Lubricant Additives

68 Lubricant Additives: Chemistry and Applications

3.3 MANUFACTURE OF PHOSPHORUS-CONTAINING

LUBRICATING OIL ADDITIVES

3.3.1 NEUTRAL ALKYL AND ARYL PHOSPHATE ESTERS

Although phosphate esters can be regarded as salts of orthophosphoric acid, they are currently not
produced from this raw material because the yields are relatively low (∼70% for triaryl phosphates).
Instead, phosphorus oxychloride (POCl 3 ) is reacted with either an alcohol (ROH), a phenol (ArOH),
or an alkoxide (RONa) as indicated in the following reactions:

3ROH POCl 33 (RO) PO 3HCl

trialkyl phosphate

→ (3.1)

3ArOH POCl 33 (ArO) PO 3HCl

triaryl phosphate

→ (3.2)

3RONa POCl 33 (RO) PO 3NaCl

trialkyl phosphate

→ (3.3)

Reactions 3.1 through 3.3 pass through intermediate steps as shown in the following reaction:

ROH POCl ROPOCl HCl RO POCl HCl RO) PO HC

ROH ROH

 32 →→() 2   →( 3 ll

(^) (3.4)
The intermediate products are called phosphorochloridates, and, if desired, it is possible to obtain
a mixture rich in a particular intermediate by changing the ratio of reactants.
In 2001, a two-stage process for the production of a tertiarybutylphenyl phosphate with low lev-
els of triphenyl phosphate (TPP) was described [53]. In this process, the POCl 3 is fi rst reacted with
suffi cient tertiarybutylphenol (C 4 H 9 ArOH) to produce mainly the diphosphorochloridate. Phenol is
then added to the reaction mixture to produce predominantly the mono-tertiarybutylphenyl diphe-
nyl phosphate (reaction 3.5). This product was said to give unusually low air entrainment values and
was therefore mainly of interest as a hydraulic fl uid base stock.
C H ArOH + POCl^493 C H ArOPOCl^492 C H ArOPO(OPh) + 2
PhOH
→→ 49 2 HHCl (3.5)
The production of mixed products can be achieved by using different alcohols or by an alcohol and
alkoxide (reaction 3.6). These materials are not in signifi cant use as AW/EP additives.
ROH POCl 32 (RO) POCl ArONa (RO) PO(OAr) 2
mixed alkyl aryl phos
 →→
pphate
NaCl (3.6)
Trialkyl (or alkoxyalkyl) phosphates can be produced by either reaction 3.1 or 3.3, although in reac-
tion 3.1, unless catalyzed, a considerably excess alcohol is required to drive the reaction to comple-
tion. The hydrogen chloride (HCl) by-product is removed as rapidly as possible—usually by vacuum
or water washing while the reaction temperature is controlled to minimize the thermal degradation
of the phosphate. In the alkoxide route (reaction 3.3), the chlorine precipitates as sodium chloride
(NaCl), somewhat simplifying the purifi cation treatment. After a water wash to remove the NaCl,
purifi cation consists of a distillation step to remove excess alcohol, an alkaline wash, and a fi nal