Once a proper gear lubricant
is selected for a given application it should suffer a minimum chemical and
physical change during use. One of the changes most likely to occur is
oxidation of the oil which ultimately will lead
to the formation of undesirable products and changes in the
characteristics of the oil. Such changes may result in the formation of
acidic products which may corrode the metal surfaces,
in an increase in viscosity of the oil, or in
production of insoluble materials. Oxidation of
lubricants is accelerated by high temperatures
or by the presence of certain catalysts,
particularly soluble metals. The immediate
effects of oxidation may appear beneficial
in that petroleum acids formed function as oiliness
agents, perhaps by the formation of monolayers of
metallic soaps. Ultimately, as oxidation of oil proceeds, the harmful
effects become evident. The degradation of the oil by oxidation may result in
not only the formation of acidic products but also asphaltenes, resins, or
other polymers. Changes in the lubricant will probably be
accompanied by increase in viscosity , darkening
in color, and the formation of sludge. Cases have
been noted where gear oils became almost solid due to oxidation.
However, oxidation of gear
lubricants can be retarded by addition of antioxidants or oxidation inhibitors.
The use of such agents in most gear oils is wise since the environment for the
lubricants is favorable for oxidation in that both air and heat are present and
thin films of the oil are in contact with the air.
The mechanism of the action of
antioxidants is generally considered to be that of chain breaking as the
additive reacts with a “hot” molecule, thus being itself oxidized. In this
process the oxidant molecule is destroyed, but with dissipation of the energy
possessed by the “hot” molecule, so that the chain reaction is broken. Thus,
the oxidation of hundreds or perhaps thousands of molecules of hydrocarbons is
prevented, since the energy would be passed on from one molecule to the next in
the normal chain reaction.
The suggestion was made by
Larsen and Diamond^35 that antioxidants may be either inhibitors or retardants,
the former acting to break reaction chains and the latter being converted into
an inhibitor during the oxidation process. Three possibilities were given by
Murphy et al.^42 for the possible disposition of such inhibitors after
they had reacted: (a) the inhibitor is oxidized to a compound which is
incapable of further antioxidant action; (b) the inhibitor is oxidized to a
compound which still exhibits antioxidant action, but generally to a reduced
extent; (c) the inhibitor is capable of regeneration. The latter type of
additive is the most desirable, provided the rate and degree of regeneration
are high.
Specific compounds suitable as
antioxidants will be suggested in a later section, but most of these agents
fall in the following bellows: (a) various types of phenols, (b) certain sulfur
bearing compounds, (c) numerous organic phosphites, and (d) certain of the
amines. A number of additives function as dual purpose agents and thus, in some
cases, a specific antioxidant may not be required.
