274 Lubricant Additives: Chemistry and Applications
9.5.4.2 Basic Alkali Metal Salts
Sulfurized products show a very strong synergistic effect in combination with basic alkali metal
salts [43], often referred to as overbased sulfonates or carboxylates. Particularly, active sulfur in
combination with overbased calcium or sodium sulfonates exhibits advantageous performance with
regard to improved load-carrying and AW properties. These additive combinations are used in
lubricants for severe metalworking operations.
It is disclosed in International Patent WO 87/06256 [41] that the load-bearing characteristics of a
grease composition and gear lubricant may be unexpectedly improved by formulating these composi-
tions with an additive mixture comprising overbased salts of alkaline earth metals or alkali metals
and at least one sulfurized organic compound. From today’s point of view, the overbased products/
sulfur combination has its advantages in some stainless steel cutting and forming operations. But the
high alkalinity of such formulations shows big compatibility problems when in contact with lubricity
esters and other types of acidic additives. Alkaline washing baths get used up quickly and need much
more frequent changes as calcium soaps built up. Welding without cleaning the metal surface is also
impossible as the high TBN sulfonates are generating high amounts of oxide ash.
9.5.4.3 Antioxidants
Inactive sulfur carriers show a synergism with aminic AOs. This effect is very distinct in low or even
sulfur-free base fl uids. Active sulfurized products do not show this synergy. On the contrary, the
active types deteriorate the oxidation stability. Table 9.6 demonstrates the oxidation stability (ASTM
D-2270, RPVOT Test) of active and inactive sulfur carriers based on the same raw materials.
The inactive product improves the oxidation stability (250 min) twice as much as the active type
(120 min). In combination with the aminic AOs, the synergistic effect is obvious. Although the inac-
tive sulfur carrier improves the AO properties of the aminic AO, the active type has a detrimental
effect and reduces the oxidation stability.
9.5.4.4 Esters/Triglycerides
Esters a re used either as base fl uids or as additives. It is impor tant to coordinate ester type and sulfur
chemistry to achieve optimum performance. Unsaturated esters show strong synergistic effects with
active sulfur, whereas inactive sulfur shows distinct synergies with saturated esters. The perfor-
mance of sulfur carriers in saturated esters is similar to their performance in mineral oil. These
synergies are widely used in the formulation of lubricants.
Combinations of active sulfur and unsaturated esters or triglycerides (mainly vegetable or
animal oils such as canola, rapeseed, tall, sunfl ower oil, and esters thereof) are very common in all
types of metalworking fl uids, in oils, as well as in water-based systems. The combination of these
products shows better EP and AW properties than the single components. This performance is
illustrated in the four-ball test (see Table 9.7).
TABLE 9.6
Synergisitc Effect of ZnDTP on Copper Corrosion
Base Oil
Hydrocracked,
Dewaxed, No
Sulfur (min)1.0% Inactive
Olefi n, 20%
S, 5% active
S (min)1.0% Active
Olefi n, 39%
S, 30% active
S (min)
Base oil 40 250 120
0.2% Aminic AO (alkylated
diphenylamine) 400 540 135