Lubricant Additives

66 Lubricant Additives: Chemistry and Applications

this product was introduced as a plasticizer for cellulose nitrate, but it was not until the 1930s that
patents began to appear claiming improved lubrication when TCP was blended with mineral oil. In
1936, this use was claimed in gear oils [3], but a detailed investigation into their behavior as AW
additives was not published until 1940 [4,5], by which time TCP was already said to be in wide-
spread use. During World War II, extensive research into phosphorus-containing additives took
place in Germany [6,7]. This research was facilitated by the recent availability of test equipment for
assessing wear and load-carrying behavior, for example, the four-ball machine [8]. The results of
the research concluded that for high load-carrying (later known as EP) performance, the molecule
must contain the following:

• A phosphorus atom


• Another active group, for example, Cl– or OH– (for attachment to the metal surface)


• At least one aryl or alkyl group (phosphoric acid was not thought to be active)

RO P OR

O

OR

RO P OH.H 2 NR

O

OR

RO P R

O

R

RO P OH

O

OR

RO P R

O

OR

RO P OR

OR

RO P OH

OR

O P O

O

O P OH

O

OH

O P O

O

O

RR

R

Trialkyl phosphate

Alkyl monoacid phosphate

Triaryl phosphate

Trialkyl phosphite

Dialkyl phosphite

Amine phosphate

Triaryl phosphite

Dialkyl alkyl phosphonate

Alkyl dialkyl phosphinate

Aryl diacid phosphate

R

FIGURE 3.2 The structures of some common phosphorus-containing lubricating oil additives.