Lubricant Additives

72 Lubricant Additives: Chemistry and Applications

are carried out at temperatures of about 60–90°C using lower than equivalent quantities of phenol.
The former process gives predominantly the monoacid phosphate with a small amount of neutral
phosphate ester. The latter produces a somewhat greater amount of neutral mixed phosphate ester
but mainly the mixed monoacid phosphate [60]. Examples of commercially available lubricating oil
additives of this chemistry are amylphenyl- and octylphenyl acid phosphates.

POCl RC H OH RC H O PO RC H O PO OH

OH

364  643 642

→()() (3.9)

RC H OPOCl^642 ArOH RC H O ArO) PO^642 RC H OArOPO OH^64
→
( ^ (3.10)

Other processes reported for the production of mixtures of mono-2-ethylhexyl and di-2-ethylhexyl
acid phosphates include the chlorination of bis-(2-ethylhexyl) phosphonate followed by hydrolysis
(reaction 3.11) or the hydrolysis of the tris-(2-ethylhexyl) phosphate.

(C H O) POH^172 C H PO Cl C H O) PO OH

Cl

817

OH

82 817 2

→()→( (3.11)

The alkyl acid phosphates are quite widely used as AW/EP additives in metalworking lubricant
applications and as corrosion inhibitors for circulatory oils.

3.3.2.2 Alkyl- and Alkylarylpolyethleneoxy Acid Phosphates

A range of polyethyleneoxy acid phosphate esters was introduced for metalworking lubricant applica-
tions in the early 1960s. These products, which consisted of both the free acids and their barium salts,
were manufactured by reacting an ethoxylated alcohol with phosphorus pentoxide (reaction 3.12).
The properties of the resulting acid phosphate mix can vary signifi cantly depending on the chain
length of the alcohol and the number of units of ethoxylation. For example, products that are only
soluble in either oil or water can be produced as well as compounds that are soluble (or dispersible)
in both media.

ROH (C H O) H 24 RO(C H O) H 24 [RO(C H O) ] PO OH RO(C H

P

24 2 2

^2

nn→→n

O 5

 442 O) PO(OH)n

(3.12)

3.3.3 AMINE SALTS OF ACID PHOSPHATES AND OF POLYETHYLENEOXY ACID PHOSPHATES

Although the acidic products are very active AW/EP additives, their acidity can lead to precipitation
problems in hard water and potential interaction with other additives. To minimize such adverse
effects, the acids are sometimes used as their neutral amine (or metal) salts. The salts are produced
by reacting an equivalent weight of the base with that of the acid (reaction 3.13). The choice of base
will depend on whether oil or water solubility is required. The use of short-chain amines will nor-
mally result in water-soluble additives, whereas using, for example, tertiaryalkyl primary amines
with a chain length of C11–14 will tend to produce oil-soluble derivatives. The chain length of the
acid phosphate also infl uences the solubility. The selection of the appropriate mixture of amine and
phosphate for a given application is largely a compromise because the most active mixtures may
also produce disadvantageous side effects, for example, on foaming and air release properties. The
fact that a neutral salt is used also does not prevent the product from titrating as an acid and from
forming a different salt in the presence of the stronger base.

RNH^2 (R O) PO(OH) (R O) PO(OH H NR)

1

12 21

1

 

 
→ 12  2 21 
^ (3.13)