Ashless Phosphorus-Containing Lubricating Oil Additives 69
distillation to remove water [54]. With the alkoxide method, any residual chloride can be removed
by water washing followed by a fi nal distillation under vacuum.
Despite early research into the alkyl phosphates, a rigorous investigation into the preparation of
the lower alkyl derivatives and their properties did not take place until 1930 [55]. In contrast to the
large range of aryl phosphates available, the range of neutral alkyl phosphates is currently limited to
tri-n-butyl phosphate and tri-iso-butyl phosphate, trioctyl phosphate, and tributoxyethyl phosphate.
Other ether phosphates [56] have been claimed in the past but, as far as is known, are not currently
manufactured.
Although the neutral trialkyl phosphates have been available for sometime, they have not been
widely used as additives for mineral oil. Those products in commercial production are used prin-
cipally as components of aircraft hydraulic fl uids, in turbine oils, rolling oils, or as solvents in
industrial processes. However, interest in these materials as AW additives for applications where
the release of phenols from the degradation of the phosphate is to be avoided currently exists. They
also offer advantages as alternatives to the acid phosphates, alkoxylated acid phosphates, and their
salts in metalworking applications, where there are concerns over instability in hard water and foam
production in use (Canter, N., Private Communication, August 2001).
Triaryl phosphates, which are the most widely used of all ashless phosphorus-based AW addi-
tives, are currently manufactured almost exclusively by reaction 3.2. Phosphorus oxychloride is
added to the reaction mass containing excess of phenol in the presence of a small amount of cat-
alyst, typically aluminum chloride or magnesium chloride, before heating slowly. The hydrogen
chloride is removed as it is formed under vacuum, followed by absorption in water. On completion
of the reaction, the product is distilled to remove most of the excess phenol(s), the catalyst residue,
and traces of polyphosphates. Finally, the product may be steam-stripped to remove volatiles includ-
ing residual phenol(s) and is dried under vacuum.
The raw material for the manufacture of triaryl phosphates was originally obtained from the
destructive distillation of coal. This process yields coal tar, which is a complex mixture of phenol
and alkyl phenols including cresols (methylphenols) and xylenols (dimethylphenols). Distillation of
this mixture (sometimes known as cresylic acids) produces feedstocks rich in cresols and xylenols,
which are then converted into the neutral phosphate. An early patent on the production of triaryl
phosphates from tar acids was issued in 1932 [26].
Unfortunately, in the 1960s, as the number of coal tar distillers declined due to the move from
coal to natural gas as a fuel, it became progressively more diffi cult to obtain cresols and xylenols
from this source. As a consequence, the phosphate manufacturers turned their attention to the use
of phenol, which was alkylated with propylene or butylene. The resultant mixtures of alkylated phe-
nols were then converted into phosphates [57,58]. To distinguish phosphates from these two sources
of raw materials, the cresol and xylenol-based products became known as natural phosphates and
the phosphates from alkylated phenols as synthetic phosphates. This distinction is no longer valid
today as synthetic cresol and xylenol are now available and used in phosphate manufacture. How-
ever, the nomenclature remains a simple way of distinguishing between the cresol/xylenol-based
products and the newer products based on phenol. As the physical and chemical properties of each
product type are slightly different, customer selection may depend on the application. For example,
if the requirement is for a product that requires good oxidation stability, then the choice would be
a tertiarybutylphenyl phosphate (TBPP), but a xylyl phosphate would be selected if the product
required the best hydrolytic stability.
3.3.1.1 Natural Phosphates
The main products available in this category are TCP and trixylyl phosphate (TXP) (Figure 3.3).
These products, based on cresols and xylenols, are complex mixtures of isomeric materials [59].
However, the variation in phosphate isomer distribution, which arises from changes to the feedstock