Tang, T., Devlin, M., Mathur, N., Henly, T. et al., "Lubricants for (Hybrid) Electric Transmissions," SAE Int. J. Fuels Lubr. 6(2):289-294, 2013, doi:10.4271/2013-01-0298.
In electric or hybrid electric transmissions, the transmission fluids can be in contact with the parts of the electric motors, for example, electrical windings in the stators in order to efficiently cool the electric motors and to insulate the electrical parts to prevent a short circuit of the electric motors. The transmission fluids must therefore have low electrical conductivities [1,2,3,4,5,6,7,8,9]. Transmission fluids contain dispersants, which can be reaction products of hydrocarbyl substituted carboxylic acids or anhydrides and amines. These dispersants can be further post-treated with boron and phosphorus compounds to improve friction and anti-wear properties. Certain dispersants, which have nitrogen content up to 10,000 ppm by weight, and boron plus phosphorus to nitrogen ((B+P)/N) weight ratios of from 0.1 to about 0.8 : 1.0, were found to be effective to provide low electrical conductivities less than 1,700 pS/m . Metal detergents provide good anti-rust performance, but increase the finished fluid electrical conductivity. A balanced dispersant/detergent additive system was found to provide a good anti-rust performance and a low electrical conductivity. Through extensive fluid engineering, a (hybrid) electric transmission fluid consisting of such an optimized dispersant/detergent additive system has been developed, which shows low electrical conductivities at various temperatures even upon aging, in addition to all other critical features, for example excellent antiwear/extreme pressure performance and anti-oxidation durability.