220 Lubricant Additives: Chemistry and Applications
The more the active sulfur present, the higher the load-carrying property. However, there is also
a direct correlation between active sulfur and copper corrosivity—the more the active sulfur, the
poorer the copper corrosion protection. More active sulfur can also lead to cleanliness and stability
challenges. Therefore, the ultimate product properties for a specifi c lubricant product will dictate
which sulfurized products to use.
Although the sulfur content may not be as high as in many sulfurized olefi ns, sulfurized esters
are attractive for their exceptionally good frictional properties in many applications. This is because
combining sulfur with fat in a lubricant additive provides a synergistic effect. In this instance, the
fat provides reduced friction, and sulfur provides wear and EP protection. Of all the elements,
sulfur probably gives the best synergistic results in combination with other components and organic
compounds. As to EP characteristics, sulfurized esters have a surface activity conferred by a small
amount of their normal free fatty acids. These are polar species that tend to be absorbed in layers of
molecular dimensions at the metal interface. The interposition of such fi lms is effective in preventing
metal seizure under conditions of EP or under conditions tending to displace the lubricating fi lm
between the bearing surfaces. Here, fi lm strength and EP phenomena are often used synonymously.
Film strength implies that metal-to-metal contact and welding are prevented as a result of the fi lm
formation (or replenishment) by the chemical reaction of the metal and an EP additive. Also, fatty
oils and sulfurized fatty oils because of their affi nity for metal surfaces are less easily displaced
from metal surfaces by water than are mineral oils.
The ferrophilic ester groups improve the EP properties. Depending on the molecular structure
and its polarity, the surface activities vary. Since the surface activity or polarity of the substances
used for sulfurization plays an equally decisive role in lubricating action, it should be taken into
serious consideration when one formulates a product for a specifi c application. Comparing sulfu-
rized triglycerides (e.g., SLO) with sulfurized monoesters (e.g., SSWO), the EP properties of the
triglycerides are better. Two factors may be responsible for this phenomenon: (1) as the triester
structure is more ferrophilic, hydrogen bridging may occur; (2) as triglycerides decompose at high
temperatures to form acrolein moieties during the lubrication process, the polymerized acrolein fi lm
can add strength to the sulfi de fi lm and improve the EP characteristics. However, this EP activity of
triglycerides has limited value due to their poor stability and oil solubility. Stability tests at elevated
temperatures show faster and heavier sludging for SLO than for SSWO. Therefore, a proper balance
of all properties is an essential part of product formulations.
Sulfurized fats or esters are used extensively in lubricants such as metalworking fl uids, tractor-
transmission fl uids, and greases.
8.2.1.3 Other Sulfur Additives
Elemental sulfur provides good EP properties; however, it leads to corrosion. It dissolves in mineral
oils up to certain levels depending on the type of base oils. Low polarity paraffi nic/naphthenic type
group II and III base oils usually have very limited solubility of elemental sulfur. Sulfurized aromatics
such as dibenzyl disulfi de, butylphenol disulfi de, diphenyl disulfi de, or tetramethyldibenzyl disulfi de
generally containing less-active sulfur improve the EP characteristics of lubricants only moderately;
they are therefore used predominantly in combinations with other sulfur or phosphorus-containing
EP additives [24,25]. Other sulfur carriers such as sulfurized nonylphenol, dialkyl thiodipropionates
(S[CH 2 CH 2 C(=O)OR] 2 ), derivatives of thioglycolic acid esters (HS–CH 2 C(=O)OR), derivatives of
thiosalicylic acid, and trithians are also available [26]. However, materials with low sulfur content
are usually less active as antiwear/EP additives, but more effective as antioxidants.
8.2.2 PHOSPHORUS ADDITIVES
Phosphorus-containing additives are used to provide protection against moderate to high pressure,
metal-to-metal contacts in boundary lubrication and EHL. Unlike sulfur additives, where their
EP activity must be balanced against performance requirements for thermal stability and mild