Lubricant Additives

132 Lubricant Additives: Chemistry and Applications

Various detergents derived from metal anions other than hydroxide and carbonate are reported
in the patent literature. The anions include sulfi tes, sulfates, thiosulfates, borates, and phosphates
[32,49,52–55]. These detergents are obtained either from the carbonate detergent by displacing the
carbonate anion with the alternative anion or by using the anion precursor during overbasing. For
example, one can obtain metal sulfi te overbased detergent either by blowing sulfur dioxide during
overbasing or by displacing carbon dioxide in a carbonate detergent with sulfur dioxide. The result-
ing metal sulfi te detergent can be oxidized to a sulfate detergent by using an oxygen source, such as
oxygen gas or peroxide, or to a thiosulfate detergent by reacting it with elemental sulfur [49,52,53].
Borate and phosphate overbased compositions can be made using boric acid or phosphoric acid
during the reaction [54,55].
Common commercial detergents are derived from calcium, magnesium, sodium, and barium.
The metals are listed in order of preference. As mentioned, neutral detergents are made by react-
ing the acid substrate with a stoichiometric amount of the metal base, and overbased detergents are
made by reacting the substrate with an excess amount of base in the presence of carbon dioxide. To
make calcium and magnesium salts from natural sulfonic acids and alkylsalicylic acids, one must
convert commercially available alkali metal (sodium and potassium) salts (see Figures 4.5 and 4.7)
into free acids by reacting them with a mineral acid and then reacting the acids with magnesium
oxide or calcium hydroxide. Alternatively, alkali metal salts can be converted directly into magne-
sium and calcium salts through a double-decomposition reaction with a metal halide, as shown in
Figure 4.8. To make the natural sulfonate detergent, one must react the mahogany acid soap with
a metal halide such as calcium chloride. The reaction converts the sodium sulfonate soap into cal-
cium sulfonate, which can be overbased if desired. Because of the extensive branching, petroleum-
derived sulfonates have better oil solubility than synthetic sulfonates of similar molecular weight.
Figure 4.9 presents the idealized structures of neutral detergents.
To make overbased detergents, one can use either a two-step process or a one-step process.
Generally, the one-step process is preferred over the two-step process. In the two-step process,
the neutral salt or the soap is made fi rst, which is subsequently overbased. In the one-step pro-
cess, the excess metal base is charged to the reaction; once the neutral salt formation is complete,
carbon dioxide blowing (carbonation) of the reaction is initiated. When carbon dioxide uptake
stops, the reaction is considered complete and it is worked up to isolate the product. Figure 4.10
summarizes the two processes. For making overbased natural sulfonates and alkylsalicylates,
one can double-decompose alkali metal salts in situ by reacting with a metal halide and overbasing.
The alkali metal halide by-product need not be removed until the overbasing is complete. It comes
out during the fi nal fi ltration, which is employed to remove any unreacted excess base and other
particulate materials.

FIGURE 4.8 Double-decomposition reaction.

R

2

SO 3 Na

R

SO 3

+ CaCl 2

2

Ca + 2NaCl

R

OH OH

2

CO 2 K

R

CO 2

+ CaCl 2

2

Ca + 2KCl