Friction modifiers (FMs) or friction reducers have been applied for several years. Originally, the application was for limited slip gear oils, automatic transmission fluids, slideway lubricants, and multipurpose tractor fluids. Such products made use of friction modification to meet requirements for smooth transition from static to dynamic condition as well as reduced noise, frictional heat, and startup torque.
Since fuel economy became an international issue, initially to reduce crude oil consumption, FMs have been introduced into automotive crankcase lubricants, as well, to improve fuel efficiency through the lubricant. In the United States, additional pressure is imposed on original equipment manufacturers (OEMs) by the corporate average fuel economy (CAFE) regulation.
Following the introduction of vehicle exhaust emission regulations in various regions around the world, emphasis on friction reduction further increased. This can be well understood if it is realized that 20–25% of the energy generated in an engine by burning fuel is lost through friction. The biggest part is lost by friction on the piston liner/piston ring interface and a smaller part by bearing and valve train friction. It is predicted that in future engines the contribution of the piston group to engine friction will increase up to 50%. The need to measure fuel savings has led to the development of the following tests:
American Petroleum Institute – API: test sequences such as VI and VIA.
International Lubricant Standards Approval Committee – ILSAC: Sequence VIB.
The Coordinating European Council – CEC: test number CEC L-54-T-96.
Association des Constructeurs Européens d’automobiles – ACEA: DBM 111 engine test.
Chemistry of Organic Friction Modifiers
Organic FMs are generally long, slim molecules with a straight hydrocarbon chain consisting of at least 10 carbon atoms and a polar group at one end. The polar group is one of the governing factors in the effectiveness of the molecule as an FM. Chemically, organic FMs can be found within the following categories:
Carboxylic acids or their derivatives, for example, stearic acid and partial esters.
Amides, imides, amines, and their derivatives, for example, oleylamide.
Phosphoric or phosphonic acid derivatives.
Organic polymers, for example, methacrylates.
Friction Modifiers Mechanisms
a. Formation of Reacted Layers:
A protective layers are formed by chemical reaction of the additive with the metal surface. The reaction has to occur under the relatively mild conditions (temperature and load) of the mixed lubrication regime. These conditions require a fairly high level of chemical activity as reflected by the phosphorus and sulfur chemistry applied.
An exception to this is stearic acid. Theoretically, the friction-reducing effect of stearic acid should decrease with increasing temperature due to desorption of the molecule from the metal surface. However, stearic acid experimentally shows a remarkable drop of friction with increasing temperature, which can only be explained by the formation of chemically reacted protective layers.
b. Formation of Absorbed Layers:
The formation of absorbed layers occurs due to the polar nature of the molecules. FMs dissolved in oil are attracted to metal surfaces by strong absorption forces, which can be as high as 13 kcal/mol. The polar head is anchored to the metal surface, and the hydrocarbon tail is left solubilized in the oil, perpendicular to the metal surface.
Next the following steps occur:
Other FM molecules have their polar heads attracted to one another by hydrogen bonding and Debye orientation forces, resulting in dimer clusters. Forces are ∼15 kcal/mol.
Van der Waals forces cause the molecules to align themselves such that they form multimolecular clusters that are parallel to one another.
The orienting field of the absorbed layer induces further clusters to position themselves with their methyl groups stacking onto the methyl groups of the tails of the absorbed monolayer.
As a result, all molecules line up, straight, perpendicular to the metal surface, leading to a multilayer matrix of FM molecules. The FM layers are difficult to compress but very easy to shear at the hydrocarbon tail interfaces, explaining the friction-reducing properties of FMs. Owing to the strong orienting forces, mentioned earlier, sheared-off layers are quite easily rebuilt to their original state.
The thickness and effectiveness of the absorbed FM films depend on several parameters, four of which are explained here.
Polar group: Polarity itself is not necessarily sufficient for adsorption; the polar group must also have hydrogen-bonding capability. Molecules with highly polar functional groups that are not capable of forming hydrogen bonds, such as nitroparaffins, do not adsorb. 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 .
Chain length: Longer chains increase thickness of the absorbed film, and the interactions between the hydrocarbon chains increase as well.
Molecular configuration: Slim molecules allow for closer packing as well as increased interaction between adjacent chains, leading to stronger films. Therefore, straight chains may be preferred.
Temperature: Temperature influences FM film 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.
c. Formation of In Situ Polymers
The formation of low-friction-type polymer films can be considered a special case. Instead of the usual solid films, fluid films are formed under influence of contact temperature (flash 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 frictional 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 films 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).
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