Acid Neutralization Rates—Why Total Base Number Doesn’t Tell the Whole Story: Understanding the Neutralization of Organic Acid in Engine Oils

The acidification of lubricating oils during engine operation, and the subsequent additive neutralization, is an important challenge for Original Equipment Manufacturers and end-users. Often the decline in Total Base Number (TBN) and increase in Total Acid Number (TAN) is measured during engine operation as an indication of the oil’s condition and lifetime. This is clearly an oversimplification given that no consideration is given to the type of acid, how corrosive it is, or the type of base and how effective it is at neutralizing. Acids can be broadly categorized into mineral acids such as sulfuric/nitric and organic acids such as acetic. Traditionally, research has focused on understanding the effects of mineral acids such as sulfuric, which can be formed during the combustion of sulfur-containing fuel. However, emissions legislation has driven a reduction in sulfur levels, and there has been an increase in the use of biofuels, such as methanol and ethanol, which typically oxidase to form corrosive short-chain organic acids. Understanding the effects of organic acids and how these can be controlled by lubricant additives is of growing importance. This work explores how the presence of such acids can be controlled by lubricant additives through appropriate control of neutralization rates. To achieve this, stopped-flow Fourier-transform infrared (FT-IR) Spectroscopy and Small-Angle Neutron Scattering (SANS) have been used to understand the reaction between an overbased detergent and organic acids. Overbased detergent particle surface area-to-volume ratio is shown to be more important than TBN for acid neutralization ability, and evidence that surfactant shell-type affects neutralization rate is communicated (phenate > salicylate > sulfonate). Calcium is more effective than magnesium at oil phase acid neutralization (due to basicity), and reactions are shown to occur on the metal particle surface, rather than the core.