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64 Lubricant Additives: Chemistry and Applications

3.7 Toxicity and Ecotoxicity ....................................................................................................... 108

3.8 The Future for Ashless Phosphorus-Based Lubricating Oil Additives ................................. 110

3.9 Lubricating Oil Formulations (General) ............................................................................... 111

3.10 Hydraulic Oils ....................................................................................................................... 111

3.11 Automotive Engine Oils ........................................................................................................ 111

3.12 Fuels ...................................................................................................................................... 113

3.13 Conclusions ........................................................................................................................... 113

References ...................................................................................................................................... 114

Appendix A: Early Patent Literature on Phosphorus-Containing Compounds ........................... 118

Neutral Phosphates ................................................................................................................ 118

Neutral Phosphites ................................................................................................................ 118

As Antioxidants .................................................................................................................... 118

Acid Phosphates/Phosphites ................................................................................................. 118

Phosphonates ......................................................................................................................... 119

Alkyl- and Arylpolyethyleneoxy-Phosphorus Compounds .................................................. 119

Amine Salts ........................................................................................................................... 119

Physical Mixtures of Phosphorus and Sulfur and Chlorine Compounds ............................. 119

Miscellaneous Phosphorus Compounds ............................................................................... 120

Appendix B: Additional Literature and Patent References on the Mechanism and

Performance of Phosphorus-Containing Additives .............................................................. 120

Neutral Phosphates ................................................................................................................120

Neutral Phosphites ................................................................................................................ 121

Alkoxylated Phosphates ........................................................................................................ 121

Amine Salts ........................................................................................................................... 121

Dialkyl Alkyl Phosphonates ................................................................................................. 121

Mixtures of Phosphorus and Sulfur Compounds .................................................................. 122

General References ............................................................................................................... 122

3.1 INTRODUCTION AND SCOPE

In any discussion of phosphorus-containing lubricating oil additives, the products that probably

come most rapidly to mind are the zinc dialkyldithiophosphates (ZDDPs)—multifunctional addi-

tives that have been widely used in both automotive and industrial oils for many years. However, a

wide variety of ashless phosphorus-containing additives are used in the lubricating oil industry. As

with the metal-containing dithiophosphates, they have been in use over a long period and, despite

considerable research into alternative chemistries, the basic structures introduced in the 1930s are

still used today. In contrast, the technology of most other additive types and that of the base stocks

themselves have steadily developed over this time.

Many different types of phosphorus-containing molecules have been examined as additives for

lubricating oils, with most attention given to their potential as antiwear (AW) and extreme- pressure

(EP) additives. Consequently, the patent literature contains a host of references to different struc-

tures displaying this characteristic. However, regardless of composition, all the additives used in

this application serve the same and speci c function of bringing phosphorus into contact with the

metal surface, where it can be adsorbed and, under certain conditions, react. The resulting surface

 lm improves the lubrication properties of both mineral and synthetic oils.

This chapter discusses the use of chemicals that contain only phosphorus to improve the

performance characteristics of oils, speci cally neutral and acid phosphates, phosphites and phos-

phonates, and the amine salts of the acids (see Figures 3.1 and 3.2 for an outline of the main classes

and their structures). These are the principal types of phosphorus compounds in current commercial

use, but other types have also been examined and claimed in the patent literature, for example, phos-

phoramidates. There are also ashless compounds in which sulfur or chlorine has been incorporated

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Ashless Phosphorus-Containing Lubricating Oil Additives 65

into the molecule as, for example, in thiophosphates and chlorinated phosphates. These are, how-

ever, outside the scope of this discussion, but the performance of mixtures of compounds separately

containing phosphorus and sulfur or chlorine will be mentioned.

In addition to examining the impact of ashless phosphorus compounds on lubrication

performance, this chapter also looks at their performance as antioxidants, rust inhibitors, and metal

passivators. Additionally, their polar nature makes them good solvents and assists the solution of

other additives in nonpolar base stocks. The versatility displayed by phosphorus-containing addi-

tives is such that usage of these products continues to grow nearly half a century after their intro-

duction, and they  nd application in the latest technological developments.

3.2 HISTORICAL BACKGROUND

Until the 1920s, additive-free mineral oils met the majority of industry’s lubrication requirements.

In the applications where their performance was unsatisfactory, an increase in viscosity or the sul-

fur content of the oils then available usually provided adequate lubrication. For very severe applica-

tions, the oil would be blended with animal or vegetable oils—for example, tallow or rapeseed oils

were used for steam engine cylinder lubrication. Fish oils were used in the early locomotive axle

boxes, while castor oil reduced friction in worm gear drives and  owers of sulfur were added to

cutting oils. However, when hypoid gears were introduced, they quickly revealed the limited lubri-

cation of oils then available. This resulted in the development of additives such as sulfurized lard

oil and lead naphthenate. These were followed by sulfurized sperm oil, an additive that eventually

became widely used in both industrial and automotive applications.

The earliest type of an organic phosphorus chemical to  nd use as a lubricating oil additive

is thought to have been a neutral triaryl phosphate, speci cally tricresyl phosphate (TCP). This

material was originally synthesized in about 1854 [1] although trialkyl phosphates were synthe-

sized slightly earlier, in about 1849 [2]. Commercial production of TCP began in about 1919, when

Phosphate esters

Phosphites and phosphonates

Alkyl or aryl phosphates

Neutral phosphates Acid phosphates

Non-ethoxylated

Ethoxyalkyloxy-

Amine salts

Neutral amine salts

Alkyl or aryl phosphites/phosphonates

Neutral phosphites/phosphonates Acid phosphites/phosphonates

FIGURE 3.1 The main classes of phosphorus-containing additives for lubricating oils.

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66 Lubricant Additives: Chemistry and Applications

this product was introduced as a plasticizer for cellulose nitrate, but it was not until the 1930s that

patents began to appear claiming improved lubrication when TCP was blended with mineral oil. In

1936, this use was claimed in gear oils [3], but a detailed investigation into their behavior as AW

additives was not published until 1940 [4,5], by which time TCP was already said to be in wide-

spread use. During World War II, extensive research into phosphorus-containing additives took

place in Germany [6,7]. This research was facilitated by the recent availability of test equipment for

assessing wear and load-carrying behavior, for example, the four-ball machine [8]. The results of

the research concluded that for high load-carrying (later known as EP) performance, the molecule

must contain the following:

• A phosphorus atom

• Another active group, for example, Cl

–

or OH

–

(for attachment to the metal surface)

• At least one aryl or alkyl group (phosphoric acid was not thought to be active)

P ORRO

OH.H

NRRO

RRO

OHRO

RRO

ORRO

OHRO

OHO

Trialkyl phosphate

Alkyl monoacid phosphate

Triaryl phosphate

Trialkyl phosphite

Dialkyl phosphite

Amine phosphate

Triaryl phosphite

Dialkyl alkyl phosphonate

Alk

l dialk

l phosphinate

Aryl diacid phosphate

FIGURE 3.2 The structures of some common phosphorus-containing lubricating oil additives.

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Ashless Phosphorus-Containing Lubricating Oil Additives 67

Subsequent to these studies, the market adopted chlorine-containing phosphates such as tris-

(2-chloroethyl) phosphate, but they were later replaced in most applications by other EP additives as

chlorine tended to produce corrosion.

The 1940s and 1950s saw signi cant development activity in the oil industry involving TCP,

and patents appeared claiming the use of this AW/EP additive in general industrial oils [9], rolling

oils [10], cutting oils [11], greases [12], rock drill lubricants [13], and aviation gas turbine lubricants

[14,15]. Some military speci cations, for example, on hydraulic oils (NATO codes H515/520/576),

were published, which initially called for the use of this additive. However, in the late 1960s, the

dif culty of obtaining good-quality feedstocks for the manufacture of natural phosphates based on

cresol and xylenol, together with the concern regarding the neurotoxicity of TCP [16,17], led to the

reformulation of many products with the less toxic synthetic triaryl phosphates based on alkylated

phenols. TCP is still used today in aviation applications, but the quality of the phosphate in terms of

its purity and freedom from the o-cresol isomers that were mainly responsible for the neurotoxicity

behavior, has signi cantly improved in the past 10–20 years.

In addition to its use as an oil additive, TCP was also used for a period in the 1960s as an igni-

tion control additive for motor gasoline to avoid preignition arising from the deposition of lead salts.

These were formed by the interaction of the lead tetraethyl antiknock additive and the alkyl halide

scavenger [18–21]. Alkyl phosphates were claimed for this application in 1970 [22].

As a result of their polar nature, neutral triaryl phosphates have also been claimed as corrosion

inhibitors for hydrocarbons [23,24], but they are unlikely to be promoted for this application today

in view of the availability of more active species, such as the acid phosphates.

The use of trialkyl phosphates as AW and EP additives has been much less extensively evalu-

ated. Although a  urry of patent activity took place in the late 1920s and 1930s covering methods

for their manufacture [25–34], there was little interest in their use as lubricating oil additives for

further 20 years. This was probably a result of the focus, in the interim period, on chlorinated

derivatives. It was not until the late 1950s that tributyl phosphate (TBP) was disclosed in blends with

isopropyl oleate [35] for use in gear oils and claimed in blends with chlorinated aromatics. In 1967,

a patent appeared claiming the use of alkyl phosphates or the amine salts of alkyl acid phosphates

in a water-based lubricating composition [36].

In addition to alkyl phosphates, various other types of phosphorus-containing compounds have

been evaluated as AW/EP additives. Patents on acid phosphates claiming their use as EP additives

for oil appeared in 1935 and 1936 [37–39], whereas the  rst publication with detailed information on

the use of ethoxylated alkyl or aryl phosphate oil additives (in metalworking applications) appeared

in 1964 [40]. Patents on the use of these products in mineral oils [41] and in synthetic esters [42]

appeared later. Alkoxylated acid phosphates were also found to have good rust inhibition properties

[43], a feature that was additionally observed for the alkyl (or aryl) acid phosphates [44,45].

Neutral amine salts of alkyl acid phosphates were claimed in 1964 [43] and, in 1969, in admixture

with neutral phosphates [46]. These are, however, just a few examples of the patent estate covering

these product groups.

The other main phosphorus-containing products to be discussed are the phosphites. The basic

chemistry of alkyl and aryl phosphites, like that of the phosphate esters, was also uncovered in the

nineteenth century. In a similar fashion, their utilization as oil additives was not exploited until

much later. Patents appeared in 1940 on the use of mixed aryl phosphites as oil antioxidants [47]

and on their activity as AW/EP additives at least as early as 1943 [48].

Isomeric with the acid phosphites are phosphonates (Figure 3.2). Dialkyl alkyl phosphonates

were claimed as lubricants in 1952 and 1953 [49,50] but not until about 1971 as friction modi ers

and EP additives [51,52].

The preceding summary focused on the use of phosphorus compounds alone. In reality, they

are widely used in admixtures with sulfur-containing materials to provide good lubrication over

a wider range of performance requirements. Examples of some of the combinations patented are

given in Appendix A.

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68 Lubricant Additives: Chemistry and Applications

3.3 MANUFACTURE OF PHOSPHORUS-CONTAINING

LUBRICATING OIL ADDITIVES

3.3.1 N

EUTRAL 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

) is reacted with either an alcohol (ROH), a phenol (ArOH),

or an alkoxide (RONa) as indicated in the following reactions:

3ROH POCl (RO) PO 3HCl

trialkyl phosphate

⫹⫹→

(3.1)

3ArOH POCl (ArO) PO 3HCl

triaryl phosphate

⫹⫹→

(3.2)

3RONa POCl (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

→→  →() ( 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

is  rst reacted with

suf cient tertiarybutylphenol (C

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  uid base stock.

C H ArOH + POCl C H ArOPOCl C H ArOPO(OPh) + 2

49 3 49 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 signi cant use as AW/EP additives.

ROH POCl (RO) POCl (RO) PO(OAr)

ArONa

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 puri cation treatment. After a water wash to remove the NaCl,

puri cation consists of a distillation step to remove excess alcohol, an alkaline wash, and a  nal

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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  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  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 dif 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

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70 Lubricant Additives: Chemistry and Applications

composition, has little impact on AW properties. Of greater importance are the actual phosphorus

content and the level of impurities present, particularly those that are acidic.

In the past, the tri-o-cresyl phosphate content was a source of much concern in view of the high

neurotoxicity of the material (see Section 3.19). However, the feedstock that is most widely used

today in the production of TCP, is predominantly a mixture of m- and p-cresol, and o-cresol levels

are extremely low.

3.3.1.2 Synthetic Phosphates from Isopropylphenols

In this case, phenol is alkylated with propylene to produce a mixture of isomers of isopropylated

phenol (Figure 3.4). Depending on the reaction conditions and the degree of alkylation, it is pos-

sible to produce a range of isopropylphenyl phosphates (IPPPs) with viscosities varying from ISO

VG 22 to VG 100. In seeking an alternative product to TCP (an ISO 32 viscosity-grade  uid), the

products with the closest phosphorus contents and viscosities (IPPP/22 and IPPP/32) are most

widely used.

3.3.1.3 Synthetic Phosphates from Tertiarybutylphenols

In a similar fashion to the manufacture of isopropyphenyl phosphates, it is possible to produce a

range of phosphates from butylated phenols prepared by the reaction of isobutylene with phenol

(Figure 3.4). The tertiarybutyl substituent is larger in size than the isopropyl substituent, and this

Tricres

hos

hate

Trix

hos

hate

FIGURE 3.3 The structures of tricresyl and trixylyl phosphate.

OH OH OHOH

+++

OH OH OH

ortho-PhenolPropylene meta-

meta- para-

para-

Isomers of isopropyl phenol

Isomers of tertiarybutyl phenol

Butylene Phenol

FIGURE 3.4 Process for the production of feedstocks used in the manufacture of synthetic phosphates.

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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 3ROH ROP(OH) (RO) POH

25 2

monoalkyl acid

phosphate

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

, C

–C

alcohols, mixtures of

10–12

and alcohols, and C

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 ArOH ArOPOCl (ArO) POCl ArOPO(OH)

322

monoaryl dia

⫹⫹→→

−

ccid

phosphate

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

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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

364 643 642

⫹⫹

⫺

⭈→ ()()

(3.9)

RC H OPOCl ArOH RC H O ArO) PO RC H OArOPO OH

64 2 64 2 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 C H PO Cl C H O) PO OH

17 2

817

817 282

⫺⫺

⭈⭈→  →() (

(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 signi 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 RO(C H O) H [RO(C H O) ] PO OH RO(C H

24 24

24 2 2

⫹⫹

nn n

→→

⭈

442

O) PO(OH)

(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 C

11–14

will tend to produce oil-soluble derivatives. The chain length of the

acid phosphate also in 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 (R O) PO(OH) (R O) PO(OH H NR)

12 21

12 2 21

⫹

⫺⫺ ⫺ ⫺

⭈→

(3.13)

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Ashless Phosphorus-Containing Lubricating Oil Additives 73

where R is an alkyl group, typically C

−

. It is also possible to use secondary and tertiary amines,

NH and R

N in the production of these salts.

3.3.4 NEUTRAL PHOSPHITE ESTERS

As with phosphate esters, it is possible to produce both neutral and acid phosphite esters. The neu-

tral triaryl phosphites are produced by reacting phosphorus trichloride with a phenol or substituted

phenol (reaction 3.14).

PCl 3ArOH (ArO) P 3HCl

triaryl phosphite

⫹⫹→

(3.14)

This is also a stepwise process occurring through the production of the monoaryl and diaryl hydro-

gen phosphite intermediates.

The production of neutral trialkyl phosphites using PCl

requires the addition of a tertiary

amine base to neutralize the acid formed (reaction 3.15). Unless the HCl is removed quickly, it can

cause the process to reverse with the production of an alkyl halide and the dialkyl acid phosphite

(reaction 3.16).

PCl 3ROH 3R N P(OR) 3R NHCl

333

trialkyl phosphite

⫹⫹ ⫹→

(3.15)

P(OR) HCl RCl (RO) POH

dialkyl acid phosphite

⫹⫹→

(3.16)

The use of mixtures of different alcohols or different phenols can result in the production of mixed

alkyl or aryl phosphites. Mixed alkylaryl phosphites can be produced by reacting triaryl phosphite

with alcohols to give a mixture of aryldialkyl- and alkyldiaryl phosphites (reaction 3.17). A com-

mercial example of such a product promoted as an oil additive is decyldiphenyl phosphite.

(ArO) P ROH (ArO) POR ArOH ArOP

alkyldiaryl phosphite

⫹⫹→→

ROH

((OR) 2ArOH

aryldialkyl phosphite

⫹

(3.17)

Because of their widespread use in the plastics industry as stabilizers for polyvinylchloride, etc.,

many different neutral phosphites are commercially available. These range from C

to C

alkyl

(normally saturated) and from C

1–9

alkaryl. In lubricant applications, the most common products

are those with alkyl chains of C

−

and C

−

alkaryl, for example, trioctyl phosphite and tris-

(2,4-ditertiarybutylphenyl) phosphite. The last type is increasingly important in view of its better

hydrolytic stability.

3.3.5 ALKYL AND ARYL ACID PHOSPHITES

Alkyl and aryl acid phosphites are manufactured by reacting together phosphorus trichloride, an

alcohol (or phenol), and water (reaction 3.18):

PCl ROH H O RO POH HCl

dialkyl acid phosphite

322

23⫹⫹ ⫹→()

(3.18)

Mixtures of alcohols, as indicated earlier, may also be used to produce di-mixedalkyl phosphites.

The commercially available dialkyl acid phosphites vary from C

to C

with use as oil additives

falling mainly in the range of C

−

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