Lubricant Additives

Detergents 135

magnesium alkoxide. This reagent can be prepared by reacting magnesium metal with an excess
of highly reactive alcohol such as methanol. However, this method is hazardous because of the
hydrogen gas by-product and costly because of the price of the magnesium metal. Once the neutral
salt or soap formation occurs, the excess alcohol is exchanged for an inert solvent, such as toluene
or mineral oil, before overbasing. Alternatively, one can use a high-temperature overbasing pro-
cedure using a low-molecular-weight alkylphenol as a promoter [59]. In the case of methylene or
sulfur-bridged phenols that are more acidic than regular alkylphenols, the reactivity toward mag-
nesium oxide is not a problem. And these compounds form neutral and overbased magnesium salts
without diffi culty. Neutral and basic calcium phenates from alkylphenols, bridged or unbridged,
are easy to make because calcium hydroxide, being a stronger base, reacts with them readily. Other
acids, that is, alkylsalicylic acids, fatty carboxylic acids, and alkenylphosphonic acids, react with
calcium and magnesium bases without any problem.
The synthetic sequences used to make common types of neutral and carbonate overbased deter-
gents are outlined in Figures 4.12 through 4.17. Sulfonate, salicylate, and carboxylate detergents are
commercially available as calcium and magnesium salts, and phosphonates are available as calcium
salts. Some specialty sulfonates, for example, NA-SUL BSB®, are also available as barium salts.
Phenate detergents are commonly available as calcium salts, and phosphonate detergents are avail-
able as both calcium and barium salts. Basic calcium sulfonates make up ~65% of the total detergent
market, followed by phenates at ~31%.

4.6 TESTING

Detergents are used in engine lubricant formulations to perform two key functions. One is to neu-
tralize the acidic by-products of lubricant oxidation and thermal decomposition, and the other is to
suspend neutral but highly polar-oxygenated species in the bulk lubricant.

FIGURE 4.12 Synthesis of neutral and basic metal sulfonates.

SO 3 H

R

SO 3 Na

R

R

S O MOS

R

O

O

O

O

Divalent metal salt of

alkylbenzenesulfonic acid (soap)

Overbased or basic sulfonate

M = Ca, Mg, Ba

x MCO 3

Natural sodium sulfonate

·

R

S OMOS

R

O

O

O

O

Excess base CO 2

Metal halide

Alkylbenzenesulfonic acid

Base (stoichiometric amount)