Lubricant Additives

74 Lubricant Additives: Chemistry and Applications

Little mention is made in the literature of the use of aryl acid phosphites, and there is no known
oil industry using ethoxylated neutral or acid phosphites. Phosphites are, however, generally unsuit-
able for applications where water contamination is likely in view of their hydrolytic instability, and
ethoxylation, certainly in respect of water-soluble products, would not offer any obvious advantage.

3.3.6 DIALKYL ALKYL PHOSPHONATES

Although these products are isomeric with the dialkyl phosphites (Figure 3.2), they are a distinct
class of materials with different properties. They are claimed as friction modifi ers as well as AW/EP
additives and are prepared by the Arbusov rearrangement in which a trialkyl phosphite is heated
with an alkyl halide, for example, an iodide (reaction 3.19):

P(OR) 32 R I R PO(OR) RI

dialkyl alkyl phosphonate

′ → ′ (3.19)

Commercially available materials range from the dimethyl methyl derivative to products based on
dodecyl phosphite, although the higher-molecular-weight products are likely to be of greatest inter-
est for oil applications. Polyethyleneoxy phosphonates, produced by the reaction of diphosphites
with epoxides, have been claimed as friction modifi ers [61], whereas diaryl hydrogen phosphonates,
such as diphenyl phosphonate, are produced by hydrolysis of the corresponding phosphite with
water.

3.4 THE FUNCTION OF LUBRICITY ADDITIVES

The earliest additives used for improving lubrication performance were known as oiliness additives
and fi lm strength additives. While these descriptions are no longer used, others are now employed.
The current terminology together with typical examples of the chemistries employed is shown in
Table 3.2.

TABLE 3.2

Different types of additives used to improve lubrication performance

Additive Description Performance Mechanism Typical Chemistries

Friction modifi er Reduces friction under
near-boundary
lubrication conditions

Physical adsorption of polar

materials on metal surfaces

Long-chain fatty acids and
esters, sulfurized fatty acids,
molybdenum compounds,
long-chain phosphites, and
phosphonates
Antiwear additive (usually
with mild EP properties)

Reduces wear at low to

medium loads

Reacts chemically with the

metal surface to from a layer

(normally a metal soap) that

reduces frictional wear at

low-medium temperature

and loads

Neutral organic phosphates

and phosphites, zinc

di-alkyldithiophosphates

Extreme-pressure additive,
also known as:
-fi lm strength additive,
-load-carrying additive
-antiscuffi ng additive

Increases the load at

which scuffi ng,

scoring, or seizure

occurs

Reacts chemically with the

metal surface to form a

layer, e.g. as a metal halide

or sulfi de which reduces

frictional wear at high

temperatures/loads

Sulfurized or chlorinated

hydrocarbons, acidic

phosphorus-containing

materials, and mixtures

thereof; some metal soaps,

e.g. of lead, antimony, and

molybdenum