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94 Lubricant Additives: Chemistry and Applications

Test conditions: 40 kg, 100 rpm, 121°C, 60 min

Four-ball wear scar diameter (mm)

0.2

0.4

0.6

0.8

0204060

Ethylene oxide (wt%)

(a)

Test conditions: 100 kg, 100 rpm, 121°C, 60 min

Four-ball wear scar diameter (mm)

0.2

0.4

0.6

0.8

0204060

Ethylene oxide (wt%)

(b)

Oleyl alcohol 40 kg

Dinonyl phenol 40 kg

Oleyl alcohol 100 kg

Dinonyl phenol 100 kg

FIGURE 3.14 Effect of ethylene oxide content on wear properties 1% of additive in 100 SUS (100°F) naph-

thenic base oil.

conditions. Of greater relevance than conventional four-ball or pin and v-block tests are actual

cutting or tapping torque tests. The results given in Table 3.12 show that (1) products with a hydro-

philic–lipophilic balance (HLB) value of 11–12 give the best results and (2) in general, the further

the value deviates from this, the worse the results become. Unfortunately, no studies appear to have

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Ashless Phosphorus-Containing Lubricating Oil Additives 95

been made on the nature of surface  lm deposited on the metal, but the adsorption mechanism

indicated previously is probably still valid.

3.5.3.3 Amine Salts of Acid Phosphates

One amine phosphate that appeared in the patent literature as early as 1934 as a corrosion inhibitor

for aqueous systems (and is still occasionally used) is triethanolamine phosphate [114]. Formed by

the neutralization of phosphoric acid with triethanolamine, this product was widely used as a cor-

rosion inhibitor for automotive antifreeze formulations for many years [115].

In 1970, Forbes and Silver [116] reported on their work investigating the effect of chemical

structure on the load-carrying properties of different phosphorus compounds. In this case, the

structures under review were di-n-butylphosphoramidates, amine salts of di-n-butyl phosphate, and

derivatives of dialkylphosphinic and alkylphosphonic acids. The results indicated that the phos-

phoramidates were more effective load-carrying additives than the neutral phosphates, TBP and

TCP, but less active than the amine salts of di-n-butyl phosphate. The evaluation of the series

of dialkylphosphinic and alkylphosphonic acid esters indicated that the AW performance related

directly to the strength of the acid from which they had been produced (Figure 3.15), suggesting that

adsorption through the polarity of the ester group was an important step in the process.

In addition to the work carried out in hydrocarbon base stocks, some testing was also performed

in a synthetic ester. This  uid enabled a comparison to be made of tetra-alkylammonium salts of

dibutylphosphate (otherwise insoluble in mineral oil), which displayed the best AW/EP properties

of all the amine phosphates tested (Table 3.13). The authors suggested that this was probably due to

the stability of the ions.

TABLE 3.12

Phosphate Ester Surfactant Ranking on Steel in a Water-Based System as a Function

of Structure

Alcohol

Phosphate

EO Units

Pin-on-Vee

Block

Rankings of

Sample

Four-Ball

Wear

Tapping

Torque

Total of

Rankings

Overall

Rating

HLB

Number

C9–16 5.5 4 1 4 9 1 13

C18 4 6 4 2 12 2 12

Nonylphenol 4 10 2 1 13 3 11

C13 10 2 9 5 16 4 14

C12 6 1 12 7 20 5 12

C8–10 6 8 7 6 21 6 11.5

C12 12 5 8 12 25 7 15

C13 6 9 6 10 25 8 11.5

C12 9 7 5 14 26 9 14

Nonylphenol 6 14 3 9 26 10 8

Phenol 6 12 13 3 28 11 15.4

Dinonylphenol 5 3 14 11 28 12 9

Nonylphenol 9.5 11 11 8 30 13 13

C13 4 13 10 15 38 14 9.7

butanediol 6 15 15 13 43 15 —

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96 Lubricant Additives: Chemistry and Applications

A further study of the mechanism of amine phosphates by Forbes and Upsdell appeared in

1973 [117]. Adsorption/reaction studies of dibutyl and di-2-ethylhexyl phosphates with either n-

octylamine or cyclohexylamine and iron powder showed that both the amine and the acid phosphate

were adsorbed onto the metal surface and that the rate and extent of their adsorption/desorption

varied with chemical structure. The higher the solubility of the iron–phosphate complex formed,

3.5

3.0

2.5

2.0

1.5

of acid

Four-ball wear scar diameter (mm)

0.3 0.4 0.5 0.6 0.65

1: n-butyl di-n-octylphosphinate

2: Di-n-butyl hexylphosphonate

3: Di-n-butyl phenylphosphonate

4: Tri-n-butyl phosphate

5: Diethyl benzyl phosphonate

6: Diethyl o-nitrophenylphosphonate

FIGURE 3.15 Effect of acid strength on AW performance. (From Forbes, E.S., Silver, H.B., J. Inst. Pet.

56(548), 90–98, 1970. With permission.)

TABLE 3.13

Four-Ball Test Results of Various Amine Dibutyl Phosphates in Diisooctyl Sebacate

EP Test AW Test

(BuO)

MHL (kg) WL (kg) ISL (kg) 30 min

WSD (mm)

after 45 min 60 min

40.5 130 125 0.38 0.38 0.36

40.0 130 115 0.37 0.38 0.39

PhNH

43.6 140 120 0.37 0.38 0.39

[nC

]NH

34.8 140 100 0.26 0.27 0.33

[nC

]

NH 37.8 150 110 0.37 0.38 0.42

[nC

]

N 54.4 150 150 0.25 0.25 0.26

[CH

]

N 56.1 165 165 0.30 0.35 0.40

None 18.6 120 55 0.57 0.61 0.64

TCP 19.8 110 60 0.31 0.33 0.35

Note: % additive = 4 milliatoms of P/100 g of  uid. (MHL, Mean Hertz Load; WL, Weld Load; ISL, Incipient Sei-

zure Load; WSD, Wear Scar Diameter.)

Source: Forbes, E.S., Silver, H.B., J. Inst. Pet., 56(548), 90–98, 1970. With permission.

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Ashless Phosphorus-Containing Lubricating Oil Additives 97

the greater was the likelihood of desorption. Furthermore, good AW performance depended on high

phosphate and amine adsorption and retention of the phosphate moiety on the surface.

The conversion of a dialkyl acid phosphite to an amine phosphate and the use of the mixed

product as a multifunctional AW/EP additive, antioxidant, and corrosion inhibitor with improved

metal passivation properties were claimed in 1997 [118].

An amine salt and TCP were studied as AW agents for different synthetic esters by Weller and

Perez [119] and compared with a sulfurized hydrocarbon. The neutral ester (TCP) generally showed

an increasing amount of wear up to 1% addition before reducing at higher levels. The amine salt,

however, rapidly reduced wear to very low levels. Friction coef cients were also consistently lower

with the amine salt.

Kristen [120] reported the effect of additive interaction between amine phosphates and a phos-

phorothionate. The additives were evaluated under FZG gear test conditions (DIN 51354). The

results showed that the additives respond differently in nonpolar and polar base stocks, speci cally

a polyalphaole n and a synthetic ester (Figures 3.16 and 3.17). In the synthetic hydrocarbon base, a

level of 0.75% amine phosphate and 0.25% phosphorothionate (or perhaps 0.5% of each) provided a

borderline FZG 12 load stage pass/fail. In comparison, 0.75% of amine phosphate ester and 1% of

phosphorothionate were required to achieve the same level of performance in the ester. Monitoring

the response of additive combinations reveals not only the most cost-effective mixtures but also any

antagonisms between additives, as were found here in the ester base at higher additive levels. Such

information is invaluable to formulators when trying to meet speci cation requirements and ensur-

ing that the performance level is consistently above the minimum limit.

Amine salts, for example, triethanolamine salts of alkyl and arylpolyethyleneoxy acid phos-

phates, are widely used in metalworking applications. Some of these products are not only

commercially available but are also produced in situ when the pH of the product is adjusted by the

addition of base to ensure the product is alkaline in use. This is to avoid corrosion and minimize

skin irritation.

0.50 1.0 1.5 2.0

0.50 1.0 1.5

Additive concentration (%)

TPPT (%)

>12

Amine phosphate

TPPT

FZG load stage failure

Amine phosphate (%)

1.0

0.5

1.5

12 load stage

>12

FIGURE 3.16 FZG performance of an amine phosphate and TPPT separately and in mixtures in an ISO VG

32 polyalphaole n. (From Kristen, U., Additive für Schmierstoffe, Expert Verlag, Renningen-Malmshaim,

German, 1994. With permission.)

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98 Lubricant Additives: Chemistry and Applications

3.5.4 NEUTRAL ALKYL AND ARYL PHOSPHITES

3.5.4.1 Use as Antiwear/Extreme-Pressure Additives

The earliest known reference to the evaluation of phosphites as AW/EP additive is in a 1950 paper

by Davey [75]. As a result of these investigations, which also included a comparison with phosphates

and the effect of incorporating chlorine into the phosphate/phosphite molecule, it was found that

• Phosphites have superior EP properties to the phosphates, and long alkyl chains are more

effective than aryl groups.

• Evaluation of TBP and TXP revealed similar optimum concentrations of between 1 and 2%

as were found in the previous study with TCP [5].

• Polar compounds such as acids or esters improve the lubricating (AW) properties of phos-

phites and phosphates by being strongly adsorbed on to the surface.

• The incorporation of chlorine or sulfur into the molecule (or the addition of small amounts

of free sulfur) improves the EP properties. Chlorine is more effective when part of an alkyl

residue, and when sulfur is added to a P/Cl compound (e.g., a chlorinated phosphite), the EP

properties are further improved.

Following the study by Davey, a number of patents appeared claiming the use of phosphites

in lubricant applications [121–124], but it was not until 1960 that a further detailed study of

the behavior of phosphites, this time by Sanin et al. [125], was published. The study emphasized

the correlation between structure and activity, and the short-chain derivatives were found to be the

most active.

In 1993, Ohmuri and Kawamura [126] carried out fundamental studies into the mechanism of

action of phosphite EP additives. They found that initial adsorption rates of phosphorus- containing

FIGURE 3.17 FZG performance of an amine phosphate and TPPT separately and in mixtures in an ISO VG 22

pentaerythritol ester. (From Kristen, U., Additive für Schmierstoffe, Expert Verlag, 1994. With permission.)

0.50 1.0 1.5 2.0

0.50 1.0 1.5

Additive concentration (%)

TPPT (%)

>12

Amine phosphate

TPPT

1.0

0.5

1.5

Amine phosphate (%)

FZG load stage failure

12 load stage

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Ashless Phosphorus-Containing Lubricating Oil Additives 99

esters depended largely on the existence of

–

OH and

–

P=O bonds in the structure. The extent of

adsorption was in uenced by the hydrolytic stability of the esters, and this process was found to

occur through reaction with water adsorbed onto the iron surface. Adsorption of the phosphites

varied depending on the degree of esteri cation; triesters were adsorbed after being decom-

posed hydrolytically to monoesters, whereas diesters were adsorbed without hydrolysis. Phos-

phite esters eventually hydrolyzed to inorganic acid regardless of the degree of esteri cation,

followed by its adsorption and conversion to the iron salt. It was suggested the adsorbing and

hydrolyzing properties of the esters depended on the arrangement of the molecules physisorbed

onto the surface.

Evaluation of a range of alkyl phosphites as EP additives in gear oils was reported by Riga and

Rock Pistillo [127]. The most effective products were those with short chains, particularly dibutyl

phosphite, which resulted in a wear layer of >1000 Å and the formation of both iron phosphate and

phosphide. Other phosphites formed only traces of phosphide, and as the chain length increased,

the resulting  lm became thinner and contained less phosphorus, possibly due to steric hindrance.

Long-chain (C

) alkyl phosphites have also been claimed as AW additives for aluminum rolling oil

[128] and, in fact, are still used in metalworking applications.

In view of the work carried out on the use of phosphates as vapor-phase lubricants, an investiga-

tion into the effect of phosphites on the frictional properties of ceramic-on-ceramic and ceramic-

on-metal surfaces was carried out in 1997 [129]. The phosphites (and other additives evaluated) had

no effect on ceramic-on-ceramic friction; in fact, short-chain phosphites signi cantly increased

friction. When several types of metal were slid against oxide ceramics, the alkyl phosphites were

found to lower the friction for each metal except copper. Apparently, the reaction products between

copper and the phosphite had adhesive properties and increased friction.

The decomposition of trimethylphosphite on a nickel surface was also studied to obtain insight

into the initial steps in the decomposition of phosphates when used as vapor-phase lubricants [130].

The main breakdown path is the cleavage of the –P–O– bond to yield the methoxy species, which

then degrades to CO and H

or reacts with the nickel surface. Following heating to 700°K, the sur-

face loses adsorbed species other than phosphorus, which is seen as a simple way for the controlled

deposition of phosphorus onto a metal surface.

3.5.4.2 Use as Antioxidants for Lubricating Oils

In addition to their use as AW/EP additives, neutral (and acid) phosphite esters have long been used

as antioxidants or stabilizer for hydrocarbons. They were originally introduced as stabilizers for

rubber and thermoplastics. Trisnonylphenyl phosphite, for example, was  rst used to stabilize sty-

rene-butadiene rubber in the early 1940s; this was shortly followed by patents claiming phosphites

as antioxidants for lubricants [48,122,123,131,132].

Phosphites function as decomposers of hydroperoxide, peroxy, and alkoxy radicals (reactions

3.20 through 3.22) rather than eliminating the hydrocarbyl-free radicals formed in the chain initia-

tion process. They also stabilize lubricants against photodegradation [133].

R OOH (RO) P (RO) P O R OH

hydroperoxide

⫹⫽⫹→

(3.20)

ROO (RO)P RO (RO)P O

alkylperoxy

radical

••

→⫹⫹⫽

(3.21)

R O (RO) P R OP(RO) RO

alkoxy radical

••

→⫹⫹

(3.22)

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100 Lubricant Additives: Chemistry and Applications

This behavior as secondary antioxidants by destroying the hydroperoxides, etc., formed in the

chain propagation process results in their use in synergistic combination with those antioxidant

types that are active as radical scavengers in the initiation process, for example, the hindered phe-

nols and aromatic amines [134–138].

Phosphites are useful additives because of their multifunctionality. However, although they

are still used as antioxidants in hydrocarbon oils, their relatively poor hydrolytic stability and

the formation of acidic compounds that could affect the surface active properties of the oil have

prompted the introduction of “hindered” phosphites with better hydrolytic stability: for example,

tris-(2,4-ditertiarybutylphenyl) phosphite or tris-(3-hydroxy-4,6-ditertiarybutylphenyl) phosphite,

and, where solubility permits, cyclic phosphites, for example, based on pentaerythritol such as bis-

(2,4-ditertiarybutylphenyl) pentaerythritol diphosphite (Figure 3.18). These types are claimed as

stabilizers or costabilizers for lubricating oils [139–142].

Table 3.14 [141] illustrates the signi cant improvement in oxidation stability shown by such

blends.

In common with most other types of phosphorus-containing products, neutral (and acid) phos-

phites have also been claimed as corrosion inhibitors [143,144].

3.5.5 ALKYL AND ARYL ACID PHOSPHITES

As might be predicted from the behavior of the other types of phosphorus-containing additives, the

acid phosphites have good AW/EP properties; the nonylphenyl acid phosphite is particularly effec-

tive [145,146]. When used in aviation gas turbine lubricants, the acid phosphites were sometimes

formulated in combination with neutral phosphates (TCP); blends of the two products showed syn-

ergy even when the amount of the phosphite was very low [147]. The acid phosphites are also

claimed to be corrosion inhibitors [144] and antioxidants [47,149,150].

The effects of structure on the AW and load-carrying properties of dialkyl phosphites was

studied by Forbes and Battersby [151] in a liquid paraf n. AW performance was best with long-

chain compounds (Figure 3.19), whereas the short-chain (highest phosphorus content) derivatives

displayed the best load-carrying performance.

Scuf ng behavior, however, appeared to reach a minimum at about a C

carbon chain length

(Figure 3.20). This parallels the behavior of the neutral phosphites. Adsorption studies also showed

that the phosphorus content of the solution was depleted in the same order of the load-carrying

performance, namely, the most active products showed the highest loss from solution. The pres-

ence of water increased the uptake of phosphorus from solution. Comparison of the performance

Tris-(2,4-ditertbutylphenyl) phosphite

Bis-(2,4-ditertbutylphenyl) pentaerythritol diphosphite

FIGURE 3.18 Structures of some commonly available hindered phosphites.

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Ashless Phosphorus-Containing Lubricating Oil Additives 101

of the phosphites against the corresponding acid phosphate revealed that the phosphites had better

load-carrying but inferior AW behavior (see Table 3.15). The authors suggest that the activity of the

phosphites is due to an initial hydrolysis to produce the following intermediate either in solution or

on the metal surface:

HO O

This reacts with the iron surface to give an iron salt that was thought to be responsible for the

AW properties of the product. Under much more extreme conditions as are found with scuf ng,

TABLE 3.14

Antioxidant Synergism between Hindered Aryl Phosphites and a Hindered Phenol

Oxidation Stability

Base

Stock Antioxidant

% Viscosity

Change

Total Acid

Number Increase

1 Hindered phenol (0.5%) 357 11.5

Hindered phosphite A (0.5%) 438 12.2

Hindered phenol (0.1%) + phosphite A (0.4%) 8.7 0.01

Hindered phenol (0.17%) + phosphite A (0.33%) 9.4 0.06

2 Hindered phenol (0.5%) 712 14.2

Hindered phosphite B (0.5%) 452 10.6

Hindered phenol (0.1%) + phosphite B (0.4%) 8.1 0.05

Hindered phenol (0.17%) + phosphite B (0.33%) 8.7 0.03

Note: Hindered phenol is tetrakis-(methylene-3,5-ditert-butyl-4-hydroxy hydrocinnamate) methane; Phosphite

A is tri-(2,4-ditert-butylphenyl) phosphite; Phosphite B is bis-(2,4-ditert-butylphenyl) pentaerythritol

diphosphite. Test conditions: IP 48 (modi ed), 200°C for 24 h, air at 15 1/h in an ISO VG 32 mineral oil.

Source: U.S. Patent 4,652,385, Petro-Canada Inc., 1987.

Ethyl

n-Butyl

2-Ethylhexyl

Lauryl

Stearyl

Cyclohexyl

Compounds blended

at 4 mmol/100g

liquid paraffin

2 4 6 8 10 12 14 16 18

0.2

0.3

0.4

0.5

0.6

0.7

Carbon chain length

Wear scar diameter (mm)

FIGURE 3.19 Effect of chain length on the four-ball AW performance of dialkyl phosphites. (From Forbes,

E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.)

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102 Lubricant Additives: Chemistry and Applications

the aforementioned salt was thought to decompose further to give the phosphorus-rich layer of the

following type:

The authors postulated the load-carrying mechanism of phosphites given in Figure 3.21.

3.5.5.1 Amine Salts of Acid Phosphites

In 1975, Barber [152] investigated the four-ball test performance of several long-chain amine salts of

short-chain acid phosphites, which were found to be very active. Unfortunately, he did not investigate

Ethyl

n-butyl

2-Ethylhexyl

Lauryl

Stearyl

Cyclohexyl

Compounds blended

at 4 mmol/100g

liquid paraffin

2 4 6 8 10 12 14 16 18

100

120

140

160

150

130

110

210

200

190

180

170

220

Carbon chain length

Initial seizure load (kg)

FIGURE 3.20 Effect of chain length on the initial seizure loads of dialkyl phosphites. (From Forbes, E.S.,

Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.)

TABLE 3.15

Comparison of the Load-Carrying Properties of Dialkyl Phosphates

and Dialkyl Phosphites at 4 mmol/100 g Base Oil

Initial Seizure

Load (kg)

Wear Scar Diameter

(60 min) mm

Diethyl phosphite 225 0.70

Diethyl phosphate 160 0.43

Dibutyl phosphite 155 0.64

Dibutyl phosphate 85 0.42

Di-2-ethylhexyl phosphite 125 0.36

Di-2-ethylhexyl phosphate 80 0.29

Dilauryl phosphite 130 0.32

Dilauryl phosphate 80 0.35

Source: Forbes, E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.

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Ashless Phosphorus-Containing Lubricating Oil Additives 103

the effect of increasing the chain length of the phosphite while reducing the length of the amine.

Most of the paper concerns the behavior of a wide range of phosphonate esters (see Section 3.16).

3.5.6 PHOSPHONATE AND PHOSPHINATE ESTERS

A large group of phosphonate esters was prepared by Barber in 1975 [152] and evaluated using the four-

ball machine. Although short-chain esters were more effective in preventing scuf ng, the most effective

products were those containing chlorine. However, even at high levels of chlorine, the performance was

still inferior to the amine phosphite reaction products reported earlier. In comparison with TCP, incipient

seizure loads were generally higher, but the weld loads were broadly similar. Unfortunately, there were

no direct comparative data under wear test conditions. A limited number of phosphinate esters were

evaluated and found (also by four-ball tests) to give similar performance to the phosphonate esters.

The activity of a range of phosphonates was studied by Sanin et al. [153], who concluded that

their effectiveness depended on their structure and the friction regime, but esters containing no

chlorine had “no effect at either low or high load.” A further study, by the same authors, of the

mechanism of activity of phosphonates again suggested the reaction of decomposition products

with iron and the formation of a protective layer. Under severe conditions, this layer is removed,

resulting in a sudden increase in friction followed by seizure or welding. Studies of the reaction of

a dibutyltrichlorophosphonate (Cl

CPO(OBu)

) indicated that a reaction took place at 405–408K

to give chlorobutane and an iron-containing polymer. At 413K, this polymer decomposed to give

FeCl

, which gave additional protection.

Phosphonate (and pyrophosphonate) esters, as their metal or amine salts, have appeared in the

patent literature over many years as AW/EP additives. Amine salts of dinonylphosphonate are, for

example, claimed in aircraft gas turbine lubricants [154], and dimethyltetradecyl phosphonate has

(a) Hydrolysis

(b) Reaction

Reaction with iron surface

Adsorption Hydrolysis in solution

Antiwear region

Antiscuff region

Hydrolysis

FIGURE 3.21 Mechanism of load-carrying action of dialkyl phosphites. (From Forbes, E.S., Battersby, J.,

ASLE Trans., 17(4), 263–270, 1974. With permission.)

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