Lubricant Additives

136 Lubricant Additives: Chemistry and Applications

All components of a lubricant—base oil, additives, and viscosity modifi er—oxidize because
of their organic nature. Oxidation of the API group I base oils is more facile than that of the group
II and group III base oils, primarily because of the presence of aromatic and sometimes olefi nic
components. These compounds oxidize to form hydroperoxides and radicals [6]. These species are
highly reactive and start the oxidation chain reaction. The result is the oxidative breakdown of all
components of the lubricant to highly oxygenated polar species, which are of low lubricant solubility.
Because of this, these materials, both acidic and neutral, tend to separate on surfaces, thereby

FIGURE 4.13 Synthesis of neutral and basic metal phenates.

O M O

R R R

O M O

R R

OH

Alkylphenol

Base

Stoichiometric

amount

Divalent metal salt of

alkylphenol (soap)

Excess base CO 2

x MCO 3

·

Overbased or basic phenate

M = Ca, Mg

FIGURE 4.14 Synthesis of neutral and basic bridged metal phenates.

Y

OH OH

R

Y

O O

M

R R

Y

O O

M

R R

Divalent metal salt of sulfur- or

methylene-bridged alkylphenol (soap)

Base

Stoichiometric

amount

Sulfur- or methylene-bridged

alkylphenol

Y = S, CH 2

M = Ca, Mg

Excess base CO 2

x MCO 3

·

Overbased or basic bridged phenate