Lubricant Additives

224 Lubricant Additives: Chemistry and Applications

8.2.2.2.3 Applications and Performance Characteristics
Dialkyl (or diaryl) hydrogen phosphites, besides being excellent antiwear agents, are considered the
most potent form of phosphorus, suited to high-torque, low-speed operations. This is the area where
antiwear processes are taken to the extreme and is one of the most important sections of the EP per-
formance spectrum. Sulfur can be quite incapable of giving protection under such conditions. Only
a phosphorus source, if active enough and in suffi cient concentration, can help here. Conversely,
phosphorus components are of little use in high-speed and shock operations where sulfur compo-
nents can be excellent. Dialkyl or diaryl phosphites are also potent antioxidants.
With dialkyl phosphites, it has been reported that oxidation produces a phosphate anion, which
tends to act as a bridging ligand to form an oligomeric iron (III) complex, that is, an iron oxide
complex resembling Structure C.

R OP O

O

OH

Fe O

O

OH

STRUCTURE C

However, there is also a weak, high-viscosity, nonsolid fi lm that increases the overall thickness
of the total fi lm at high speeds [24,31].
Dialkyl phosphites are widely used in gear oils, automatic transmission fl uids (ATF), and many
other applications. Spiro bicyclodiphosphites are also reported to be used in continuously variable
transmission fl uids [32] (Structure D).

O

O

P

O

P

O

R O O R 1

STRUCTURE D

8.2.2.3 Dialkyl Alkyl Phosphonates

Dialkyl alkyl phosphonates [R–P(=O)(OR) 2 ] are stable organic phosphorus compounds that are
miscible with ether, alcohol, and most organic solvents. Besides being used as additives in sol-
vents and low-temperature hydraulic fl uids, they can also be used in heavy metal extraction, solvent
separation, and as preignition additives to gasoline, antifoam agents, plasticizers, and stabilizers.
Dialkyl alkyl phosphonates are prepared from either dialkyl hydrogen phosphites or trialkyl phos-
phites as described in reactions (Michaelis–Arbuzov reaction).

(RO) 3 P + R′X ⇒ (RO) 2 P(=O)R′ + RX (8.14)

(RO) 2 P(=O)H + R′OH + CCl 4 ⇒ (RO) 2 P(=O)R′ + H 2 O (8.15)

(RO) 2 P(=O)H + NaOH ⇒ (RO) 2 P∙O∙Na + R′X ⇒ (RO) 2 P(=O)R′ + NaX (8.16)

In principle, the thermal isomerization of all phosphites to phosphonates can be carried out. The
stability of these compounds varies greatly; however, depending on the nature of the R group, other
products may be formed during heating. For R = methyl, complete conversion occurs at 200°C in
18 h, but for R = butyl, the compound is stable at 223°C. It is thought by some that isomerization of
phosphites may be possible only if traces of phosphonate are already present as an impurity [33].

(RO) 3 P + Heat ⇒ (RO) 2 P(=O)R (8.17)