Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 105

or, in the case of neutral phosphates, the cleavage of the C–O bond to release an aryl radical

and a residual –P–O• radical.

• Either reaction of the –P–OH or –P–(OH) 2 with the metal surface to form an iron salt,
  possibly followed by further hydrolysis to release the remaining hydrocarbon moieties and
  reaction of the new –P–OH groups with the surface to form polyphosphate, or the reaction
  of the residual –P–O• radical with the iron surface to form a succession of Fe–O–P– bonds
  leading to the formation of polyphosphate.


• Products that contain –P–C bonds (e.g., the phosphonates and particularly the phosphi-
  nates) are less likely to operate by a mechanism involving hydrolysis, and the stability of
  the P–C bond might be expected to prevent or delay the formation of the phosphorus-rich
  surface layer with an adverse effect on EP properties. However, the same stability could
  result in better friction-modifi cation properties. The fact that phosphinates and phospho-
  nates are active as AW/EP additives suggests that the –P=O bond is also involved in the
  surface adsorption process, but that either the nature of the surface fi lm may be different or
  a polyphosphate fi lm is produced as a result of the scission of a –P–C bond.


• The formation of amine salts results in an increase in activity, possibly as a result of the
  stability of the ion and improved adsorption on the metal surface.

The mechanism of formation of phosphide, which is reported in many instances, has not yet been
clarifi ed but might possibly involve the amorphous carbon that then acts as a reducing agent on the
phosphate/phosphite layer as it forms on the surface.
These conclusions lead, as a broad generalization, to the order in activity and impact on surface
chemistry/stability as shown in Figure 3.23.
The preceding comments are, however, a simplifi cation of the situation. Depending on the
length of the alkyl or alkaryl chain, if the iron salts that are formed are soluble in the oil they may
desorb from the surface, leading to poor AW/EP performance. Interaction with other surface-active
materials will inevitably infl uence the performance of AW/EP additives, whereas depletion in use
due to oxidation, etc., will also affect performance.

3.6 MARKET SIZE AND COMMERCIAL AVAILABILITY

Information on the market size for ashless phosphorus-containing AW/EP additives is limited. An
approximate total market of ∼10,000 tpa is broken down, as given in Table 3.16. The data exclude
the use of phosphites as antioxidants in oil applications, which is separately estimated to be between
100 and 200 tpa.
The wide use of phosphorus-containing AW/EP additives is due, in addition to their good lubric-
ity performance, to the following features of value to formulators:

• Ashless


• Low odor, color, and volatility


• Low acidity/noncorrosive (applies to the neutral esters only)

Amine phosphites

Amine phosphates

Acid phosphites

Acid phosphates

Neutral phosphites

Neutral phosphates

Neutral phosphonates

Improvement in

EP properties

Improvement in

AW properties

Impact on

stability, etc.

FIGURE 3.23 An approximate ranking of the effect of structure on the AW, EP, and stability properties of
the base stock.