92 Lubricant Additives: Chemistry and Applications
environmental concerns associated with the use of chlorinated hydrocarbons, their possible replace-
ment by mixtures of phosphorus and sulfur compounds has been investigated [66].
Mixtures of monophosphoric and diphosphoric acid esters were compared with a dithiophos-
phate acid amide in macroemulsions using a variety of EP tests. Performance in drilling and tapping
tests (which are regarded as the conditions most closely simulating cutting performance) indicated
that the dithiophosphate amide gave the best performance, whereas the monoacid and diacid phos-
phates produced levels of performance similar to or better than that of the chlorparaffi n alone.
Traditionally, the acid phosphates in commercial use have high acid numbers (200 –300 mg KOH /g).
As a consequence, in addition to their use as AW/EP additives, they are used as corrosion inhibitors
[110], and certain structures are promoted as copper passivators [110]. A recent development has
been the availability of aryl phosphate-based products that have a relatively low level of acidity
(typically 10–15 mg KOH/g) while offering a combination of good AW/EP performance with rust
prevention and oxidation inhibition. The multifunctionality of this product type offers opportunities
for the simplifi cation of additive packages and use in a wide range of hydraulic and circulatory oils,
metalworking, and gear applications, whereas the lower level of acidity reduces the potential for
additive interaction and the promotion of foaming, etc.
Increased activity in alkyl acid phosphates has been reported in the patent literature. This arises
from the use of long-chain alcohols (C 16 −C 18 ) to produce an acid phosphate ester mix with a high
monoacid content (preferably greater than 80:20% monoacid–diacid ratio) [111]. With this acid
distribution, it has been possible to achieve lower wear than for the conventional ethoxylated alkyl
phosphates with a monoacid to diacid ratio of 60:40%.
3.5.3.2 Alkyl and Alkarylpolyethleneoxy Acid Phosphates
Polyethyleneoxy acid phosphates are a potentially very large class of compounds. Not only are varia-
tions possible in the type of alcohol or phenol chosen but also in the type of alkoxylation (although
ethylene oxide [EO] is invariably used) and the EO content. Products of this process were originally
claimed to be more active than the non-ethoxylated variety, but the latest advances in the latter
types [111] suggest this may no longer be the case.
Depending on the choice of raw materials, the fi nished product may be oil- or water-soluble
or water-dispersible. Alkyl and (alk)arylpolyethyleneoxy acid phosphate esters acids containing
<55% EO were found to be oil-soluble; products with an EO content of more than 60% were
water-soluble as the free acids and their amine salts, whereas products with 40–60% of EO
were both oil- and water-soluble or water-dispersible [40]. The free acids are used in oil applica-
tions, whereas amine (usually triethanolamine) or metal salts of the acids are used in aqueous
applications. The alcohols and phenols initially selected for evaluation were lauryl and oleyl
alcohols and nonyl, dinonyl, and dodecyl phenol. Other raw materials used today include C 8 −C 10
alcohols, 2-ethyl hexanol, tridecanol, cetyl-oleyl mixed alcohols, and phenol. The products are
nonionic surfactants with excellent wetting and emulsifi cation properties, and certain types do
not support bacterial growth. They are also good corrosion inhibitors—an important factor for
their use in metalworking applications. The higher EO-content products tend to produce a heavy
and stable foam, and materials containing ∼45% EO are therefore preferred for metalworking
applications [40].
The effect of the alcohol or phenol and the impact of EO content on the wear behavior in a
naphthenic oil can be seen in Figures 3.13 and 3.14, respectively [112].
The performance of the product based on oleyl alcohol is interesting in that it does not appear to
change with EO content, yet is simultaneously capable of producing materials that vary from oil- to
water-soluble. However, the four-ball or pin and v-block tests, although widely used as screening
tests for the metal-working application, are not considered to be capable of predicting the per-
formance under cutting conditions. This is confi rmed in the paper given in 1995 by Werner et al.
[113], which compares the performance of different ethoxylated acid phosphates under various test