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.
