Lubricant Additives

222 Lubricant Additives: Chemistry and Applications

The thermal stability of triaryl phosphates is considerably superior to that of the trialkyl esters,
which degrade thermally by a mechanism analogous to that of the carboxylic esters (reaction 8.7).

C

H

R CH 2 O

P

R O OR

OR

C CH 2

R

R

O

P

OR

+

HO OR

(8.7)

With respect to hydrolytic stability, aryl phosphate esters are superior to the alkyl esters.
Increasing chain length and degree of branching of the alkyl group leads to considerable improve-
ment in hydrolytic stability. However, the more the substituent is sterically hindered, the more
diffi cult it is to prepare the ester. Alkylaryl phosphates tend to be more susceptible to hydrolysis
than the triaryl or trialkyl esters.
The low-temperature properties of phosphate esters containing one or more alkyl substituents
tend to be reasonably good. Many triaryl phosphates are fairly high-melting point solids, but an
acceptable pour point can be achieved by using a mixture of aryl components. Coal tar fractions,
used to make natural phosphate esters, are already complex mixtures and give esters with satisfac-
tory pour points.
Phosphate esters are very good solvents and are extremely aggressive toward paints and a wide
range of plastics and rubbers. When selecting suitable gasket and seal materials for use with these
esters, careful consideration is required. The solvency power of phosphate esters can be advanta-
geous in that it makes them compatible with most other common additives and enables them to be
used as carriers for other less-soluble additives to generate additive slurry.

8.2.2.2 Phosphites

Phosphites are the main organophosphorus compounds used to control oxidative degradation
of lubricants. They eliminate hydroperoxides and peroxy and alkoxy radicals, retard the dark-
ening of lubricants over time, and also limit photodegradation. In addition to their important
role as antioxidants, phosphites are also found to be useful antiwear additives. Dialkyl hydro-
gen phosphites and diaryl hydrogen phosphites are neutral esters of phosphorus acid. These
materials have two rapid equilibrating forms: the keto form, (RO) 2 P(=O)H, and the acid form,
(RO) 2 P–O–H. Physical measurements indicate that they exist substantially in the keto form,
associated in dimeric or trimeric groupings by hydrogen bonding. Trialkyl phosphites and tria-
ryl phosphites are neutral trivalent phosphorus esters. These materials are clear, mobile liquids
with characteristic odors.

8.2.2.2.1 Chemistry and Manufacture
Phosphites are produced by reaction of phosphorus trichloride with alcohols or phenols as given by

3ROH + PCl 3 + 3NH 3 ⇒ P(OR) 3 + 3NH 4 Cl (8.8)

When mixed alkylaryl phosphites are produced, the reactants phenol and alcohol are added sepa-
rately with the reaction temperature being controlled carefully. High-molecular-weight phosphites
can be produced from transesterifi cation reaction of either alcohols or phenols with trimethyl phos-
phite under catalytic (acidic) conditions.

P(OCH 3 ) 3 + 3ROH ⇒ P(OR) 3 + 3CH 3 OH (8.9)

With acid catalyzed hydrolysis, dialkyl or diaryl hydrogen phosphites can be produced from trialkyl
or triaryl phosphites as shown in reactions 8.10 through 8.12.