Analytical Instrumentation
13
improved dropping point [1]. This property of increased dropping point is shown in Table 2, where the lithium grease containing 4 % wt. WS2 was observed to have the highest dropping point [1]. The antiwear performance was evaluated using steel-steel and copper-copper fraction pairs lubricated by each grease [1]. A scanning electron microscope was then used to examine the surface and the wear scar on the steel and copper plates [1]. Table 2 illustrates the physical and chemical properties of the tested greases and clearly illustrates an amplifi ed dropping point as the percent weight of the tungsten disulfi de increased in the IL-PANI/WS2 grease [1]. Also included in this table are contact resistance and volume resistivity. Contact resistance describes the resistance occurring when two conductors contact each other [13]. Grease is only slightly conductive, and a depressed contact resistance shows heightened electrical conductivity [1]. This trend is expected and was sought after when choosing tungsten disulfi de in this experiment and is shown in the decreased contact resistance as tungsten disulfi de percent weight increased in the grease composite [1].
Figure 2 Coeffi cient of Friction vs. Grease Type. Adaptation from [12]
other purposes due to the multipurpose nature of these greases [11]. Therefore, different dropping points correspond to different uses, and higher dropping points are sought after for use in engine lubrication due to an engine’s typically high operating temperature (where coolant reaches approximately 90.6-104.4 degrees C) [4].
Research has shown that specifi c nanoparticle additives yield improved dropping point temperatures. This is seen in the research of N. M. Ramli et al., which studied the synergistic effect of molybdenum disulfi de (MoS2) and butylated hydroxytoluene (BHT) in lithium complex grease [12]. The greases tested included two industrial greases, denoted Industry K and Industry S, which were purchased and two lithium complex greases, which were produced with different weight percentages of MoS2 and BHT. The fi rst grease, denoted LCG 01/18, contained only the BHT additive, with a weight percentage of 0.13% [12]. The second lithium complex grease produced contained both BHT and MoS2 and was denoted LCG 01/5 [12]. Both additives were held at 0.13% weight, meaning the grease contained 0.13% wt BHT and 0.13 % wt MoS2 [12]. These greases were then tested for their tribological properties, including dropping point, worked penetration, and oil separation according to the ASTM standards D556, D217 and D6184, respectively [12]. The results of these tests are shown below in Table 1 and a vast improvement in dropping point in the nano- additive greases with respect to the Industry S grease can be seen [12]. However, the Industry K grease beat out all the other tested greases when only considering the dropping point.
Although the Industry K grease was observed to have the highest dropping point, the improvement in dropping point from Industry S grease to the two lithium complex greases is large and is greatest in the LCG 1/5 grease. It was additionally found that the LCG 1/18 experienced the lowest coeffi cient of friction when tested using the four-ball test, which examines the wear scar on a surface lubricated by the grease being tested. The result of the test is shown in Figure 2 below, which plots the coeffi cient of friction versus grease type [12].
Due to the observed reduction in the coeffi cient of friction, and heightened dropping point in the LCG 1/18 grease, it was concluded that the two additives (MoS2 and BHT) have a synergetic effect and vastly improve all tribological parameters tested, specifi cally in terms of dropping point [12]. This fi nding shows that these two additives, when included in the same grease thickener, can improve chemical and physical properties more effectively than these additives can achieve alone. Thus, the MoS2 additive independently improves the dropping point, as shown by LCG 1/18, as well as synergistically with BHT, as shown by LCG 1/5.
These additives are a preliminary step to improving both the tribological characteristics of lithium complex grease and its effi ciency in lubricating mechanical systems in automobiles.
Similarly, research into tungsten disulfi de additive in lithium complex grease also yielded improved dropping point. Tungsten disulfi de (WS2) is a layered solid lubricant often used as a solid or fl uid lubricant and is preferable over molybdenum disulfi de and graphene due to its better conductivity [1]. Research by Yanqui et al. investigates the use of ionic liquid-polyaniline/
tungsten disulfi de (IL-PANI/WS2) composite in lithium complex grease. The IL-PANI/WS2 additive was tested against lithium grease containing only one of the additives (grease containing only IL-PANI and a separate grease containing only WS2) and tribological characteristics of the different greases were compared [1]. It was determined that the greases containing both additives (IL-PANI/WS2) exhibited better antiwear performance, electrical conductivity performance, and an
which grease retains its structure and quality. Knowing this temperature is important in deciding when a certain grease can be applied and utilized in a mechanical component system, such as an internal combustion engine. While not applicable for ICEs currently, by integrating additives such as tungsten disulfi de and carbon nanofi bers, greases with low dropping points containing improved antiwear or anticorrosion properties may have future applications. Additionally, additives placed in greases that are
WWW.PETRO-ONLINE.COM
This observed increase in dropping point proportional to percent weight of WS2 is explained by both tungsten disulfi de and PANI having large specifi c surface areas [1]. The increased surface area hinders the fl ow of liquid molecules and thereby delays the liquid from dropping out of the grease. This synergetic effect of WS2 and PANI represents a viable option for additives to improve dropping point while simultaneously improving additional tribological properties, such as viscosity, antiwear, and corrosion.
Simply, the dropping point is the maximum temperature at
Figure -3
Dr. Raj Shah, explaining the dropping point and other grease tests during the NLGI training course held at Koehler Instrument Company’s HQ in NY
Table – 2 Formulated and Industrial Grease Characterization. Adapted from [1]
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76
