Tribology Online, Vol. 11, No. 5 (2016) Pp. 639-645

Tribology Online, 11, 5 (2016) 639-645. ISSN 1881-2198 DOI 10.2474/trol.11.639

Article

Study into Structural Impact of Novel Calcium Complex Grease Delivering High Temperature Performance

Kazuya Watanabe* and Keiji Tanaka

Showa Shell Sekiyu Co. Ltd. 4052-2 Nakatsu, Aikawa-cho, Aikoh-gun, Kanagawa 243-0303, Japan *Corresponding author: [email protected]

( Manuscript received 30 October 2015; accepted 16 August 2016; published 15 September 2016 ) ( Presented at the International Tribology Conference Tokyo 2015, 16-20 September, 2015 )

We newly developed the prototype calcium complex grease (Cx-S) which delivered a better temperature performance than traditional calcium complex greases and could keep its structure even over 200°C. This was achieved by combining the common calcium complex thickener technology with a new concept. In this report, the bearing life of several grease types was evaluated by ASTM D 1741. It was demonstrated that the bearing life of Cx-S wasn’t as long as urea grease, although it had excellent anti-oxidation performance. Additionally, leakage rate was calculated by the amounts of leaked grease from bearings during the test. Therefore, it was suggested that a physical factor such as shear stability of the grease is likely to cause shorter bearing life. Then, by using behenic acid instead of stearic acid - to improve shear stability of Cx-S - it was found that behenic type calcium complex grease (Cx-B) forms much thicker and stronger fiber micelles than stearic type (Cx-S). As a result, the shear stability of Cx-B was strongly improved and bearing life was extended by 50% compared to Cx-S.

Keywords: calcium complex grease, heat resistance, oxidation stability, leakage, bearing life, behenic acid

As a result of various investigations, we’ve newly 1. Introduction developed a prototype calcium complex grease which got a better temperature performance and could keep its Due to enhanced progress within various industries structure even above 200°C [5]. However, it was found such as automotive, wind turbine, steel and so on, that the bearing life of prototype calcium complex grease requirements are becoming severe year by year. These wasn’t as long as urea grease, although it had excellent days, long life greases under high temperature condition oxidation stability [6]. We evaluated the difference in are highly demanded in the grease market. Therefore, oxidation process of several types of greases during the market share of poly urea greases and lithium complex bearing life test and aimed for extended bearing life of greases being used in high temperature applications are prototype calcium complex grease. recently increasing [1]. On the other hand, traditional calcium complex greases which preferably used in 2. Experimental methods former times are now representing commodity product [2]. But, they did not deliver a satisfying performance in 2.1. Typical properties terms of heat resistance e. g. dropping point and bearing Penetration and dropping point were measured to life under high temperature condition [3]. However, the evaluate properties of greases by using JIS K 2220 7 big advantages of calcium greases are low cost and and JIS K 2220 8, respectively. availability in large quantity as well as they are easy to 2.2. Bearing life test handle. Lithium greases and lithium complex greases Bering life test was carried out on bearing life tester might be more expensive as lithium price recently as defined by ASTM D 1741. Table 1 shows the test increased due to high demands of lithium for batteries [4] conditions. Then, three factors below were measured; and for urea greases, some ingredients of urea thickener  Bearing life; such as aniline, cyclohxyl amine and so on [2]. This is the Each test was carried out three times and the major reason we have made the decision for developing corresponding average was calculated. novel calcium greases.  Carbonyl absorbance ratio;

Table 1 Test conditions of bearing life evaluation (ASTM D 1741) Grease amount, Load, N Rotating speed, Temp., Determination Bearing type Cycle g Radial Axial min−1 °C of life 20 h running Over torque No. 6306 6.0 ±0.1 221 178 3,500 140 /4 h shutdown Temp. increase

It is well known as an indicator of oxidation degradation [7]. It can be calculated from the ratio between the absorption of carbonyl group (>CO) caused by oxidation/degradation of greases (wave number: 1700 ~ 1710 cm−1) and methylene group (-CH2-) derived from its hydrocarbon oscillation (wave number: 720 cm−1) with FT-IR.  Leakage rate; It was calculated via amounts of leaked grease from bearing after the test. 2.3. Roll stability test Shear stability was measured on Roll stability test as defined by ASTM D 1831. Test temperature was room temperature and 100°C. Change rate of penetration Fig. 1 Properties of traditional calcium complex before and after testing was calculated. grease

3. Results Table 2 Decomposition point of calcium soap on 3.1. Traditional calcium complex grease TG-DTA Current calcium complex greases in the market are Ca acetate Ca benzoate using a thickener such as calcium salts composed of two Decomposition point, 330 ~ 350 390 ~ 410 different types of fatty acids (stearic and acetic acids) °C [2]. Figure 1 shows penetration and dropping point of this calcium complex grease as a function of mole ratio higher heat resistance and also was easy to handle. of calcium stearate/acetate. It can be shown that Stearic acid and benzoic acid reacted with calcium penetration and dropping point are varying by mole dioxide in paraffinic base oil, and then heated gradually. ratio of calcium stearate/acetate. This makes the choice But, it was found that the reaction product separated of mole ratio very critical to the final grease itself from base oil due to poor solubility and therefore a performance. However, dropping point shows around complex soap couldn’t be formed. The reason is as 200°C at a maximum, and it has limitation in optimizing follows. the composition of grease [5]. Therefore, we studied to Benzoic acid got a poor solubility in paraffinic base improve the heat resistance of traditional calcium oil at around 100°C, which is the temperature of complex grease below. saponification reaction. However, the melting point of benzoic acid is 122°C which seems to be bit too high as 3.2. Prototype calcium complex grease well. Several components were studied to improve the Basically, calcium complex soap is composed of solubility of benzoic acid in base oil. The components long chain fatty acid and heat resistant component. As needed to act as a grease thickener, to show a low heat resistant component, short chain fatty acid or melting point and to have a high polarity. Therefore, Aromatic carboxylic acid can be used. Generally, it is acetic acid seemed to be an option since it met the needs well known that heat resistance properties of greases and was industrially widely used. such as dropping point are depending on the Consequently, the prototype calcium complex grease decomposition point of the thickener component [2]. As was obtained by combining stearic acid, benzoic acid a result of several studies, it was found that calcium and acetic acid and optimizing the component soap composed of aromatic carboxylic acids and compositions and the process of manufacturing. Figure hydroxide got a high heat resistance due to a benzene 2 shows penetration and dropping point of this grease. It ring. Table 2 shows the decomposition points of calcium was found that this grease - which was generated of acetate or benzoate on TG-DTA. The decomposition three types of acids - became harder with lower point of calcium benzoate was around 400°C which was thickener dosage and improved heat resistance around 60°C higher than the one of calcium acetate. performance than calcium stearate grease and traditional Therefore, it was decided to add calcium benzoate as a calcium complex grease [5]. third component into thickener system, because it got

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Table 3 Composition and properties of greases Methods Cx-S Li Lx U Prototype Lithium Aliphatic Thickener - Lithium Ca complex Complex Diurea Type - Paraffinic mineral oil Base oil Vk40, mm2/s ASTM D445 99.05 Additive (Anti-oxidant) - p, p’ -dioctyldiphenylamine (0.5%) Unworked 272 266 271 279 Penetration ASTM D217 Worked 281 269 279 275 Dropping point °C ASTM D566 >270 187 >270 262

Bearing life of prototype calcium complex grease (Cx-S) is 360 hours which is longer than lithium grease (Li) and lithium complex grease (Lx) but shorter than urea grease (U) [6]. 3.3.2 Carbonyl absorbance ratio Figure 4 shows the change of carbonyl absorbance ratio as a function of test duration. The higher value indicates that it is more oxidized. Lithium grease (Li) and lithium complex grease (Lx) were oxidized early in the test, while the prototype calcium complex grease (Cx-S) was hardly oxidized. 3.3.3 Leakage rate Fig. 2 Properties of prototype calcium complex Figure 5 shows the change of leakage rate as a grease compared to traditional calcium type function of test duration. Almost all amounts of lithium greases

3.3. Bearing life test Table 3 shows the composition and properties of several types of greases. In this report, four types of greases, prototype calcium complex grease (Cx-S), lithium grease (Li), lithium complex grease (Lx), and aliphatic di-urea grease (U) were prepared: all these greases were composed of each type of thickener, paraffinic mineral oil and anti-oxidant. The penetration of all greases is NLGI grade No.2. Also, prototype calcium complex grease (Cx-S), lithium complex grease (Li) and urea grease (U) get much higher heat resistance than lithium grease (Li). Fig. 4 Changes of carbonyl absorbance ratio 3.3.1 Bearing life First, Fig. 3 shows the result of bearing life test.

Fig. 3 Bearing life of four types of greases Fig. 5 Changes of leakage rate

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grease (Li) and prototype calcium complex grease (Cx-S) leaked from bearing just when reaching end of bearing life. As for lithium grease (Li), the leakage rate was higher due to the thickener structure which was destroyed by oxidative degradation and couldn’t keep base oil. But, as for prototype calcium complex grease (Cx-S), one challenging question remained: Why did almost all amounts of this grease leak from bearing, although hardly oxidized?. 3.4. Process until reaching bearing life Figure 6 shows the process until reaching bearing life [2]. It is well known that the process until end of bearing life is determined by two factors: a chemical factor influencing oxidation and a physical factor impacting Fig. 8 Penetration change of prototype mechanical shear performance [8]. Generally both - calcium complex grease before chemical and physical - factors influence bearing life of and after roll stability test greases. However, as for prototype calcium complex grease (Cx-S), it was found that softening of grease under (Fig. 8). It doesn’t mean that thickener structure is high temperature condition led mainly to shorter bearing destroyed by heat and thickener micelles are easier to life as follows. clump under high temperature, which means softening of Then, shear stability was evaluated by roll stability this grease. As a result, this grease led to leakage which test (ASTM D 1831). Figure 7 shows change rate of reduced the bearing life performance. Therefore, it is penetration at room temperature and at 100°C. It had suggested that the key point is to prevent clumping of good shear stability at room temperature, but got much thickener micelle. softer at 100°C. However, this soft grease became hard again when being homogenized with three roller mill 3.5. Improvement of shear stability and bearing life Basically, it is proposed that within the thickener the high polarity hydrophilic functions cause the clumping effect. Therefore, there might be two solutions to prevent clumping. One is by adding dispersant additive to disperse thickener into base oil. The other one is by using longer chain fatty acid such as behenic acid for changing polarity in thickener structure. Figure 9 shows the change rate of penetration depending on three greases. Grease1 is prototype calcium complex grease (Cx-S). Grease2 with additional dispersant additive, and Grease3 is similar to Grease1 but with behenic acid as oil-soluble component (Cx-B). It was found that shear Fig. 6 Process until end of bearing life stability at 100°C was strongly improved especially when adding behenic acid. Furthermore, as shown in Fig. 10, bearing life of behenic type calcium complex grease (Cx-B) was extended by 50% compared to

Fig. 7 Change rate of penetration of prototype calcium complex Fig. 9 Shear stability comparison of grease after roll stability test three greases

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fiber micelle with Transmission Electron Microscope (TEM). Fiber formations of stearic type (Cx-S) look much thinner, while behenic type (Cx-B) looks much thicker and stronger. Observations can be explained as follows. Figures 13,14 are showing the model of the fiber formation of stearic type calcium complex grease (Cx-S). White one representing hydrogen, black carbon, red oxygen, and

Fig. 10 Bearing life of behenic type Ca complex grease (Cx-B) compared to prototype Ca complex grease (Cx-S) and urea grease prototype calcium complex grease (Cx-S) and delivered nearly same performance as urea grease [9].

4. Discussion Fig. 13 Chemical compounds which compose Figures 11,12 show fiber formations of two greases stearic type calcium complex grease (Cx-S) and the circle portions of these figures show a typical (a) Acetic calcium mono-stearate (b) Acetic calcium mono-benzoate (c) Calcium di-acetate

Fig. 11 Fiber formation of stearic type calcium complex grease (Cx-S)

Fig. 14 Model of fiber formation of stearic type calcium complex grease (Cx-S) (a) Unit micelles Fig. 12 Fiber formation of behenic type (b) Single fiber micelle calcium complex grease (Cx-B) (c) Bundle of 3 to 4 fiber micelles

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green calcium in that molecular model (Figs. 13,14). In this report, XRD (X-Ray Diffraction) and Solid-state NMR (Nuclear Magnetic Resonance) techniques were used to analyze the chemical structure of calcium complex soap. General methods of structural analysis such as GC (Gas Chromatography), LC (Liquid Chromatography) and Liquid-state NMR can’t be used because calcium soaps are very slightly soluble in typical solvents and not easily gasify. In addition, novel calcium complex soap is complex in structure because it is a mixture of several calcium soaps in which calcium hydroxide reacts with three kinds of acids to form, as described above. Therefore, XRD and especially Solid-state NMR-ROSY (Relaxation Ordered Spectroscopy) method were selected. On XRD, it can be guessed whether a mixture which composes novel calcium complex soap is a physical or chemical one by Fig. 15 Chemical compounds which compose the difference in the crystalline of each components of behenic type calcium complex grease the mixture. And, Solid-state NMR-ROSY is a (Cx-B) technique to separate the overlapping 13C chemical shift (a) Acetic calcium mono-behenate spectra of solid mixtures via an inverse Laplace (b) Acetic calcium mono-benzoate transform by using the difference in 1H longitudinal (c) Calcium di-acetate magnetization relaxation time of each components of the mixture (HT1) [10]. As a result, it was suggested that it was mainly composed of three compounds, acetic calcium mono- stearate (Fig. 13(a)), acetic calcium mono-benzoate (Fig. 13(b)), and calcium di-acetate (Fig. 13(c)), which were combinations derived from the thickener formulation of prototype calcium complex grease (Cx-S). And it was guessed that these compounds would be coordinating in a manner to represent the unit micelle, and then the fiber micelle would be formed in a line, as it can be seen from Fig. 14(a). The diameter of a single fiber micelle can be calculated from molecular length of these compounds. The diameter of a single fiber micelle of stearic type calcium complex grease (Cx-S) is probably Fig. 16 Model of fiber formation of behenic type around 5nm as shown in Fig. 14(b), since molecular calcium complex grease (Cx-B) length of acetic calcium mono-stearate is around 2nm (a) Single fiber micelle which is calculated from its intermolecular distance. As (b) Bundle of 7 to 9 fiber micelles a result, as shown in Fig. 14(c), 3 to 4 fiber micelles are likely to bundle together with a joined diameter of 6 to 7 nm which is measured from a typical fiber micelle in 5. Conclusions Fig. 11. It was demonstrated that the prototype calcium On the other hand, Figs. 15,16 show the model of complex grease (Cx-S) has not only excellent heat the fiber formation of behenic type calcium complex resistance properties such as enhanced dropping point grease (Cx-B), which contains acetic calcium mono- and oxidation stability, but also high performance in bbehenate, as it can be seen from Fig. 15(a). Since it has other grease properties. However, bearing life of a longer carbon backbone chain than stearate, this single prototype calcium complex grease (Cx-S) wasn’t as long fiber micelle got a better higher oil-solubility and as urea grease. Finally, this report demonstrates strongly stronger van der Waals’ force between the fibers as it the difference of oxidation process depending on several can be seen from Fig. 16(a). This prevents clumping as types of greases during a bearing life test. The results discussed above. As shown in Fig. 16(b), 7 to 9 fiber are summarized below; micelles are likely to bundle together with a joined  Bearing life of lithium grease (Li) and lithium diameter of 20 to 30 nm which is measured from a complex grease (Lx) is shorter than prototype typical fiber micelle in Fig. 12. That’s why this fiber is calcium complex grease (Cx-S) and aliphatic likely to be thicker and stronger. As a result, shear di-urea grease (U). stability and bearing life have been highly improved.  As for lithium grease (Li) and lithium complex

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