Lubricant Additives

204 Lubricant Additives: Chemistry and Applications

2. Partial esters, specifi cally glycerol mono-oleate (GMO) (Figure 7.6). GMO is a reaction
   product of glycerin (natural alcohol with three hydroxyl groups) and olein (as mentioned
   previously). Investigations have shown that the alpha version (terminal hydroxyl groups
   esterifi ed) is the active component rather than the beta one (middle hydroxyl group esteri-
   fi ed). Special production techniques are required to manufacture high-alpha-containing
   products.

The mode of action of both product groups is based on the formation of adsorbed layers that can easily
be sheared off, leading to reduced friction. It is expected that further research will result in new and
improved types to cope with more severe requirements with regard to friction retention over time.
Within the group of metallo-organic compounds, molybdenum dithiocarbamate [Mo(dtc) 2 ]
seems almost exclusively to be used to obtain friction reduction (Figure 7.7). Research [7,20] has
shown that the friction-reducing activity of Mo(dtc) 2 is based on the exchange of functional groups
with zinc dialkyldithiophosphates [Zn(dtp) 2 ] (Figure 7.8).
It was found that oxidation affects these exchange reactions signifi cantly and that the most
effective friction reduction is achieved at the later stages of oxidation when the concentrations of
the single-exchange product [Mo(dtc)(dtp)] and the double-exchange product [Mo(dtp) 2 ] are high.
When both products are nearly consumed by oxidation, friction reduction ceases.

7.8 FRICTION MODIFIER PERFORMANCE

Literature suggests that FMs act both in the BL and ML regimes [7,16,21]. Their mode of action
should depend on FM chemistry and prevailing engine conditions. It is further suggested that
organic FMs are most active in the mixed regime, whereas metallic types are predominantly active

NH 2

O

O

O

OH

OH

Oleylamide Glycerol mono-oleate (GMO)

FIGURE 7.6 Organic FMs—structural drawings.

R

N C

R

S

S

Mo

S

S

Mo

S

S

C N

R

R

OO

FIGURE 7.7 Molybdenum dithiocarbamate—

structural drawing.

FIGURE 7.8 Molybdenum dithiocarbamate—

exchange of functional groups.

Mo(dtc) 2

Mo(dtp) 2

Mo(dtc)(dtp)

+

+

+

Zn (dtp) 2

Zn (dtp)(dtc)

Zn(dtc) 2

A

B

C