Lubricant Additives

Sulfur Carriers 267

9.4.1.1.5 Antioxidant
Sulfurized products with low active sulfur content are suitable to improve the AO behavior of
lubricants. This is particularly important if hydrocracked, almost sulfur-free base fl uids are used.
During the synthesis of these oils, the natural sulfur (mainly heterocycles, inactive) is removed. The
reintroduction of inactive sulfur carriers improves the oxidation stability, especially in combination
with other secondary AOs.

9.4.1.1.6 Lubricity
Lubricity can be described as friction reduction under low-pressure conditions. Under these condi-
tions, physical adsorbed lubricating fi lms are effective (see Section 9.4.2.1.4). Inactive sulfurized
triglycerides are widely used to improve the lubricity of a lubricant. In general, the lubricity of sul-
fur carriers increases with the polarity. Sulfurized olefi n (no lubricity) < sulfurized ester (medium
lubricity) < sulfurized triglyceride (high lubricity). Special products with enhanced lubricity are
based on synergistic raw material blends such as triglyceride/long-chain alcohol, triglyceride/fatty
acid, and triglyceride/olefi n.

9.4.1.1.7 Color
The color of sulfurized compounds is mainly infl uenced by the production method and virtue of the
raw materials. Light color is not only a matter of cosmetics but also a quality feature. Light-colored
products manufactured with high-pressure hydrogen sulfi de processes or by mercaptan oxidation
do not have remaining unsaturated double bonds, and therefore, they show better oxidation stability
in general.

9.4.2 PHYSICAL PROPERTIES

9.4.2.1 Effect of Additive Structure on Properties

9.4.2.1.1 Raw Materials
The selection of the raw materials and the production process determine the chemical structure
of the compound. The physical properties of a sulfurized product are dependent on the chemical
structure. An overview is given in Table 9.2.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.5 1 1.5 2 2.5 3 3.5 4

% Sulfur in oil

Four-ball wear scar (mm)

A

B

C

D

Type Total Sulfur ActiveSulfur Activity (ASTM D-1662)(%)

A Hydrocarbon 40 36 90

B Hydrocarbon (^20525)
C Triglyceride (^10) 0.5 5
D Triglyceride 18 10.5 58
CHART 9.3 Infl uence of activity on AW performance.