20 Lubricant Additives: Chemistry and Applications
1.10.1.3.2 Aldehyde or Ketone Formation
RR HCOii→RCHO R (1.8)
RR R CO →RR CO R i (1.9)
The chain-branching steps begin with the cleavage of hydroperoxide into an alkoxy radical (RO•)
and a hydroxy radical (HO•). This reaction has high activation energy and is only signifi cant at tem-
peratures >150°C. Catalytic metal ions accelerate the process. The resulting radicals will undergo a
number of possible reactions: (a) the alkoxyl radical abstracts hydrogen from a hydrocarbon to form
a molecule of alcohol and a new alkyl radical according to reaction 1.6, (b) the hydroxyl radical fol-
lows the pathway of reaction 1.7 to abstract hydrogen from a hydrocarbon molecule to form water
and a new alkyl radical, (c) a secondary alkoxyl radical (RR′HCO•) may decompose through reac-
tion pathway 1.8 to form an aldehyde, and (d) a tertiary alkoxy radical (RR′R′′CO•) may decompose
to form a ketone (reaction 1.9).
The chain-branching reaction is a very important step to the subsequent oxidation state of
the oil as not only will a large number of alkyl radicals be formed that expedites the oxidation
process, but also the lower-molecular-weight aldehydes and ketones generated will immediately
affect the physical properties of the lubricant by decreasing oil viscosity and increasing oil vola-
tility and polarity. Under high-temperature oxidation conditions, the aldehydes and ketones can
undergo further reactions to form acids and high-molecular-weight species that thicken the oil
and contribute to the formation of sludge and varnish deposits. Detailed mechanisms will be
discussed in Section 1.10.3.
1.10.1.4 Chain Termination
RR RRii→ (1.10)
RiiR OO →ROOR (1.11)
As oxidation proceeds, oil viscosity will increase due to the formation of high-molecular-weight
hydrocarbons. When oil viscosity has reached a level that diffusion of oxygen in oil is signifi cantly
limited, chain termination reactions will dominate. As indicated by reactions 1.10 and 1.11, two
alkyl radicals can combine to form a hydrocarbon molecule. Alternatively, an alkyl radical can
combine with an alkyl peroxy radical to form a peroxide. This peroxide, however, is not stable and
can easily breakdown to generate more alkyl peroxy radicals. During the chain-termination pro-
cesses, formation of carbonyl compounds and alcohols may also take place on the peroxy radicals
that contain an extractable α-hydrogen atom:
RR CHOO RR CHOOOOHCR RO (^2) RR C O RR CH OH
i←→
→
(1.12)
1.10.2 METAL-CATALYZED LUBRICANT DEGRADATION
Metal ions are able to catalyze the initiation step as well as the hydroperoxide decomposition in
the chain-branching step [184] through a redox mechanism illustrated in the following section. The
required activation energy is lowered for this mechanism, and thus, the initiation and propagation
steps can commence at much lower temperatures.