TS-141
Relationship between the Chemistry of Overbased Detergents and of Dispersants used in Marine Lubricants, and the Neutralization Ability Measured by NAMO Technique
Jean-Philippe ROMAN*
ABSTRACT
The method NAMO (Neutralization Ability of Marine Oils) yields to determine the neutralization speed in thin film, of marine lubricants that contain overbased additives. In order to understand the effect of the chemistry of such additives on the neutralization process, single overbased detergents, e,g. sulfonates, phenates and salicylates, have been studied. In addition, the effects of base oils used and the effect of dispersants used to formulate the marine lubricants have also been studied.
It has been shown that from one type of detergent to another, in the same BN range, significant variations of neutralization speed exist. It also has been shown that the soap content, the nature of the alkylate and the making process are contributing to neutralization speed variations. It has finally been shown that neutralization speed can increase more than two times when changing the nature of the base oil or when combining a bis-succinimide dispersant with the detergent.
Key Words: Neutralization, Overbased, Sulfonate, Phenate, Salicylate, Bis-succinimide, Base Number, NAMO
1. INTRODUCTION
The Marine Lubricants used in low-speed and medium-speed diesel engines generally contain a major part of overbased additives. Their role, for a part, is to neutralize the sulfuric acid generated during the combustion process by means of their alkalinity reserve.
The sulfuric acid is produced from the sulfur contained in the Heavy Fuel Oil (HFO). It is able to condense into the engine if the temperature and the pressure conditions correspond to the conditions of this phenomenon. It finally leads to a corrosive wear of major parts of the engine such as the liners, when reaching the cylinder wall through the oil film.
Various models have been proposed to explain the neutralization reaction between the acid and the base species. For Hosonuma [1] and for Van Helden [2], the process depends strongly on the physical aspect, such as the particle size or the transportation of the acid in the oil film. These assumptions have been later on supported by Davis [3] who enhanced the fact that, afier reacting with a portion of the available basicity, some residual acid could co-exist with the remaining base species. More recently, when using the new experimental NAMO technique [4], which applies a thin film of lubricant, the author has revealed that the neutralization process actually takes place in three distinct steps [5]. A mechanism based on the physical formation of colloidal particles in the oil film has been proposed.
This mechanism supports the fact that the duration of the neutralization depends on the probability of contact between overbased particles and acid droplets. This probability of contact depends for a part on the size of the particles and of the droplets, which are controlled by the nature of the detergents and dispersants. NAMO experiments conducted with full formulated marine lubricants have demonstrated that some detergent chemistries are much more efficient than others to quickly neutralize the acid in the oil film of the engine. This has been confirmed by Wu [6] who has shown that the shrinking of acid droplets in lubricating oil depends on the acid nature, on the acid concentration and on the oil formulation.
On one side it can be seen that much data from the literature focuses on the fact that detergents and dispersants are the most influent parameters of the neutralization reaction. On another side marine engine users are still facing problems of corrosive wear even if engine builders have increased the operating temperature of engines. This supports the fact that improvements in overbased additive chemistry or oil formulation are still a demand of the market. As a consequence it is still necessary to better understand the effect of surfactant chemistry on the neutralization process.
As NAMO method has shown its efficiency and its accuracy to discriminate the marine lubricants in terms of neutralization speed, it has been used for the present study. Experiments have been conducted with single additives of sulfonate, phenate and salicylate types and dispersants have been combined with sulfonates in order to determine and to compare their neutralization speeds. In some cases the base oils used to blend the additives have been changed in order to see their effect.
* Marine Lubricants R&D Project Manager
Centre de Recherche ELF Solaize, BP22 69360 SOLAIZE - FRANCE
