 0.973 0.975
Polydimethyl-siloxanes
Table 2 Properties of titanium complex greases prepared by using polydimethylsiloxane (PDMS) and mineral oil (KN4010) as the mixed base oil Samples.
PDMS
PDMS contents in mixed base oil/wt%
Dropping points/°C
Penetration/mm
Copper corrosion
Oil separation/%
Evaporation losses/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
None H201-100 H201-100 H201-100 H201-100 H201-100 H201-350 H201-350 H201-350 H201-350 H201-350 H201-500 H201-500 H201-500 H201-500 H201-500
0 5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
315 317 317 320 301 279 319 329 321 299 283 325 316 302 287 276
23.0 25.7 26.4 26.9 28.5 29.5 21.3 22.0 25.9 26.3 27.1 18.5 22.2 21.4 24.6 23.5
2a 2a 2a 1b 1b 1b 2a 1b 1b 1b 1b 2a 1b 1b 1b 1b
3.7 4.2 3.9 4.4 4.8 6.3 2.7 2.3 2.5 3.4 5.8 2.1 2.2 3.0 4.9 6.0
3.0 3.0 3.2 3.1 3.2 3.1 3.1 3.3 2.9 3.1 3.0 3.0 2.9 2.8 3.2 3.2
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
molecular weight of 6250 was used as a co-base oil, the as-prepared titanium complex grease had the maximum dropping point of 320 °C at the polydimethylsiloxane content of 15%. When polydimethylsiloxane (H201-350) with the average molecular weight of 13680 was used as a co-base oil, the as-prepared titanium complex grease had the maximum dropping point of 329 °C at the polydimethylsiloxane content of 10%. For polydimethylsiloxane (H201-500) with the largest average molecular weight of 17520, the resultant titanium complex grease had the maximum dropping point of 325 °C at the polydimethylsiloxane content of 5%. To obtain the maximum dropping point, the optimal content of polydimethylsiloxane decreased upon the increase in their average molecular weights. It could be explained as that the molecular weight of polydimethylsiloxane affected its compatibility with the mineral oil (KN 4010), resulting in the change in the dropping points. For the penetration parameter, it was found that the penetration values of the resultant titanium complex greases slightly increased upon increasing the content of polydimethylsiloxane. The penetration value was affected by the molecular weight of polydimethylsiloxane. The effect of polydimethylsiloxanes on the penetration values was roughly in an order of H201-100 N H201-350 N blank N H201-500. The penetration values of the titanium complex greases prepared using the polydimethylsiloxane (H201-100) with the smallest average molecular weight as a co-base oil were larger than those using the polydimethylsiloxanes (H201-350 and H201-500) as co-base oils. The high viscosity decreased the penetration value to some extent. The addition of polydimethylsiloxane effectively improved the anti-corrosion property of titanium complex grease, probably due to the formation of polydimethylsiloxane film on the copper sheet surface, which was certified by the surface element analysis of friction pairs. The oil separation values slightly increased with the increase in polydimethylsiloxane contents. When the polydimethylsiloxane (H201-100) with the smallest average molecular weight was used as a co-base oil, the oil separation values of titanium complex greases were larger than that prepared without the use of polydimethylsiloxane. However, when the polydimethylsiloxanes, H201-350 and H201–500, with the large average molecular weights were used as co-base oils, the oil separation values of the resultant titanium complex greases with the polydimethylsiloxane contents of 5%–20% and 5%–15% were obviously less than that prepared without the use of polydimethylsiloxane. Considering that the viscosity of polydimethylsiloxane (H201-350) was close to that of mineral oil, the compatibility between polydimethylsiloxane and mineral oil had effect on the oil separation property of the resultant titanium grease. The evaporation loss values of the titanium greases prepared with or without the use of polydimethylsiloxane were comparable. Polydimethylsiloxane had no obvious impact on the evaporation loss values of the resultant titanium greases because they are difficult to evaporate. 3.2. Friction and wear analysis 3.2.1. Friction coefficient of titanium complex grease on steel/steel surface To compare the effect of molecular weight of polydimethylsiloxane on the anti-friction property of titanium complex grease, the polydimethylsiloxanes, H201-100, H201-350, and H201-500, with the same content of 10% were used as the co-base oils. The friction coefficients of the resultant titanium complex greases were ca. 0.143, 0.098, and 0.115, respectively (Fig. 1). The friction coefficients of the titanium complex greases with the use of the polydimethylsiloxanes, H201-350 and H201-500, as the co-base oils were obviously lower than those of the controlled sample. The use of polydimethylsiloxane (H201-350) as the co-base oil gave the lowest friction coefficient. The structure of polydimethylsiloxane had obvious effect on the anti-friction property
0.20
10% PDMS (100) 10% PDMS (500)
0.18
Friction coefficient
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0% PDMS 5% PDMS (350) 10% PDMS (350) 15% PDMS (350)
0.16 0.14 0.12 0.10 0.08 0
600
1200
1800
2400
3000
3600
Test time/ s Fig. 1. Friction coefficients of titanium complex greases prepared with mineral oil (KN 4010) and polydimethylsiloxane (PDMS) as the mixed base oil. The PDMS contents in the mixed base oil were 0–20 wt%.
of titanium complex grease. Polydimethylsiloxane with large molecular weight (or long chain length) decreased the friction coefficient. Considering that the use of polydimethylsiloxane (H201-350) as a co-base oil exhibited good anti-friction property, the effect of polydimethylsiloxane (H201-350) content on the anti-friction property of titanium grease was also investigated. A minimum friction coefficient of 0.098 was obtained when the polydimethylsiloxane (H201-350) content was 10%. However, when the contents were 5% and 15%, the friction coefficients of the titanium greases increased to 0.114 and 0.136, respectively. The results revealed that both molecular weight and addition content of polydimethylsiloxane had effect on the anti-friction property of the resultant titanium complex grease. 3.2.2. SEM analysis on the worn surface of tested steel ball Because the use of polydimethylsiloxane (H201-350) as a co-base oil endowed the resultant titanium greases with good anti-friction property, the morphologies of wear scars lubricated by these greases at 392 N for 60 min were measured (Fig. 2). At lower polydimethylsiloxane contents of 5% and 10%, the diameters of the wear scars were 0.567 and 0.492 mm, respectively. When the polydimethylsiloxane content was increased to 15%, the diameter of the wear scar increased to 0.627 mm, which was close to that (0.657 mm) of the wear scar caused by using the controlled grease. The results revealed that at a lower content of polydimethylsiloxane (H201-350), the as-prepared titanium complex greases exhibited good anti-wear property as compared to the controlled one, being in agreement with the results obtained by friction coefficient measurement. It was reported that the use of vegetable oil as a co-base oil in titanium complex grease and alkylphenyl diphosphates as additives in lithium complex grease and polyurea grease could effectively improve the lubricating and anti-wear properties of these greases [10,11]. In our present work, the use of polydimethylsiloxane as a co-base oil in titanium complex grease not only improved its lubricating and anti-wear properties but also obviously increased its dropping point as compared with the controlled grease sample. 3.2.3. EDS analysis on the worn surface of tested steel ball Fig. 3 shows the EDS spectra of the worn surfaces of tested steel balls lubricated by the titanium complex grease prepared by using polydimethylsiloxane (H201–350,10%) as the co-base oil and the controlled grease. The elements C, O, Si, Ti, Cr, and Fe, on the worn surfaces were detected. The mole fractions of surface elements are listed in Table 3. It was found that the O and Si contents on the worn surface of the steel ball lubricated by the titanium complex grease prepared by using polydimethylsiloxane and mineral oil as the mixed base oil were larger
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
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Fig. 2. SEM images of wear scars lubricated by different titanium complex greases at 392 N for 60 min. a, titanium complex grease based on mineral oil (KN 4010); b, c, and d, titanium complex greases based on polydimethylsiloxane (PDMS, H201-350) and KN 4010 mixed base oils, PDMS contents were 5 wt%, 10 wt%, and 15 wt%, respectively.
10000
10000
a Fe O Cr C Si Ti
6000
4000
2000
0
b
8000
Intensity
Intensity
8000
Fe O Cr C Si Ti
6000
4000
2000
0
1
2
3
4
5
6
7
8
9
10
11
Energy /keV
0
0
1
2
3
4
5
6
7
8
9
10
11
Energy /keV
Fig. 3. EDS spectra of worn surfaces of tested steel balls lubricated by titanium complex greases: a, titanium complex grease based on KN 4010 and 10% polydimethylsiloxane (H201-350) mixed base oil; b, titanium complex grease based on KN 4010 base oil.
Table 3 Elements on the worn surfaces of the tested steel balls Elements
C O Si Ti Cr Fe
Sample A
Sample B
Mole fractions/%
Mole fraction/%
14.95 35.73 4.06 1.23 0.94 43.09
19.00 24.16 0.73 4.14 1.13 50.84
Note: Samples A and B are the worn surfaces of the steel balls using titanium complex grease with polydimethylsiloxane as the co-base oil and controlled grease as the lubricants.
than those on the worn surface of the steel ball lubricated by the controlled titanium complex grease, respectively. However, the C and Ti contents on the worn surface of the steel ball lubricated by the titanium complex grease based on mixed base oil were less than those lubricated by the controlled titanium complex grease, respectively. The results revealed that polydimethylsiloxane was present between the friction pairs when the titanium complex grease based on the mixed base oil was used as the lubricant. Combining with the friction coefficient and wear scar diameter measurement, it was suggested that when the titanium complex grease based on the mixed base oil was used as the lubricant, a polydimethylsiloxane film probably formed on the friction surface, giving good lubricating property.
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J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
4. Conclusions A new category of titanium complex grease was developed by using titanium tetraisopropanolate, 12-hydroxystearic acid, and benzoic acid as the raw materials of titanium complex thickener and naphthenic mineral oil (KN 4010)/polydimethylsiloxane as the mixed base oil. The titanium complex greases exhibited low friction coefficient, high dropping point, and high anti-wear property as compared with the titanium complex grease prepared without the use of polydimethylsiloxane. The molecular weight of polydimethylsiloxane and the polydimethylsiloxane content in base oil significantly affected the tribological properties of the as-prepared titanium complex greases. According to the EDS analysis, lubricating films of polydimethylsiloxane were probably formed on the surfaces of the friction pairs to give a good lubricating property. Acknowledgements The authors sincerely thank Mr. Shen H. and Ms. Xie J. at Wuxi Hanmei Advanced Materials Co., Ltd. for supporting the grease test. The research was financially supported by the Wuxi 530 Project (20130529010040). References [1] C. Lea, Energy savings through use of advanced biodegradable lubricants, Ind. Lubr. Tribol. 59 (2007) 132–136. [2] W.J. Bartz, Lubricants and the environment, Tribol. Int. 31 (1998) 35–47. [3] W.J. Bartz, Ecotribology: Environmentally acceptable tribological practices, Tribol. Int. 39 (2006) 728–733. [4] E.L. Plumer, Formulation, Characterization and Performance of Aluminum Complex Amino-acid Grease, NLGI Spokesman, 28, 1964 142–145. [5] R.J. Muir, High Performance Calcium Sulphonate Complex Lubricating Grease, NLGI Spokesman, 52, 1988 140–146. [6] W. Macwood, R.J. Muir, Calcium Sulphonate Grease, One Decade Later, NLGI Spokesman, 63, 1998 24–37. [7] D. Liu, M. Zhang, G. Zhao, et al., Tribological behavior of amorphous and crystalline overbased calcium sulfonate as additives in lithium complex grease, Tribol. Lett. 45 (2012) 265–273.
[8] M. Washo, Frictional Characteristics of Greased Rolling Contact Bearings, NLGI Spokesman, 57, 1994 456–462. [9] S. Okamura, M. Tyota, Long Life Urea Grease for High Temperature and High Speed Application, NLGI Spokesman, 56, 1992 94–100. [10] J. Yin, J. Qu, B. Song, et al., Friction and wear behavior of titanium complex grease with admixture base oil, Key Eng. Mater. 419 (2010) 57–60. [11] X. Wu, G. Zhao, J. Liu, et al., Tribological properties of alkylphenyl diphosphates as high-performance antiwear additive in lithium complex grease and polyurea grease for steel/steel contacts at elevated temperature, Ind. Eng. Chem. Res. 53 (2014) 5660–5667. [12] J.E. Martin-Alfonso, G. Moreno, C. Valencia, Influence of soap/polymer concentration ratio on the rheological properties of lithium lubricating greases modified with virgin LDPE, J. Ind. Eng. Chem. 15 (2009) 687–693. [13] A. Kumar, B.D. Mittal, M.P. Singh, et al., Ecofriendly Titanium Complex Grease, NLGI Spokesman, 61, 1997 22–28. [14] A. Kumar, E. Sayanna, A.S. Verma, et al., A New Generation High Performance Titanium Complex Grease, NLGI Spokesman, 58, 1994 25–30. [15] A. Kumar, S.C. Nagar, B.D. Mittal, et al., Titanium Complex Grease for Girth Gear Applications, NLGI Spokesman, 63, 1999 15–19. [16] A. Kumar, S.C. Nagar, K.P. Naithani, et al., Enhancing Further Performance Properties of Titanium Complex Grease, NLGI Spokesman, 62, 1998 20–27. [17] V.J. Gatto, M.A. Grina, Effects of base oil type, oxidation test conditions and phenolic antioxidant structure on the detection and magnitude of hindered phenol/ diphenylamine synergism, Lubr. Eng. 55 (1999) 11–20. [18] L. Yadgarov, V. Petrone, R. Rosentsveig, Tribological studies of rhenium doped fullerene-like MoS2 nanoparticles in boundary, mixed and elasto-hydrodynamic lubrication conditions, Wear 297 (2013) 1103–1110. [19] A. Rothschild, J. Sloan, R. Tenne, Growth of WS2 nanotubes phases, J. Am. Chem. Soc. 122 (2000) 5169–5179. [20] J. Chen, Tribological properties of polytetrafluoroethylene, nano-titanium dioxide, and nano-silicon dioxide as additives in mixed oil-based titanium complex grease, Tribol. Lett. 38 (2010) 217–224. [21] B.K. Sharma, A. Adhvaryu, J.M. Perez, et al., Soybean oil based greases: influence of composition on thermo-oxidative and tribochemical behavior, J. Agric. Food Chem. 53 (2005) 2961–2968. [22] J. Chen, Synthesis, characterization, and tribological behavior of neopentyl polyol ester-based and mixed oil-based titanium complex grease, Tribol. Lett. 40 (2010) 149–154. [23] G. Lai, S. Xing, Synthesis Process and Application of Organic Silicon Products, 2nd ed. Chemical industry press, Beijing, 2010 (in Chinese). [24] B. Helal, B.S. Karadi, Artificial lubrication of joints: Use of silicone oil, Ann. Phys. Med. 9 (1968) 34–40.
Contents lists available at ScienceDirect
Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Article
Impact of polydimethylsiloxanes on physicochemical and tribological properties of naphthenic mineral oil (KN 4010)-based titanium complex grease☆ Jitai Li, Chu Zhai, Hengbo Yin ⁎, Aili Wang, Lingqin Shen Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 15 June 2018 Received in revised form 21 August 2018 Accepted 3 September 2018 Available online 8 September 2018 Keywords: Complexes Surface Polymers Titanium complex grease Polydimethylsiloxane Tribological properties
a b s t r a c t Titanium complex greases were prepared by using naphthenic mineral oil and polydimethylsiloxane as the mixed base oil. The effect of polydimethylsiloxane molecular weight and polydimethylsiloxane content in mixed base oil on the physicochemical and tribological properties of titanium complex greases was investigated. As compared to the sole mineral oil-based titanium complex grease, the use of polydimethylsiloxane (H201-350) as a co-base oil increased the dropping point from 310 to 329 °C, decreased the oil separation from 3.7% to 2.3%, reduced the corrosion extent, and obviously improved the tribological properties. When the mixed oil-based titanium complex grease was used as a lubricant, lubricating films of polydimethylsiloxane were probably formed on the surfaces of friction pairs, giving good lubricating property. © 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.
1. Introduction Lubricating grease plays a significant role in the industrial production to reduce friction energy loss, protect machinery from wear, and prolong the operation life [1–3]. Since 1960s, Al-, Ca-, and Li-complex greases and polyurea greases were investigated and commercially used in the world [4–9], these greases with high dropping points (N230 °C) exhibit good lubrication performance for a long operation time. However, these commercial greases are able to leak, overflow or exhaust from machinery and equipment during the operation at high temperature and pressure, causing high friction and wear and subsequently making damage to the operating machinery and equipment [10]. With the development in the metallurgical, automobile, and aviation industries, increasing numbers of mechanical equipment have worked at high temperature and extreme pressure [11,12]. It is a great challenge to let lubricating greases work under these severe conditions. Kumar et al. [13–16] invented mineral oil-based titanium complex greases and disclosed their preparation methods and tribological properties. Titanium complex greases have several advantages, such as shearing reversibility, good at low and high working temperatures ranging from − 20 °C to 150 °C, anti-wear under extreme pressure, ☆ Supported by the National Natural Science Foundation of China (21506078, 21506082) and China Postdoctoral Science Foundation (2016M591786, 2016M601739). ⁎ Corresponding author. E-mail address: [email protected] (H. Yin).
high corrosion and oxidation resistances, and compatibility with other greases [14,15]. As the evaluation index of working temperature, the dropping point of mineral oil-based titanium complex grease was up to 294 °C, higher than those of Li- and Al-complex greases of 263 °C and 275 °C, respectively [14]. Additives were commonly used in the greases to improve their properties. Antioxidants, such as diphenylamine and phenol, were used to improve the oxidation resistance and extend lubricating life time [17]. Tackifiers, such as polyisobutene and emulsion, were used to improve the adhesion performances. Anti-friction and anti-wear agents, such as WS2, MoS2, nano-SiO2, and TiO2, were used to reduce the friction coefficient, protect rubbing surface, and improve load-bearing capacity [18–20]. However, base oils have great impact on the properties of complex greases. It was reported that vegetable oil-based titanium complex greases exhibited high dropping point around 330 °C, higher than mineral oil-based greases. Unfortunately, due to the poor thermo-oxidative stability of vegetable oil, the vegetable oil-based titanium complex greases cannot work at a high temperature for a long time [13,21,22]. Polydimethylsiloxanes, a series of synthetic oils from organosilicone industry, with superior thermal stability, high oxidation resistance, high waterproofness, and wide working temperature range have been widely used in medical treatment, electromechanical, and coating fields [23]. To the best of our knowledge, the use of polydimethylsiloxane as a lubricant has rarely been reported [24] and the use of polydimethylsiloxane as a co-base oil for the titanium complex grease has not been reported.
https://doi.org/10.1016/j.cjche.2018.09.002 1004-9541/© 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
In the present work, polydimethylsiloxanes with different molecular weights were mixed with naphthenic mineral oil (KN 4010) as mixed base oils for the preparation of titanium complex greases. The effect of polydimethylsiloxane molecular weight and polydimethylsiloxane content in the mixed base oil on the physicochemical and tribological properties of titanium complex greases was investigated. The presence of polydimethylsiloxane significantly improved the tribological properties of titanium complex greases. 2. Experimental 2.1. Materials Polydimethylsiloxanes (H201-100, H201-350, H201-500) were supplied by Jiangsu Hongda New Materials Co. Ltd. Naphthenic mineral oil (KN 4010) and 12-hydroxystearic acid were supplied by Wuxi Hanmei Advanced Materials Co., Ltd. The physical properties of polydimethylsiloxanes and KN 4010 are listed in Table 1. Titanium tetraisopropanolate was purchased from Yangzhou Lida Resin Co., Ltd. Benzoic acid was of reagent grade and was purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used through all the experiments. 2.2. Preparation of titanium complex grease The preparation process of titanium complex grease is illustrated as follows. Naphthenic mineral oil (KN 4010) and polydimethylsiloxane (0%–25 wt% in the mixed base oil) were used as a mixed base oil. The mixed base oil (125 g), 12-hydroxystearic acid (75.7 g), and benzoic acid (30.8 g) were mixed and heated to 90 °C in a 1 L three-necked round-bottom flask equipped with a mechanical stirrer and a condenser. Titanium tetraisopropanolate (143.2 g) was added into the mixture and the saponification reaction was carried out at 150 °C for 8 h. After the saponification reaction, the formed isopropanol was evaporated. And then, mixed base oil (125 g) was added and the temperature was increased to the refining temperature of 210 °C at a heating rate of 20 °C·h−1 and kept at 210 °C for 5 min. Nitrogen was used as shielding
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gas throughout the whole heating process. The as-prepared greases were dumped into a stainless mug and cooled down to the room temperature. A little amount of water (5 ml) was added to the as-prepared titanium complex greases when they were grinded to obtain the titanium complex grease samples. 2.3. Characterization of titanium complex grease Dropping points of greases were determined on an XH–141 dropping point tester according to the grease dropping point test method of GB/T 3498-2008 (similar to ASTM D2265). Penetrations of the greases were determined on an XH–140A grease penetration tester according to the standard of GB/T 269-1991 (similar to ASTM D217). Oil separation was tested using an FDH–4701 steel mesh tester according to the standard of SH/T 0324-1992 (similar to ASTM D6184). Evaporation loss and copper corrosion were characterized according to the standards of GB 7325-1987 and GB 7326-1987 (similar to ASTM D972 and ASTM D4048), respectively. Friction and wear behaviors (tribological properties) were examined on an MS–10JB four-ball tester according to the standard of SH/T 02042004 (similar to ASTM D2266) at a rotating speed of 1450 r·min−1 , a load of 392 N, and working temperature of 75 °C for 60 min. The test steel balls with the diameter of 12.7 mm and the hardness of 60 HRC were made of GCr15 bearing steel. The morphology of the worn surface was observed on a Zeiss Evo–18 scanning electron microscope. The surface elements (C, O, Si, Ti, Cr, and Fe) on the worn surface were measured on an Oxford Max–20 energy disperse spectroscopy at a current intensity of 300 Pa for 90 s. 3. Result and Discussion 3.1. Physicochemical properties of titanium complex greases The penetration degrees, dropping points, copper corrosions, oil separation contents, and oil evaporation contents of the titanium complex greases are listed in Table 2. For the dropping points, it was found that when polydimethylsiloxane (H201-100) with the smallest average
Table 1 Physical properties of mineral oil and polydimethylsiloxanes Reagents
Brands
Viscosity, 20 °C/mm2 ·s−1
Flashing point/°C
Molecular weight
Refractive index
Density/g·ml−1
Mineral oil
KN 4010 H201–100 H201–350 H201–500
335 100 350 500
220 N300 N300 N300
– 6250 13,680 17,520
1.492 1.401 1.403 1.404
0.905 0.969 0.973 0.975
Polydimethyl-siloxanes
Table 2 Properties of titanium complex greases prepared by using polydimethylsiloxane (PDMS) and mineral oil (KN4010) as the mixed base oil Samples.
PDMS
PDMS contents in mixed base oil/wt%
Dropping points/°C
Penetration/mm
Copper corrosion
Oil separation/%
Evaporation losses/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
None H201-100 H201-100 H201-100 H201-100 H201-100 H201-350 H201-350 H201-350 H201-350 H201-350 H201-500 H201-500 H201-500 H201-500 H201-500
0 5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
315 317 317 320 301 279 319 329 321 299 283 325 316 302 287 276
23.0 25.7 26.4 26.9 28.5 29.5 21.3 22.0 25.9 26.3 27.1 18.5 22.2 21.4 24.6 23.5
2a 2a 2a 1b 1b 1b 2a 1b 1b 1b 1b 2a 1b 1b 1b 1b
3.7 4.2 3.9 4.4 4.8 6.3 2.7 2.3 2.5 3.4 5.8 2.1 2.2 3.0 4.9 6.0
3.0 3.0 3.2 3.1 3.2 3.1 3.1 3.3 2.9 3.1 3.0 3.0 2.9 2.8 3.2 3.2
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
molecular weight of 6250 was used as a co-base oil, the as-prepared titanium complex grease had the maximum dropping point of 320 °C at the polydimethylsiloxane content of 15%. When polydimethylsiloxane (H201-350) with the average molecular weight of 13680 was used as a co-base oil, the as-prepared titanium complex grease had the maximum dropping point of 329 °C at the polydimethylsiloxane content of 10%. For polydimethylsiloxane (H201-500) with the largest average molecular weight of 17520, the resultant titanium complex grease had the maximum dropping point of 325 °C at the polydimethylsiloxane content of 5%. To obtain the maximum dropping point, the optimal content of polydimethylsiloxane decreased upon the increase in their average molecular weights. It could be explained as that the molecular weight of polydimethylsiloxane affected its compatibility with the mineral oil (KN 4010), resulting in the change in the dropping points. For the penetration parameter, it was found that the penetration values of the resultant titanium complex greases slightly increased upon increasing the content of polydimethylsiloxane. The penetration value was affected by the molecular weight of polydimethylsiloxane. The effect of polydimethylsiloxanes on the penetration values was roughly in an order of H201-100 N H201-350 N blank N H201-500. The penetration values of the titanium complex greases prepared using the polydimethylsiloxane (H201-100) with the smallest average molecular weight as a co-base oil were larger than those using the polydimethylsiloxanes (H201-350 and H201-500) as co-base oils. The high viscosity decreased the penetration value to some extent. The addition of polydimethylsiloxane effectively improved the anti-corrosion property of titanium complex grease, probably due to the formation of polydimethylsiloxane film on the copper sheet surface, which was certified by the surface element analysis of friction pairs. The oil separation values slightly increased with the increase in polydimethylsiloxane contents. When the polydimethylsiloxane (H201-100) with the smallest average molecular weight was used as a co-base oil, the oil separation values of titanium complex greases were larger than that prepared without the use of polydimethylsiloxane. However, when the polydimethylsiloxanes, H201-350 and H201–500, with the large average molecular weights were used as co-base oils, the oil separation values of the resultant titanium complex greases with the polydimethylsiloxane contents of 5%–20% and 5%–15% were obviously less than that prepared without the use of polydimethylsiloxane. Considering that the viscosity of polydimethylsiloxane (H201-350) was close to that of mineral oil, the compatibility between polydimethylsiloxane and mineral oil had effect on the oil separation property of the resultant titanium grease. The evaporation loss values of the titanium greases prepared with or without the use of polydimethylsiloxane were comparable. Polydimethylsiloxane had no obvious impact on the evaporation loss values of the resultant titanium greases because they are difficult to evaporate. 3.2. Friction and wear analysis 3.2.1. Friction coefficient of titanium complex grease on steel/steel surface To compare the effect of molecular weight of polydimethylsiloxane on the anti-friction property of titanium complex grease, the polydimethylsiloxanes, H201-100, H201-350, and H201-500, with the same content of 10% were used as the co-base oils. The friction coefficients of the resultant titanium complex greases were ca. 0.143, 0.098, and 0.115, respectively (Fig. 1). The friction coefficients of the titanium complex greases with the use of the polydimethylsiloxanes, H201-350 and H201-500, as the co-base oils were obviously lower than those of the controlled sample. The use of polydimethylsiloxane (H201-350) as the co-base oil gave the lowest friction coefficient. The structure of polydimethylsiloxane had obvious effect on the anti-friction property
0.20
10% PDMS (100) 10% PDMS (500)
0.18
Friction coefficient
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0% PDMS 5% PDMS (350) 10% PDMS (350) 15% PDMS (350)
0.16 0.14 0.12 0.10 0.08 0
600
1200
1800
2400
3000
3600
Test time/ s Fig. 1. Friction coefficients of titanium complex greases prepared with mineral oil (KN 4010) and polydimethylsiloxane (PDMS) as the mixed base oil. The PDMS contents in the mixed base oil were 0–20 wt%.
of titanium complex grease. Polydimethylsiloxane with large molecular weight (or long chain length) decreased the friction coefficient. Considering that the use of polydimethylsiloxane (H201-350) as a co-base oil exhibited good anti-friction property, the effect of polydimethylsiloxane (H201-350) content on the anti-friction property of titanium grease was also investigated. A minimum friction coefficient of 0.098 was obtained when the polydimethylsiloxane (H201-350) content was 10%. However, when the contents were 5% and 15%, the friction coefficients of the titanium greases increased to 0.114 and 0.136, respectively. The results revealed that both molecular weight and addition content of polydimethylsiloxane had effect on the anti-friction property of the resultant titanium complex grease. 3.2.2. SEM analysis on the worn surface of tested steel ball Because the use of polydimethylsiloxane (H201-350) as a co-base oil endowed the resultant titanium greases with good anti-friction property, the morphologies of wear scars lubricated by these greases at 392 N for 60 min were measured (Fig. 2). At lower polydimethylsiloxane contents of 5% and 10%, the diameters of the wear scars were 0.567 and 0.492 mm, respectively. When the polydimethylsiloxane content was increased to 15%, the diameter of the wear scar increased to 0.627 mm, which was close to that (0.657 mm) of the wear scar caused by using the controlled grease. The results revealed that at a lower content of polydimethylsiloxane (H201-350), the as-prepared titanium complex greases exhibited good anti-wear property as compared to the controlled one, being in agreement with the results obtained by friction coefficient measurement. It was reported that the use of vegetable oil as a co-base oil in titanium complex grease and alkylphenyl diphosphates as additives in lithium complex grease and polyurea grease could effectively improve the lubricating and anti-wear properties of these greases [10,11]. In our present work, the use of polydimethylsiloxane as a co-base oil in titanium complex grease not only improved its lubricating and anti-wear properties but also obviously increased its dropping point as compared with the controlled grease sample. 3.2.3. EDS analysis on the worn surface of tested steel ball Fig. 3 shows the EDS spectra of the worn surfaces of tested steel balls lubricated by the titanium complex grease prepared by using polydimethylsiloxane (H201–350,10%) as the co-base oil and the controlled grease. The elements C, O, Si, Ti, Cr, and Fe, on the worn surfaces were detected. The mole fractions of surface elements are listed in Table 3. It was found that the O and Si contents on the worn surface of the steel ball lubricated by the titanium complex grease prepared by using polydimethylsiloxane and mineral oil as the mixed base oil were larger
J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
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Fig. 2. SEM images of wear scars lubricated by different titanium complex greases at 392 N for 60 min. a, titanium complex grease based on mineral oil (KN 4010); b, c, and d, titanium complex greases based on polydimethylsiloxane (PDMS, H201-350) and KN 4010 mixed base oils, PDMS contents were 5 wt%, 10 wt%, and 15 wt%, respectively.
10000
10000
a Fe O Cr C Si Ti
6000
4000
2000
0
b
8000
Intensity
Intensity
8000
Fe O Cr C Si Ti
6000
4000
2000
0
1
2
3
4
5
6
7
8
9
10
11
Energy /keV
0
0
1
2
3
4
5
6
7
8
9
10
11
Energy /keV
Fig. 3. EDS spectra of worn surfaces of tested steel balls lubricated by titanium complex greases: a, titanium complex grease based on KN 4010 and 10% polydimethylsiloxane (H201-350) mixed base oil; b, titanium complex grease based on KN 4010 base oil.
Table 3 Elements on the worn surfaces of the tested steel balls Elements
C O Si Ti Cr Fe
Sample A
Sample B
Mole fractions/%
Mole fraction/%
14.95 35.73 4.06 1.23 0.94 43.09
19.00 24.16 0.73 4.14 1.13 50.84
Note: Samples A and B are the worn surfaces of the steel balls using titanium complex grease with polydimethylsiloxane as the co-base oil and controlled grease as the lubricants.
than those on the worn surface of the steel ball lubricated by the controlled titanium complex grease, respectively. However, the C and Ti contents on the worn surface of the steel ball lubricated by the titanium complex grease based on mixed base oil were less than those lubricated by the controlled titanium complex grease, respectively. The results revealed that polydimethylsiloxane was present between the friction pairs when the titanium complex grease based on the mixed base oil was used as the lubricant. Combining with the friction coefficient and wear scar diameter measurement, it was suggested that when the titanium complex grease based on the mixed base oil was used as the lubricant, a polydimethylsiloxane film probably formed on the friction surface, giving good lubricating property.
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J. Li et al. / Chinese Journal of Chemical Engineering 27 (2019) 944–948
4. Conclusions A new category of titanium complex grease was developed by using titanium tetraisopropanolate, 12-hydroxystearic acid, and benzoic acid as the raw materials of titanium complex thickener and naphthenic mineral oil (KN 4010)/polydimethylsiloxane as the mixed base oil. The titanium complex greases exhibited low friction coefficient, high dropping point, and high anti-wear property as compared with the titanium complex grease prepared without the use of polydimethylsiloxane. The molecular weight of polydimethylsiloxane and the polydimethylsiloxane content in base oil significantly affected the tribological properties of the as-prepared titanium complex greases. According to the EDS analysis, lubricating films of polydimethylsiloxane were probably formed on the surfaces of the friction pairs to give a good lubricating property. Acknowledgements The authors sincerely thank Mr. Shen H. and Ms. Xie J. at Wuxi Hanmei Advanced Materials Co., Ltd. for supporting the grease test. The research was financially supported by the Wuxi 530 Project (20130529010040). References [1] C. Lea, Energy savings through use of advanced biodegradable lubricants, Ind. Lubr. Tribol. 59 (2007) 132–136. [2] W.J. Bartz, Lubricants and the environment, Tribol. Int. 31 (1998) 35–47. [3] W.J. Bartz, Ecotribology: Environmentally acceptable tribological practices, Tribol. Int. 39 (2006) 728–733. [4] E.L. Plumer, Formulation, Characterization and Performance of Aluminum Complex Amino-acid Grease, NLGI Spokesman, 28, 1964 142–145. [5] R.J. Muir, High Performance Calcium Sulphonate Complex Lubricating Grease, NLGI Spokesman, 52, 1988 140–146. [6] W. Macwood, R.J. Muir, Calcium Sulphonate Grease, One Decade Later, NLGI Spokesman, 63, 1998 24–37. [7] D. Liu, M. Zhang, G. Zhao, et al., Tribological behavior of amorphous and crystalline overbased calcium sulfonate as additives in lithium complex grease, Tribol. Lett. 45 (2012) 265–273.
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