Lubricant Additives

Detergents 131

Not all promoters are effective for all overbasing reactions, and one has to experiment to select
the right promoter system. For low-temperature overbasing (≤100°C), alcohol–water mixtures are
commonly used; for high-temperature overbasing (≥100°C), low-molecular-weight alkylphenols
are used. The structure of the fi nal detergent from the two processes is believed to be different
and hence the performance in certain tests. The role of promoters in the overbasing reaction is not
well understood. One explanation regarding their role is based on their preferential reaction with
the base to form an alkoxide or a phenoxide. This species then transfers the metal to the substrate,
thereby facilitating salt formation and overbasing. The other explanation is based on their acting as
a surfactant and a wetting agent. This improves contact between the base and the substrate, thereby
assisting the reaction to occur. The second explanation is defi nitely more plausible than the fi rst.
However, in high-temperature overbasing reactions, usually carried out under anhydrous condi-
tions, the fi rst explanation may have merit.
Although a number of metals can be used to make neutral salts (soaps), only a fewer metals have
the ability to result in oil-soluble basic detergents. The common metals that can be used for this
purpose include lithium, sodium, and potassium in group I and magnesium, calcium, strontium, and
barium in group II of the periodic table. Aluminum is the only overbasable metal in group III. Over-
based salts of transition metals such as zinc, copper, cadmium, molybdenum, copper, manganese,
cobalt, nickel, and iron, from sulfonic acids, alkylphenols, and naphthenic acids are also reported in
the patent literature [19,50,51]. The ability to overbase relates to a metal’s base strength: the higher
the basic character, the easier it is to overbase. For group I metals, where basic character increases
from lithium to sodium to potassium, it is easier to overbase potassium than lithium. In group II
metals, the basic character increases from magnesium to calcium to strontium to barium; hence, it
is easier to overbase barium than magnesium.

FIGURE 4.7 Synthesis of alkylsalicylic acids and alkenylphosphonic acids.

R

R

R R

R

R 1

R

R

R

R

R 1

R

R R

R

R 1

RR

R

R

R 1

OH

Alkylphenol

R

OH

Potassium alkylsalicylate

COOK

R

OH

Alkylsalicylic acid

COOH

KOH + KCI

CO 2

HCI

+ P 2 S 5

Phosphorus pentasulfide

P 2 S 5

PolyisobutyleneP 2 S 5 adduct

P OH

OH

S

H 2 O ∆

Polyisobutenylphosphonic acid