236 Lubricant Additives: Chemistry and Applications
The chlorine compounds act and function in that they coat the metal surface with a metal
chloride fi lm under the infl uence of high pressure at point of lubrication and in the presence of
traces of moisture. FeCl 2 melts at 672°C and has low shear strength when compare with steel.
The effect of chlorine compounds depends on the reactivity of the chlorine atom, temperature,
and concentration. Hydrogen chloride formed in the presence of larger quantities of moisture can
cause severe corrosion of the metal surfaces. As the corrosion hazards increase along with the EP
properties with increasing reactivity of the chlorine atoms, a compromise must be found in the
development of chlorine-containing additives.
Chlorinated paraffi ns such as trichlorocetane represent a group of important EP additives used
in the past. They can signifi cantly increase the load stages in the FZG test with increasing con-
centration. The chain length has practically very little infl uence on the EP effect; on the contrary,
the load-carrying capacity increases with increasing degree of chlorination. In practice, chlori-
nated paraffi ns with ~40 to 70 wt% chlorine are used; however, they are sensitive to moisture and
light and can easily evolve hydrogen chloride [97]. Compounds such as phenoxy-propylene oxide,
amines, or basic sulfonates neutralize hydrogen chloride and thus act as stabilizers.
Good results are also obtained with chlorinated fatty acids and their derivatives; particularly
those with trichloromethyl groups in the end position, since the additives with CCl 3 groups are
particularly effective.
Owing to their high stability, chlorinated aromatics have less favorable EP properties than
the chlorinated aliphatics. Alkylaromatics with chlorinated side chains improve the load-carrying
capacity much more than those chlorinated in the ring; the effi ciency increases with the number of
carbon atoms in the side chain. Chlorinated fatty oils and esters as well as chlorinated terpenes and
amines have also been patented as EP additives.
Sulfur–chlorine additives were found to be satisfactory for gear lubrication in passenger cars
in the mid-1930s. Apparently, this type of additive could satisfy the high-speed and moderate-load
operation of passenger cars used in that time period. When sulfur and chlorine are combined in the
organic molecule, sulfur somewhat reduces the corrosive tendency of chlorine; on the contrary, the
EP properties of the combined moieties are improved in comparison with the individual compounds.
Chlorinated alkyl sulfi des, sulfurized chloronaphthalenes, chlorinated alkyl thiocarbonates, bis-(p-
chlorobenzyl) disulfi de, tetrachlorodiphenyl sulfi de, and trichloroacrolein mercaptals [Cl 2 C=CCl–
CH(SR′)–SR′′, where R′ and R′′ are alkyl or aryl] must be mentioned in this class. Reaction products
of olefi ns and unsaturated fatty acid esters with sulfur chlorides contain highly reactive β-chlorosul-
fi des, which due to their reactive chlorine and sulfur atoms give very good EP agents, yet show more
or less strong corrosive tendencies. However, severe wear was frequently encountered in truck axles
where performance under high-torque, low-speed conditions is of greater importance. Later on, the
presence of chlorine, although a good EP agent, was found to be detrimental to lubricant thermal
stability. Hence, for the past 30 years, chlorine has not been used in gear oils.
Chlorinated trioleyl phosphate, condensation products of chlorinated fatty oils with alkali
salts of dithiophosphoric acid diesters, and reaction products of glycols with PCl 3 are examples of
chlorine–phosphorus additives used in earlier years.
The most serious drawback for chlorine antiwear and EP additives is in the environmental
area. Legislation around the industrial world limits the chlorine content of many lubricants to parts
per million. Therefore, except for the cutting oil industry, which is also under pressure to change,
chlorine additives are not considered a viable option for modern lubricants.
8.2.9 NONTRADITIONAL ANTIWEAR/EXTREME-PRESSURE ADDITIVES
Traditional sulfur, phosphorus, and halogen-related compounds are considered to be the dominant
antiwear/EP additives in the marketplace. However, as environmental concerns escalate, the future
trends will favor products that diminish potential hazard and disposal problems. Recent clean fuel