Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 71

reduces the overall level of alkylation in the molecule necessary to achieve the same viscosity,
resulting in the presence of more unsubstituted phenyl groups. Again, the TBPPs from this range,
which are used as AW additives in mineral oil, are those closest in phosphorus content and viscosity
to TCP, that is, TBPP/22 and TBPP/32 (Table 3.1).

3.3.2 ACID PHOSPHATE ESTERS

3.3.2.1 Alkyl and Aryl Acid Phosphates (Non-ethoxylated)

The manufacture of acid phosphates, particularly alkyl acid phosphates, is also based on technology
that has its roots in the nineteenth century but was commercialized only during the past 50 years.
The process involves the reaction of phosphorus pentoxide with an alcohol in the absence of water
(reaction 3.7).

P O 25 3 ROH ROP (OH) 2 (RO) POH

monoalkyl acid

phosphate

2

dialkyl ac

→

iid

phosphate

(3.7)

The ratio of monophosphate to diphosphate is usually 40–50% monophosphate and 50–60% diphos-
phate, with very small amounts of phosphoric acid (⩽1%). Small amounts of neutral ester may
also be produced. Products commercially available are made from C 5 , C 7 –C 9 alcohols, mixtures of
C10 –12 and alcohols, and C 18 alcohols.
Monoaryl and diaryl acid phosphates (also known as monoaryl and diaryl hydrogen phosphates)
are by-products in the manufacture of triaryl phosphates and may be produced by stopping the reac-
tion before completion and hydrolyzing the intermediate phosphorochloridates (reaction 3.8).

POCl 32 ArOH ArOPOCl (ArO) POCl 2 OH ArOPO(OH) 2

monoaryl dia

→→

−

ccid

phosphate

2

diaryl monoacid

phosphate

(ArO) PO OH

(3.8)

Di(alkyl)aryl monoacid phosphates, soluble in mineral oil, are reported to be produced by react-
ing phosphorus oxychloride with an alkylated phenol in the presence of base (reaction 3.9) or by the
reaction of monoarylphosphorodichloridate with an alkylated phenol (reaction 3.10). The reactions

TABLE 3.1

Phosphorus Contents and Viscosity Levels for Neutral Trialkyl

and Triaryl Phosphate AW Additives

Phosphate Ester

Phosphorus

Content (%)

Typical Viscosity

at 40°C (cSt)

TiBP 11.7 2.9

TOP 7.8 7.9

TBEP 7.1 6.7

TCP 8.3 25

TXP 7.8 43

IPPP/22 8.3 22

IPPP/32 8.0 32

TBPP/22 8.5 24

TBPP/32 8.1 33

TBPP/100 7.1 95