Ashless Antiwear and Extreme-Pressure Additives 221
corrosivity toward copper-containing alloys, phosphorus additives usually possess very good
corrosivity control. Owing to totally different mechanisms involved in surface fi lm formation rates
and fi lm strengths, phosphorus additives cannot replace sulfur additives in many applications and
vice versa. Typically, phosphorus additives are extremely effective in applications with slow sliding
speeds and high surface roughness.
8.2.2.1 Phosphate Esters
Phosphate esters have been produced commercially since the 1920s and have gained importance
as lubricant additives, plasticizers, and synthetic base fl uids for compressor and hydraulic oils.
They are esters of alcohols and phenols with a general formula O=P(OR) 3 , where R represents alkyl,
aryl, alkylaryl, or very often, a mixture of alkyl and aryl components. The physical and chemical
properties of phosphate esters can be varied considerably depending on the choice of substituents,
and these can be selected to give optimum performance for a given application. Phosphate esters
are particularly used in applications that benefi t from their high-temperature stability and excellent
fi re-resistance properties in addition to their adequate antiwear properties [27].
8.2.2.1.1 Chemistry and Manufacture
Phosphate esters are produced by reaction of phosphoryl chloride with alcohols or phenols as shown
in reaction 8.6.
3ROH + POCl 3 ⇒ O=P(OR) 3 + HCl (8.6)
Early production of phosphate esters was based on the so-called crude cresylic acid fraction or tar acid
derived by distillation of coal tar residues. This feedstock is a complex mixture of cresols, xylenols,
and other heavy materials and includes signifi cant quantities of ortho-cresol. The presence of high
concentrations of ortho-cresol results in an ester that has been associated with neurotoxic effects,
and this has led to the use of controlled coal tar fractions, in which the content of ortho-cresol and
other ortho-n-alkylphenols is greatly reduced. Phosphate esters using coal tar fractions are generally
referred to as natural as opposed to synthetic, where high-purity raw materials are used.
The vast majority of modern phosphate esters are synthetic, using materials derived from pet-
rochemical sources. For example, t-butylated phenols are produced from phenols by reaction with
butylene. The reaction of alcohol or phenol with phosphoryl chloride yields the crude product, which
is generally washed, distilled, dried, and decolorized to yield the fi nished product. Low-molecular-
weight trialkyl esters are water-soluble, requiring the use of nonaqueous techniques. When mixed
alkylaryl esters are produced, the reactant phenol and alcohol are added separately. The reaction
is conducted in a stepwise process and the reaction temperature is kept as low as possible to avoid
transesterifi cation reactions from taking place.
The most commonly used phosphate esters for antiwear performance features are tricresyl
phosphates (TCP), trixylenyl phosphates (TXP), and tributylphenyl phosphates (TBP).
8.2.2.1.2 Physical and Chemical Properties
The physical properties of phosphate esters vary considerably according to the mix and type of
organic substituents, the molecular weights, and structural symmetry, all proving to be particu-
larly signifi cant. Consequently, phosphate esters range from low-viscosity, water-soluble liquids to
insoluble high-melting solids.
As mentioned previously, the use of phosphate esters as synthetic base fl uids arises mostly from
their excellent fi re resistance and superior lubricity, but is limited due to their hydrolytic and thermal
stability, low-temperature properties, and viscosity index. Although phosphate esters are widely used
as antiwear additives for lubricants, the concerns about hydrolytic stability, thermal stability, and of
course, satisfactory antiwear properties are equally important. In that sense, triaryl phosphates are
dominant over trialkyl phosphates, because their hydrolytic–thermal stability is much better.