64 Lubricant Additives: Chemistry and Applications
3.7 Toxicity and Ecotoxicity ....................................................................................................... 108
3.8 The Future for Ashless Phosphorus-Based Lubricating Oil Additives ................................. 110
3.9 Lubricating Oil Formulations (General) ............................................................................... 111
3.10 Hydraulic Oils ....................................................................................................................... 111
3.11 Automotive Engine Oils ........................................................................................................ 111
3.12 Fuels ...................................................................................................................................... 113
3.13 Conclusions ........................................................................................................................... 113
References ...................................................................................................................................... 114
Appendix A: Early Patent Literature on Phosphorus-Containing Compounds ........................... 118
Neutral Phosphates ................................................................................................................ 118
Neutral Phosphites ................................................................................................................ 118
As Antioxidants .................................................................................................................... 118
Acid Phosphates/Phosphites ................................................................................................. 118
Phosphonates ......................................................................................................................... 119
Alkyl- and Arylpolyethyleneoxy-Phosphorus Compounds .................................................. 119
Amine Salts ........................................................................................................................... 119
Physical Mixtures of Phosphorus and Sulfur and Chlorine Compounds ............................. 119
Miscellaneous Phosphorus Compounds ............................................................................... 120
Appendix B: Additional Literature and Patent References on the Mechanism and
Performance of Phosphorus-Containing Additives .............................................................. 120
Neutral Phosphates ................................................................................................................ 120
Neutral Phosphites ................................................................................................................ 121
Alkoxylated Phosphates ........................................................................................................ 121
Amine Salts ........................................................................................................................... 121
Dialkyl Alkyl Phosphonates ................................................................................................. 121
Mixtures of Phosphorus and Sulfur Compounds .................................................................. 122
General References ............................................................................................................... 122
3.1 INTRODUCTION AND SCOPE
In any discussion of phosphorus-containing lubricating oil additives, the products that probably
come most rapidly to mind are the zinc dialkyldithiophosphates (ZDDPs)—multifunctional addi-
tives that have been widely used in both automotive and industrial oils for many years. However, a
wide variety of ashless phosphorus-containing additives are used in the lubricating oil industry. As
with the metal-containing dithiophosphates, they have been in use over a long period and, despite
considerable research into alternative chemistries, the basic structures introduced in the 1930s are
still used today. In contrast, the technology of most other additive types and that of the base stocks
themselves have steadily developed over this time.
Many different types of phosphorus-containing molecules have been examined as additives for
lubricating oils, with most attention given to their potential as antiwear (AW) and extreme- pressure
(EP) additives. Consequently, the patent literature contains a host of references to different struc-
tures displaying this characteristic. However, regardless of composition, all the additives used in
this application serve the same and specifi c function of bringing phosphorus into contact with the
metal surface, where it can be adsorbed and, under certain conditions, react. The resulting surface
fi lm improves the lubrication properties of both mineral and synthetic oils.
This chapter discusses the use of chemicals that contain only phosphorus to improve the
performance characteristics of oils, specifi cally neutral and acid phosphates, phosphites and phos-
phonates, and the amine salts of the acids (see Figures 3.1 and 3.2 for an outline of the main classes
and their structures). These are the principal types of phosphorus compounds in current commercial
use, but other types have also been examined and claimed in the patent literature, for example, phos-
phoramidates. There are also ashless compounds in which sulfur or chlorine has been incorporated