Ashless Antiwear and Extreme-Pressure Additives 245
Not all phosphorus-containing additives behave the same in engine oils. Furthermore, even
within the same ZDDP family, not all ZDDP respond the same to after-treatment devices as evi-
denced by their relative volatility performance. Data indicated that volatilized phosphorus showed
very low statistical dependence on either oil volatility or phosphorus concentration in the fresh oil.
Rather the data seemed to indicate that the chemistries of the phosphorus-containing additives and
their formulation with other additives were the controlling cause of phosphorus volatility and, by
extension, emission level. Selby’s Phosphorus Emission Index (PEI) and Sulfur Emission Index
(SEI) shed some insights into the volatility impact on emission issues and better S/P volatility con-
trol than the current ZDDP that is highly desired for future ashless antiwear additives [107–109].
Chlorine in lubricants and other materials is becoming an increasing environmental concern.
Legislation around the industrial world limits the chlorine content of many lubricant products
to 50 ppm or less. The Montreal Protocol mandated a gradual phase-out of the use of chlorine-
containing refrigerants, such as hydrochlorofl uorocarbon (HCFC) and chlorofl uorocarbon (CFC),
and replacement with alternative hydrofl uorocarbons (HFC). Increased wear occurred in the refrig-
eration compressor when HFC refrigerants were substituted for CFC, and the cause of this increased
wear was believed to be inferior antiwear capability of the alternative HFC refrigerant as the envi-
ronmental gas, compared to that for CFC [110]. This offers some opportunities for the development
of new ashless antiwear additives for refrigeration compressor oils.
The cutting oil industry is facing similar ecological pressures, and future changes to reduce
or eliminate chlorine are expected. The most signifi cant opportunity is perhaps driven by human
health and waste disposal issues concerning the use of chlorinated paraffi ns. Chlorinated paraffi ns
are used extensively as EP additives in metalworking fl uids. The National Toxicology Program
(NTP) listed chlorinated paraffi ns, derived from C12 feedstocks and chlorinated at 60%, as a
suspect carcinogen. Although few metalworking fl uids are formulated with this class of chlorinated
paraffi n, the image of chlorinated paraffi ns in general has suffered due to uncertainties about future
NTP reclassifi cation of all such additives.
Gear additives are another area of concern. Because of the problems associated with chlorine
additives, their use in gear oils has been greatly reduced. However, a number of processes for
making gear additives utilize chlorine or chlorine-containing reagents at some point in the reaction
sequence. Small amounts of chlorine still remain in the fi nal product. The complete removal of
chlorine is therefore expected to become an important priority, but will be diffi cult to attain in the
near future.
Finally, the use of metallic antiwear/EP additives is diminishing due to the infl uence of environ-
mental concerns. Heavy metals are considered pollutants, and their presence is no longer welcomed
in the environment. Given equal performance and costs, ashless antiwear additives will be preferred
for many future lubricants.
In the future, the lubricant additive business will continue to grow and will need more ash-
less antiwear/EP additives [111]. Possible new markets include biodegradable lubricants, biodiesel
fuel–friendly lubricants, advanced transportation lubricants, robotics, ceramics, and space technol-
ogy lubricants. Traditional markets in engine oils, ATFs, marine, aviation, gear, hydraulic, circu-
lating oils, metalworking, and other industrial lubricants are also expanding. Healthy growth for
nonconventional base oils (groups II–V) is expected in many of these areas. Clearly advanced ashless
antiwear additives with environment-friendly features, excellent stability, and unique performance
properties, especially for nonconventional base oils, will be the additives of choice for increasingly
demanding lubricant applications.
ACKNOWLEDGMENT
I thank Pat Dedert and Elvin Hoel for assistance in the literature search and also many of my
colleagues, especially Dr. Douglas Deckman, Dr. David Blain, Dr. Steven Kennedy, Dr. Andy
Horodysky, and Dr. Andy Jackson, for their valuable comments.