202 Lubricant Additives: Chemistry and Applications
Hence, these do not function as friction-reducing additives. However, polarity plays a
major role among the various lateral surface interactions through strong electrostatic
dipole–dipole interactions. These may be either repulsive or attractive, depending on the
orientation of the adsorbed dipoles with respect to the surface [19].- Chain length. Longer chains increase thickness of the absorbed fi lm, and the interactions
between the hydrocarbon chains increase as well [18]. - Molecular confi guration. Slim molecules allow for closer packing as well as increased
interaction between adjacent chains, leading to stronger fi lms. Therefore, straight chains
may be preferred. - Tem pera t ure. Temperature infl uences FM fi lm thickness and tenacity. Adsorption of friction-
reducing compounds to the metal surface does occur at relatively low temperatures. AW
additives form protective layers by chemical reactions for which higher temperatures are
needed.
If the temperature is too high, enough energy might be provided to desorb the friction-
reducing molecules from the metal surface.
7.4.1.3 Formation of In Situ Polymers
The formation of low-friction-type polymer fi lms can be considered a special case. Instead of the
usual solid fi lms, fl uid fi lms are formed under infl uence of contact temperature (fl ash temperature)
and load. Another difference is that the polymers are developed at the interface between metal
asperities without reacting with the metal surface.
The requirements of such polymers are
- Polymers must have relatively low reactivity. Polymerization must be generated by fric-
tional energy. - The polymers formed must be mechanically and thermally stable and should not be soluble
in the lubricant. - The polymers must develop a strong bond to the metal surface either by absorption or by
chemical bonding. - The formation and regeneration of fi lms must be fast to prevent competitive adsorption by
other additives.
Examples of polymer-forming FMs are
Partial complex esters, for example, a sebacic acid/ethylene glycol partial ester methacrylates
Oleic acid (olein), which may be explained through thermal polymerization (formation of
dimers and higher oligomers)7.5 CHEMISTRY OF OTHER FRICTION MODIFIERS
Within this group, the following categories can be distinguished by chemical type:
- Metallo-organic compounds
- Oil-insoluble materials
Classifi cation by type appears in Table 7.5.
7.5.1 METALLO-ORGANIC COMPOUNDS
Molybdenum dithiophosphate, molybdenum dithiocarbamate, and molybdenum dithiolate as well
as copper-oleate, copper-salicylate, and copper-dialkyldithiophosphate are examples of friction-
reducing metallo-organic compounds.