Ashless Phosphorus-Containing Lubricating Oil Additives 67
Subsequent to these studies, the market adopted chlorine-containing phosphates such as tris-
(2-chloroethyl) phosphate, but they were later replaced in most applications by other EP additives as
chlorine tended to produce corrosion.
The 1940s and 1950s saw signifi cant development activity in the oil industry involving TCP,
and patents appeared claiming the use of this AW/EP additive in general industrial oils [9], rolling
oils [10], cutting oils [11], greases [12], rock drill lubricants [13], and aviation gas turbine lubricants
[14,15]. Some military specifi cations, for example, on hydraulic oils (NATO codes H515/520/576),
were published, which initially called for the use of this additive. However, in the late 1960s, the
diffi culty of obtaining good-quality feedstocks for the manufacture of natural phosphates based on
cresol and xylenol, together with the concern regarding the neurotoxicity of TCP [16,17], led to the
reformulation of many products with the less toxic synthetic triaryl phosphates based on alkylated
phenols. TCP is still used today in aviation applications, but the quality of the phosphate in terms of
its purity and freedom from the o-cresol isomers that were mainly responsible for the neurotoxicity
behavior, has signifi cantly improved in the past 10–20 years.
In addition to its use as an oil additive, TCP was also used for a period in the 1960s as an igni-
tion control additive for motor gasoline to avoid preignition arising from the deposition of lead salts.
These were formed by the interaction of the lead tetraethyl antiknock additive and the alkyl halide
scavenger [18–21]. Alkyl phosphates were claimed for this application in 1970 [22].
As a result of their polar nature, neutral triaryl phosphates have also been claimed as corrosion
inhibitors for hydrocarbons [23,24], but they are unlikely to be promoted for this application today
in view of the availability of more active species, such as the acid phosphates.
The use of trialkyl phosphates as AW and EP additives has been much less extensively evalu-
ated. Although a fl urry of patent activity took place in the late 1920s and 1930s covering methods
for their manufacture [25–34], there was little interest in their use as lubricating oil additives for
further 20 years. This was probably a result of the focus, in the interim period, on chlorinated
derivatives. It was not until the late 1950s that tributyl phosphate (TBP) was disclosed in blends with
isopropyl oleate [35] for use in gear oils and claimed in blends with chlorinated aromatics. In 1967,
a patent appeared claiming the use of alkyl phosphates or the amine salts of alkyl acid phosphates
in a water-based lubricating composition [36].
In addition to alkyl phosphates, various other types of phosphorus-containing compounds have
been evaluated as AW/EP additives. Patents on acid phosphates claiming their use as EP additives
for oil appeared in 1935 and 1936 [37–39], whereas the fi rst publication with detailed information on
the use of ethoxylated alkyl or aryl phosphate oil additives (in metalworking applications) appeared
in 1964 [40]. Patents on the use of these products in mineral oils [41] and in synthetic esters [42]
appeared later. Alkoxylated acid phosphates were also found to have good rust inhibition properties
[43], a feature that was additionally observed for the alkyl (or aryl) acid phosphates [44,45].
Neutral amine salts of alkyl acid phosphates were claimed in 1964 [43] and, in 1969, in admixture
with neutral phosphates [46]. These are, however, just a few examples of the patent estate covering
these product groups.
The other main phosphorus-containing products to be discussed are the phosphites. The basic
chemistry of alkyl and aryl phosphites, like that of the phosphate esters, was also uncovered in the
nineteenth century. In a similar fashion, their utilization as oil additives was not exploited until
much later. Patents appeared in 1940 on the use of mixed aryl phosphites as oil antioxidants [47]
and on their activity as AW/EP additives at least as early as 1943 [48].
Isomeric with the acid phosphites are phosphonates (Figure 3.2). Dialkyl alkyl phosphonates
were claimed as lubricants in 1952 and 1953 [49,50] but not until about 1971 as friction modifi ers
and EP additives [51,52].
The preceding summary focused on the use of phosphorus compounds alone. In reality, they
are widely used in admixtures with sulfur-containing materials to provide good lubrication over
a wider range of performance requirements. Examples of some of the combinations patented are
given in Appendix A.