Ashless Phosphorus-Containing Lubricating Oil Additives 111
to the application. The potential in other market segments including those traditionally using ZDDP
is discussed in greater detail in the following:
3.9 LUBRICATING OIL FORMULATIONS (GENERAL)
The current trend toward the use of groups II and III mineral oil base stocks for general industrial appli-
cations, with improved antioxidant response but inferior lubricity as a result of the removal of aromat-
ics and sulfur compounds, could encourage the wider use of phosphorus. The lack of competition for
the surface, which has previously been shown for TCP in stocks containing naphthenics and aromatics,
should result in the increased activity of phosphorus compounds. Their use may also be benefi cial due
to their ability to aid the dissolution of additives that might otherwise have limited solubility.
In Europe, legislation (Directive 2000/769/EC) implemented in 2004 requires a substantial
reduction in sulfur dioxide (SO 2 ) emissions from the combustion of waste materials including waste
oil. This may result in a move to lower sulfur levels in lubricating oils (including metalworking oils)
and a possible replacement by phosphorus to restore the level of AW/EP performance.
3.10 HYDRAULIC OILS
In recent years, there has been a move toward the use of ashless hydraulic oils. This is mainly for
two reasons. First, as a result of the sensitivity of ZDDPs toward moisture and the resulting deposi-
tion of zinc oxide/sulfi de. This deposit can adversely affect the fi lterability of the oil and reduce oxi-
dation stability. Second, there is increasing concern regarding the environmental behavior of heavy
metals. Regulatory controls, however, are likely to extend further to cover metals such as zinc, as
in the Great Lakes Initiative between the United States and Canada. As the zinc cannot be easily
removed from waste at the effl uent plant, there has been a focus on the reduction in use levels.
Concern has also been expressed in certain countries regarding the smell of sulfur arising from
the degradation of the ZDDP when the hydraulic oil is used, for example, in elevators (Dixon, R.,
Shell Global Solutions, Private Communication, November 2007).
3.11 AUTOMOTIVE ENGINE OILS
Vehicle emissions legislation (e.g., in the United States, Europe, and Japan) now exists to control
and substantially reduce the levels of particulates, hydrocarbons, carbon monoxide, and oxides of
nitrogen in the engine exhaust. The engine manufacturers have met these requirements by a variety
of design changes that impact the composition of oils and fuels in the following ways:
- The introduction of catalytic converters to oxidize the hydrocarbon and carbon monoxide
components to carbon dioxide and water, and reduce the nitric oxide (NO) to nitrogen, has
been very successful in reducing emissions. When they operate at their normal operat-
ing temperature and optimum level of effi ciency, they are almost 100% effi cient and most
of the remaining emissions occur in the time before the catalyst reaches “light-off” tem-
perature. Many studies into reducing this period to achieve yet lower emissions have been
conducted. Although much success has been achieved, further progress may be hindered
by the formation of a deactivating fi lm on the catalyst surface by the phosphorus from the
ZDDP antioxidant and AW/EP additive. As a consequence, there is pressure to reduce the
phosphorus content of engine oils to minimize catalyst fouling. Currently, oil specifi ca-
tions such as ILSAC GF-4 and ACEA Cx limit the phosphorus content of both diesel and
gasoline engine oils to 0.05–0.09% with the actual level being linked to the amount of
catalyst used and the expected service interval. Further reductions below 0.05% are being
considered, but there is a concern that such a low level could adversely affect the durabil-
ity of certain engine parts, for example, the valve train and timing chain, as reducing the
ZDDP content also reduces the wear protection. However, a recent study [173] suggests that