Lubricant Additives

134 Lubricant Additives: Chemistry and Applications

As mentioned, common metals that can be used to make neutral or basic detergents include
sodium, potassium, magnesium, calcium, and barium. Calcium and magnesium fi nd most extensive
use as lubricant additives, with a preference for calcium due to its lower cost. The use of barium-
derived detergents is being curbed due to concerns for barium’s toxicity. Technically, one can use
metal oxides, hydroxides, and carbonates to manufacture neutral (nonoverbased) detergents; for
nonoverbased detergents, oxides and hydroxides are the preferred bases. Sodium hydroxide, cal-
cium hydroxide, and barium hydroxide are often used for sodium, calcium, and barium detergents.
For magnesium detergents, however, magnesium oxide is the preferred base.
During the synthesis of calcium detergents, overbasing is usually stopped before all the metal
base is converted into calcium carbonate. As a result, the excess base is present as a mixture of
calcium hydroxide and calcium carbonate. The calcium carbonate predominates because, if the
reaction is overblown with carbon dioxide, the amorphous calcium carbonate, which is desired, is
converted into crystalline calcium carbonate. Of low solubility in the overbased system, crystalline
calcium carbonate falls out of solution, and one obtains an oil-insoluble gel-like product. Although
such products are of little use as lubricant additives, they are useful as rheology control agents in
coatings. The challenge is to make them on a consistent basis. Lubrizol supplies such products
derived from alkylbenzenesulfonic acids as its Ircogel® product line. Gelled carboxylates and solid
calcium micellar complexes have also been reported in the patent literature [56–58].
Basic detergents contain reserve base, which is entrained into the detergent in a colloidal form.
The base, such as the carbonate, is believed to be encapsulated by soap molecules. In this arrange-
ment, the polar head group (sulfonate, phenate, or carboxylate) of the soap associates with the car-
bonate, and the hydrocarbon portion of the soap associates with the oil (see Figure 4.11). The base
neutralizes acids that result from oxidation of the fuel and the lubricant and from the oxidation and
thermal decomposition of thermally labile additives.
Some detergents are marketed as neutral or nonoverbased. However, most of them have a small
amount of reserve base present. In other words, they are overbased to some degree. This implies that
no effort was made to overbase them, and their reserve base is due to the presence of the unreacted
base used to make them. For example, commercially available neutral sulfonates have a TBN of 30
or less, and the base is commonly present as a hydroxide such as calcium hydroxide. Conversely,
basic or overbased detergents have a much higher base number, that is, they typically have a TBN
of 200–500, and the base is commonly present as a metal carbonate.
Calcium-based phenate detergents are easier to make than magnesium-based detergents
because alkylphenols are weak acids, and their reaction with magnesium oxide, a weak base,
is not facile. To make the neutral salt, one must react the alkylphenol with a strong base such as

FIGURE 4.11 Micelle structure of detergents.

S

O

O

O

S

O

O

O

Ca

Hydrocarbon group

Sulfonate head group

Neutral calcium sulfonate

CaCO 3

Basic sulfonate inverse micelle structure; size: 100− 150 Å