Lubricant Additives

Sulfur Carriers 265

olefi ns are nonpolar and show a relative poor affi nity for metal surfaces (see Section 9.4.2.1.4). As
the polarity increases from olefi n < ester < triglyceride, the EP performance increases in the same
order. This behavior is demonstrated in a simple four-ball EP test. Chart 9.1 shows the four-ball weld
load (DIN 51350 Part 2) of sulfurized additives over the sulfur level in oil. The products with a high
polarity (C and D) show considerably higher EP loads than the nonpolar additives (A and B).
The content of active sulfur is only of minor importance on the EP performance, but the polarity
and chemical structure play a major role.

9.4.1.1.3 Activity
Active sulfur is the amount of sulfur available for a reaction at a certain temperature. A common
method for its determination is ASTM D-1662 [34]. The amount of active sulfur is determined by
reacting copper powder with the sulfurized product for 1 h at 149°C. Depending on the raw mate-
rials and on the sulfurization method, the active sulfur content can vary greatly. The activity is a
function of the temperature. Chart 9.2 shows typical active sulfur contents of sulfurized products
based on different chemistry and sulfurization methods.
The activity depends mainly on the sulfur chain in the molecule. Mono- and disulfi des are
not aggressive against yellow metals. Pentasulfi des are highly reactive and, therefore, suitable for
heavy-duty machining of steel The long-term inhibition of these products against yellow metals is
hardly possible. Long-chain sulfur bridges in polysulfi des (A) are thermally less stable than short
sulfur bridges, where sulfur is linked to the carbon atom of the raw material. For this reason,
the reaction with the metal surface is possible at relatively low temperatures. Mono and disulfi des
show only a medium activity, because sulfur will be released only at higher temperatures [35]. The
active sulfur at a given temperature is an indication of the ability of the product to provide suffi -
cient reactive sulfur to form metal sulfi des. Published work on the mechanism of the infl uence of

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

0 0.5 1.5 2.5 3 3.5 4.5 5 5.5 6.5

% Sulfur in oil

Four-ball weld load (N)

A

B

C

D

Type Total Sulfur Active Sulfur Activity (%)

A Olefin 40 36 90

B Ester 17 8 47

C Triglyceride 10 0.5 5

D Triglyceride 18 9 50

162 4

CHART 9.1 Infl uence of raw materials on EP performance.