Lubricant Additives

98 Lubricant Additives: Chemistry and Applications

3.5.4 NEUTRAL ALKYL AND ARYL PHOSPHITES

3.5.4.1 Use as Antiwear/Extreme-Pressure Additives

The earliest known reference to the evaluation of phosphites as AW/EP additive is in a 1950 paper
by Davey [75]. As a result of these investigations, which also included a comparison with phosphates
and the effect of incorporating chlorine into the phosphate/phosphite molecule, it was found that

• Phosphites have superior EP properties to the phosphates, and long alkyl chains are more
  effective than aryl groups.


• Evaluation of TBP and TXP revealed similar optimum concentrations of between 1 and 2%
  as were found in the previous study with TCP [5].


• Polar compounds such as acids or esters improve the lubricating (AW) properties of phos-
  phites and phosphates by being strongly adsorbed on to the surface.


• The incorporation of chlorine or sulfur into the molecule (or the addition of small amounts
  of free sulfur) improves the EP properties. Chlorine is more effective when part of an alkyl
  residue, and when sulfur is added to a P/Cl compound (e.g., a chlorinated phosphite), the EP
  properties are further improved.

Following the study by Davey, a number of patents appeared claiming the use of phosphites
in lubricant applications [121–124], but it was not until 1960 that a further detailed study of
the behavior of phosphites, this time by Sanin et al. [125], was published. The study emphasized
the correlation between structure and activity, and the short-chain derivatives were found to be the
most active.
In 1993, Ohmuri and Kawamura [126] carried out fundamental studies into the mechanism of
action of phosphite EP additives. They found that initial adsorption rates of phosphorus- containing

FIGURE 3.17 FZG performance of an amine phosphate and TPPT separately and in mixtures in an ISO VG 22
pentaerythritol ester. (From Kristen, U., Additive für Schmierstoffe, Expert Verlag, 1994. With permission.)

0 0.5 1.0 1.5 2.0

0 0.5 1.0 1.5

Additive concentration (%)

TPPT (%)

12

10

8

6

4

>12

Amine phosphate

TPPT

1.0

0.5

0

1.5

Amine phosphate (%)

FZG load stage failure

12 load stage

11

10