Charging Ability of Pure Hydrocarbons and Lubricating Oils

Abstract

An experiment using a purpose-built test rig to examine the effect of oil chemistry on the charging ability of various oils revealed that the charge produced depends on two extrinsic properties; the electric field (used to charge the droplet) and the droplet radius. A normalization parameter was developed from these dependencies and used to compare the results from the tests described herein.

Four classes of liquids were tested. These were pure hydrocarbon liquids (hexadecane, decane, and squalane), base oils (both mineral and synthetic stocks), formulated oils (mineral base oil with additives), and commercial oils (fully formulated mineral and synthetic oils, available over-the-counter).

The charging for both pure hydrocarbons and base oils indicates that the charged species have a low mobility and thus are likely to be large, bulky molecules. For this reason, the charging of the base oils is noticeably time dependent and the charge imparted on the oil did not always reach saturation. The charging (rate) of base oils appears to follow the level of impurities (particularly sulfur). Generally, both pure hydrocarbons and base oils tend to charge more when a positive field is applied.

Conductivity values help to explain the measured charge levels. This would be expected as conductivity is related to the ionic species (charge carriers) present. The majority of the formulated and commercial oils tested reached charge saturation for all conditions tested, resulting in a normalized value of around 0.1 pC mm^{− 1} V^{− 1} , comparable with a theoretical calculated value. The formulated and commercial oils that did not reach saturation have conductivities comparable with base oils and therefore behave in a fashion similar to the base oils.

Keywords:

• Additives
• Electrostatic
• Mineral Basestocks
• Lubricants

ACKNOWLEDGMENTS

This work was funded by the U.S. Office of Naval Research (ONR). The authors thank Dave Copp of BP for help and supply of oils to test.

Manuscript received by STLE June 23, 2003

Final manuscript approved January 19, 2004

Review led by Tom Karis