Lubricant Additives

130 Lubricant Additives: Chemistry and Applications

alkenylphosphonic acids and partially hydrolyzed (contain residual sulfur) alkenylthiophosphonic
acids. The reactions to synthesize alkylsalicylic acids and alkenylphosphonic and thiophosphonic
acids are shown in Figure 4.7.
Recently, the development of basic detergents that are not derived from organic acids has been
reported [47,48]. Substrates that can be overbased include organic amines and polyamines, ethers,
and organic sulfi des (sulfurized olefi ns). Only alkali metal–derived overbased materials have
been reported.

4.5 SYNTHESIS OF NEUTRAL AND BASIC DETERGENTS

To make detergents, the organic acids are reacted with a metal base, such as a metal oxide or a
metal hydroxide. In general, the reaction between the organic acid and the inorganic base is not
good because of poor contact between the two reactants. A number of compounds, called pro-
moters, are used to facilitate salt formation and the subsequent carbonation or a related reaction.
Common promoters include ammonium hydroxide; low-molecular-weight carboxylic acids, such as
formic acid and acetic acid; low-molecular-weight alkylphenols; and other polar compounds, such
as nitroalkanes and imidazolines. A comprehensive list of such agents is provided elsewhere [49].
Most of these reagents are used in combination with water, except for high-temperature overbasing
reactions where water will not stay in. In such cases, alcohols, such as 2-ethylhexanol or iso-octyl
alcohol, and alkylphenols that have high boiling points are used. When water is present as part of
the promoter system, it either is added or results from the neutralization reaction. The promoters
are surfactants, that is, they contain a hydrophilic moiety, such as a hydroxyl group or a carboxylic
acid functionality and a reasonably sized alkyl group to impart a somewhat hydrophobic character
to the molecule.

FIGURE 4.6 Synthesis of alkylphenol substrates.

R

R

R

OH

OH

Alkylphenol

OH

R

OH

OH

R R

OH OH

Phenol

+ Olefin

Acid

Alkylphenol

(S)x

Sulfur-coupled alkylphenol or phenol sulfide

CH 2 O

S or SCI 2

Methylene-coupled

alkylphenol

R = alkyl group, typically C 6 −C 12

X = 1− 3