Ashless Phosphorus-Containing Lubricating Oil Additives 99
esters depended largely on the existence of –OH and –P=O bonds in the structure. The extent of
adsorption was infl uenced by the hydrolytic stability of the esters, and this process was found to
occur through reaction with water adsorbed onto the iron surface. Adsorption of the phosphites
varied depending on the degree of esterifi cation; triesters were adsorbed after being decom-
posed hydrolytically to monoesters, whereas diesters were adsorbed without hydrolysis. Phos-
phite esters eventually hydrolyzed to inorganic acid regardless of the degree of esterifi cation,
followed by its adsorption and conversion to the iron salt. It was suggested the adsorbing and
hydrolyzing properties of the esters depended on the arrangement of the molecules physisorbed
onto the surface.
Evaluation of a range of alkyl phosphites as EP additives in gear oils was reported by Riga and
Rock Pistillo [127]. The most effective products were those with short chains, particularly dibutyl
phosphite, which resulted in a wear layer of >1000 Å and the formation of both iron phosphate and
phosphide. Other phosphites formed only traces of phosphide, and as the chain length increased,
the resulting fi lm became thinner and contained less phosphorus, possibly due to steric hindrance.
Long-chain (C 12 ) alkyl phosphites have also been claimed as AW additives for aluminum rolling oil
[128] and, in fact, are still used in metalworking applications.
In view of the work carried out on the use of phosphates as vapor-phase lubricants, an investiga-
tion into the effect of phosphites on the frictional properties of ceramic-on-ceramic and ceramic-
on-metal surfaces was carried out in 1997 [129]. The phosphites (and other additives evaluated) had
no effect on ceramic-on-ceramic friction; in fact, short-chain phosphites signifi cantly increased
friction. When several types of metal were slid against oxide ceramics, the alkyl phosphites were
found to lower the friction for each metal except copper. Apparently, the reaction products between
copper and the phosphite had adhesive properties and increased friction.
The decomposition of trimethylphosphite on a nickel surface was also studied to obtain insight
into the initial steps in the decomposition of phosphates when used as vapor-phase lubricants [130].
The main breakdown path is the cleavage of the –P–O– bond to yield the methoxy species, which
then degrades to CO and H 2 or reacts with the nickel surface. Following heating to 700°K, the sur-
face loses adsorbed species other than phosphorus, which is seen as a simple way for the controlled
deposition of phosphorus onto a metal surface.
3.5.4.2 Use as Antioxidants for Lubricating Oils
In addition to their use as AW/EP additives, neutral (and acid) phosphite esters have long been used
as antioxidants or stabilizer for hydrocarbons. They were originally introduced as stabilizers for
rubber and thermoplastics. Trisnonylphenyl phosphite, for example, was fi rst used to stabilize sty-
rene-butadiene rubber in the early 1940s; this was shortly followed by patents claiming phosphites
as antioxidants for lubricants [48,122,123,131,132].
Phosphites function as decomposers of hydroperoxide, peroxy, and alkoxy radicals (reactions
3.20 through 3.22) rather than eliminating the hydrocarbyl-free radicals formed in the chain initia-
tion process. They also stabilize lubricants against photodegradation [133].
R OOH^1 (RO) P (RO) P O R OH
hydroperoxide
33
→ ^1
(3.20)ROO^1 (RO)P RO (RO)P O
alkylperoxy
radical3
1
3
••→
(3.21)R O^1 (RO) P R OP(RO) RO
alkoxy radical
3
1
2
••→ (3.22)