Lube-Tech
such, boron, most commonly in the form of boric acid, has been used as a lubricant modifier as early as the 1880’s [5]. There was a particular increase in the use of borates beginning in the 1940’s with the increased use of ester chemistry and polyol chemistry in lubricants [6].
During this time, the solubility of the boric acid in lubricating compositions was increased via chemical modification through dehydration coupling reactions to esters, polyols and long chain alcohols [7]. While a beneficial step in the use of boron as an additive in mineral based lubricants, these simple borate esters are known to suffer from hydration in the presence of moisture, and often revert to boric acid, which can result in both fallout and increased acidity in lubricants. To combat this hydrolytic stability issue, amide stabilised borate esters were introduced [8]. It has been demonstrated that borates of this structure have a significantly increased hydrolytic stability relative to other borates [9].
In the current ICE lubricant market, borates have found a home as anti-wear agents, extreme pressure modifiers, and anti-corrosion agents. All of these properties are of key interest to EV applications, but the use of borated additives in EV based lubricants has been mixed due to potential concerns of a significant increase in electrical conductivity in the lubricant. Some studies indicate that organoborates may possess very high conductivity [10]. Polyamine based dispersants, and particularly borated polyamine based dispersants are understood to be significant conductivity promoters and have been employed in such manner [11]. In contrast, other work has shown that borates may possibly be employed to reduce conductive deposits in lubricating compositions [12]. The discrepancy in behaviour is most likely due to the ligand composition rather than the presence of boron itself within the lubricant, and may falsely lead to the exclusion of boron based additives from future EV formulations.
28 LUBE MAGAZINE NO.180 APRIL 2024
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.151 page 2
A second additive class of some concern are molybdenum based organometallics. Molybdenum additives, particularly in the form of molybdenum dithiocarbamates (Mo-DTCs) have been uses as excellent friction modifiers in lubricating formulations going back to the 1960’s [13]. Non-sulphur containing oil-soluble molybdate were also entering the lubricant landscape as both molybdate salts of organic acids [14], and as esters of long chain alcohols [15]. As with the borate chemistry, hydrolytic stability of many of these early molybdate ester additives was an issue. Using a similar approach to the stabilised borate chemistry, the incorporation of electron donating nitrogen can increase the hydrolytic stability of the organomolybdate [16]. By introducing the nitrogen in the form of an amide, resistance to hydration is further increased [17].
Both molybdates and borates used in hydrocarbon lubricants have a high degree of polarisability [18]. There is a common understanding that polarisability and conductivity are related and that an increase in polarisability corresponds in an increase in conductivity [19]. This understanding extends to lubricant formulations, where increased concentrations of highly polarisable additives will significantly increase the conductivity of a lubricant [20]. Yet evidence exists showing that contribution of additives such as molybdenum dithiocarbamates (Mo-DTCs) to the overall electrical conductivity of an EV fluid may be low, and in some instance these additives may act to lower the overall electrical conductivity of the system [21].
In this experiment combinations of three friction modifiers, based on molybdenum and boron, are explored to assess their effects on low friction onset, wear, corrosion, and conductivity in an EV style formulation. The borate compound in this study is a hydrolytically stable ester amide based on a naturally sourced C12
ester (B-AMIDE). The two
molybdate additives represent two separate molecular architectures. One molybdate friction modifier
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
