268 Lubricant Additives: Chemistry and Applications
9.4.2.1.2 Polymerization
During the sulfurization process, the molecules of the raw materials are linked through sulfur.
Depending on the structure of the raw material, two or more raw material molecules will be linked.
Triglycerides such as lard oil and soybean oil do polymerize and form solid, rubberlike products, if
the polymerization is not controlled through chain terminators such as esters or olefi ns, containing
only one double bond. Olefi ns with only one double bond do not polymerize. Two molecules are
linked by a sulfur chain where length depends on the production process. Esters behave in a similar
way but due to varying amounts of multiple unsaturated compounds in natural esters, some polym-
erization takes place. Dark sulfurized products not only show less oxidation stability compared
with light-colored, completely saturated compounds but will also resume polymerization after the
production process is fi nished.
9.4.2.1.3 Solubility
The solubility is mainly a function of the polarity of the product. As the polarity increases from
olefi n < ester < triglyceride, the solubility decreases. Polarity as well as the grade of polymeriza-
tion determines the solubility. In general, sulfurized olefi ns have excellent solubility in solvents and
all mineral oils. Depending on the sulfurization method, esters can exhibit good solubility even in
group II and group III base oils if their polymerization grade is controlled during production. Sul-
furized triglycerides are, in general, limited in their solubility due to their high polarity. But even
more, the grade of polymerization plays a predominant role. A controlled reaction/polymerization
can lead to light-colored products that will be soluble in paraffi nic base oils, whereas uncontrolled
polymerization will lead to dark-colored products, soluble only in oils with higher polarity and
aromatic content such as, for example, naphthenic base oils.
9.4.2.1.4 Polarity
Polarity determines the adhesion of a sulfurized product to the metal surface. The polarity depends
on the raw materials used for the sulfurization. The organic portion of the molecule is responsible
for the polarity and the affi nity of the sulfurized product to the metal surface [35]. As the polarity
increases from sulfurized hydrocarbon < sulfurized ester < sulfurized triglyceride, the affi nity
(physical adsorption) to metal surfaces also increases. Therefore, sulfurized products based on tri-
glycerides, fatty acids, or alcohols provide superior lubricity compared with sulfur carriers based
on less polar esters or nonpolar olefi ns.
9.4.2.1.5 Viscosity
Viscosity of a sulfurized product depends on the type of raw material used for the sulfurization
and polymerization grade. A higher degree of polymerization (molecular weight) results in higher
viscosity. The raw materials determine the viscosity index (VI) of a sulfur carrier. Short-chain
sulfurized olefi ns show low VIs, whereas sulfurized triglycerides have VI above 200.
TABLE 9.2
Physical Properties of Sulfurized Products
Ester Triglyceride Olefi n
Polymerization Low High Very low
Solubility Good Fair–good Very good
Polarity Moderate High Low
Viscosity Low High Very low
Biodegradability Good Excellent Poor