Lubricant Additives

4 Lubricant Additives: Chemistry and Applications

1.11.2 Bulk Oil Oxidation Test ............................................................................................. 33
1.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310)..................................... 33
1.11.2.2 IP 48 Method .............................................................................................. 34
1.11.2.3 IP 280/CIGRE ............................................................................................ 34
1.11.3 Oxygen Update Test ................................................................................................... 34
1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) ........................ 34
1.12 Experimental Observations ..................................................................................................... 34
1.13 Antioxidant Performance with Base Stock Selection ............................................................. 37
1.14 Future Requirements ............................................................................................................... 38
1.15 Commercial Antioxidants ....................................................................................................... 39
1.16 Commercial Metal Deactivators .............................................................................................41
References ........................................................................................................................................ 41

1.1 INTRODUCTION

Well before the mechanism of hydrocarbon oxidation was thoroughly investigated, researchers had
come to understand that some oils provided greater resistance to oxidation than others. The differ-
ence was eventually identifi ed as naturally occurring antioxidants, which varied depending on crude
source or refi ning techniques. Some of these natural antioxidants were found to contain sulfur- or
nitrogen-bearing functional groups. Therefore, it is not surprising that, certain additives that are
used to impart special properties to the oil, such as sulfur-bearing chemicals, were found to provide
additional antioxidant stability. The discovery of sulfurized additives providing oxidation stability
was followed by the identifi cation of similar properties with phenols, which led to the development
of sulfurized phenols. Next, certain amines and metal salts of phosphorus- or sulfur-containing
acids were identifi ed as imparting oxidation stability. By now numerous antioxidants for lubricating
oils have been patented and described in the literature. Today, nearly all lubricants contain at least
one antioxidant for stabilization and other performance-enhancing purposes. Since oxidation has
been identifi ed as the primary cause of oil degradation, it is the most important aspect for lubricants
that the oxidation stability be maximized.
Oxidation produces harmful species, which eventually compromises the designated functiona-
lities of a lubricant, shortens its service life, and to a more extreme extent, damages the machinery it
lubricates. The oxidation is initiated upon exposure of hydrocarbons to oxygen and heat and can be
greatly accelerated by transitional metals such as copper, iron, nickel, and so on. when present. The
internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation
with heat and engine metal parts acting as effective oxidation catalysts. Thus, in-service engine oils
are probably more susceptible to oxidation than most other lubricant applications. For the preven-
tion of lubricant oxidation, antioxidants are the key additive that protects the lubricant from oxida-
tive degradation, allowing the fl uid to meet the demanding requirements for use in engines and
industrial applications.
Several effective antioxidant classes have been developed over the years and have seen use in
engine oils, automatic transmission fl uids, gear oils, turbine oils, compressor oils, greases, hydraulic
fl uids, and metal working fl uids. The main classes include oil-soluble organic and organometallic
antioxidants of the following types:

1. Sulfur compounds


2. Sulfur–nitrogen compounds


3. Phosphorus compounds


4. Sulfur–phosphorus compounds


5. Aromatic amine compounds


6. Hindered phenolic (HP) compounds


7. Organo–copper compounds