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54 Lubricant Additives: Chemistry and Applications

and alkyl mercaptan. A white precipitate will also form, which has been determined to be a low-

sulfur- containing zinc pyrophosphate. The oil phase will contain varying amounts of S,S,S-tri -

alkyltetrathiophosphate, O,S,S-trialkyltrithiophosphate, and O,O,S-trialkyldithiophosphate depend-

ing on the alkyl chain and the extent of degradation. The decomposition products of ZDDPs made

from secondary alkyl alcohols, straight-chain primary alkyl alcohols, and branched primary alkyl

alcohols appear similar in content but differ in proportions. This implies a similar mechanism for

both primary and secondary ZDDP decomposition.

O-alkyl thiphosphate esters are powerful alkylating agents. The P–O–R group is susceptible

to nucleophilic attack, thus producing an alkylated nucleophile and thiophosphate anion. The

incoming nucleophile initiates the reaction by an attack on the alpha carbon. This shows a kinetic

dependence on alkyl structure

−

ROP

+ O

−

Nu−R+

(2.9)

Steric hindrance to the approach of the nucleophile will play a large rate-controlling factor here.

The only nucleophile initially present is the dithiophosphate itself. The decomposition is initiated

by one dithiophosphate anion attacking another, possibly on the same zinc atom:

−

(2.10)

The resulting di-anion then attacks the triester, producing O,S-dialkyldithiophosphate anion

SSR

−

(2.11)

resulting in the migration of an alkyl group from oxygen to sulfur. This anion then, in a route analo-

gous to the dialkyldithiophosphate anion, reacts with itself in a nucleophilic attack to effect another

alkyl transfer from oxygen to sulfur producing O,S,S-trialkyldithiophosphate

RS SR

(2.12)

The net effect of the above reactions is a double alkyl migration from oxygen to sulfur

−

(2.13)

The major gases associated with ZDDP decomposition are dialkylsul de (RSR), alkyl mercaptan

(RSH), and ole n. The relative amounts of each of these gases depend on whether the alkyl group in

the ZDDP is primary, branched primary, or secondary [9]. In the presence of mercaptide anion (RS

–

)

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Zinc Dithiophosphates 55

from the intermediate zinc mercaptide (Zn[RS

]), O,S,S-trialkyldithiophosphate will react with mer-

captide to produce alkyl mercaptan and results in the following structure:

−

(2.14)

The nucleophilic phosphoryl oxygen (P=O) will then attack another phosphorus atom to produce a

P–O–P bond as in the following reaction:

−

SRP

OP P

−

(2.15)

A mercaptide anion subsequently cleaves the P–O–P bond at the original P–O site, giving rise to a

net exchange of one atom of oxygen for one atom of sulfur between the two phosphorus atoms:

−

SR SRP

P O

−

OPP

(2.16)

This gives rise to a net reaction for conversion of Structure 2.14 to S,S,S-trialkyltetrathiophosphate,

dialkylsul de and S-alkylthiophosphate di-anion as shown in the following reaction:

−

++R

(2.17)

The dialkylsul de and S,S,S-trialkyltetrathiophosphate decomposition products are soluble in oil.

The S-alkylthiophosphate decomposition product can also react with itself by way of a phos-

phoryl nucleophilic attack and elimination of mercaptide anion as in Reaction 2.15. This process

will continue until a zinc pyro- and polypyrophosphate molecule with low sulfur content is formed.

The chain will continue to extend until the product precipitates out of solution.

The decomposition of primary alkyl ZDDPs can be accurately described as discussed earlier.

ZDDPs made from branched primary alcohols will decompose in a similar fashion, although at a

much slower rate. This can be explained by the fact that the alpha carbon of the branched primary

alkyl group, being more sterically hindered than the unbranched primary alkyl group, will be less

susceptible to nucleophilic attack, as described in Reaction 2.9. The increased steric hindrance from

beta carbon branching will also decrease the amount of successful mercaptide anion attack on the

branched alkyl P–O–R bond, resulting in less dialkylsul de formation and a higher yield of mercap-

tan, an ole n by-product (through a competing protonation or elimination reaction with mercaptide

anion). Lengthening the alkyl chain will have a much less pronounced effect on thermal stability

than branching at the beta carbon due to the greater steric hindrance derived from the latter.

The decomposition of secondary alkyl ZDDPs, although similar to primary decomposition,

shows that ole n formation is much more pronounced. The increase in elimination over nucleo-

philic substitution in secondary ZDDPs over primary ZDDPs is easily explained by the fact that

elimination is accelerated by increasing the alkyl substitution around the double bond formed.

Thus, secondary alkyl groups will favor a thermal decomposition into ole ns and phosphate

acids at the expense of the sulfur–oxygen interchange noted earlier. In a similar but much more

pronounced way, tertiary ZDDP decomposition will be dominated by facile production of ole n

through elimination. This occurs at even moderate temperatures, making their use in commercial

applications prohibitive.

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56 Lubricant Additives: Chemistry and Applications

Aryl ZDDPs, due to the stability of the aromatic ring, are not susceptible to nucleophilic

attack. Thus, the initial thermal decomposition reaction described in Reaction 2.9 cannot occur.

Also the formation of ole n from an acid-catalyzed elimination reaction cannot occur. Aryl ZDDPs

are, therefore, very thermally stable.

A rating of various ZDDPs in terms of thermal stability would, therefore, be aryl > branched

primary alkyl > primary alkyl > secondary > tertiary. The varying amounts of decomposition

products that depend on the heat history and the alkyl or aryl chain involved will directly control the

amount of EP and wear protection the ZDDP will provide in a given circumstance [10].

Hydrolysis of ZDDP begins with cleavage of the carbon–oxygen bond of the thiophosphate

ester, with the hydroxide anion displacing the thiophosphate-anion-leaving group. The stability

of the intermediate alkyl cation determines the ease with which this cleavage takes place. The

secondary alkyl cation is more stable and more easily formed than the primary alkyl cation;

therefore, hydrolysis of secondary ZDDP occurs more easily than hydrolysis of primary ZDDP.

For the case of an aryl ZDDP, the carbon–oxygen bond cannot be broken, and the site of hydrolytic

attack is the phosphorus–oxygen bond with the displacement of phenoxide anion with hydroxide

anion. The order of hydrolytic stability is, therefore, primary > secondary > aryl.

2.5 OXIDATION INHIBITION

Base oils used in lubricants degrade by an autocatalytic reaction known as auto-oxidation. The

initial stages of oxidation are characterized by a slow, metal-catalyzed reaction with oxygen to form

an alkyl-free radical and a hydroperoxy-free radical as seen in the following reaction:

RH O R HOO

⫹⫹

→

(2.18)

This reaction is propagated by the reaction of the alkyl-free radical with oxygen to form an alkylp-

eroxy radical. This radical further reacts with the base oil hydrocarbon to form alkyl hydroperoxide

and another alkyl radical as seen in the following reaction:

R O ROO ROOH R

⫹⫹

→→

(2.19)

This initial sequence is followed by chain branching and termination reactions forming high-

molecular-weight oxidation products [11].

The antioxidant functionality of ZDDP is ascribed to its af nity for peroxy radicals and

hydroperoxides in a complex pattern of interaction.

The initial oxidation step of ZDDP by hydroperoxide is the rapid reaction involving the oxidative

formation of the basic ZDDP salt as seen in the following reaction:

R′OHZn

OOOHR′Zn

(RO)













(RO)













(RO)













(2.20)

In this reaction, 1 mol of alkyl hydroperoxide converts 4 mol of neutral ZDDP to 1 mol of basic ZDDP

and 2 mol of the dialkyldithiophosphoryl radical (which subsequently reacts to produce the disul-

 de) [12]. The rate of hydroperoxide decomposition slows during an induction period during which

the basic zinc thermally breaks down into the neutral ZDDP and zinc oxide [6]. This is followed

by the neutral ZDDP further reacting with hydroperoxide to produce more dialkyldithiophosphoryl

disul de and more basic ZDDP. When the concentration of the basic ZDDP becomes low enough,

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Zinc Dithiophosphates 57

a  nal rapid neutral salt-induced decomposition of the hydroperoxide will occur in which the

dialkyldithiophosphoryl radical will not react with itself to form the disul de but will react with

hydroperoxide to form the dialkyldithiophosphoric acid as seen in the following reaction [13]:

R′OOH(RO)

•

R′OO

•

(RO)

PSH +

(2.21)

The dialkyldithiophosphoric acid then rapidly reacts with alkyl hydroperoxide, producing oxidation

products that are inactive in oxidation chain reactions. The simplest reaction scheme for the reduction

of the hydroperoxide is seen in the following reaction:

R′OH H

OR′OOH

(RO)PSH

+++

(RO)













(2.22)

Oxidation products include the disul de mentioned earlier, the analogous mono- and trisul des,

and compounds of the form (RO)

(RS)

3–n

P=S and (RO)

(RS)

3–n

P=O [3]. These products show

little activity as either oxidation inhibitors or antiwear agents.

The literature also reveals an ionic process that will produce more dialkyl-dithiophosphoric

acid as seen in the following reaction:

R′OO

•

R′OO

−

(RO)PS

(RO)

PS Zn

(RO)

•

(2.23)

followed by Reaction 2.21 [14].

At low concentrations of ZDDP, hydrolysis of the ZDDP to the zinc basic double salt and

dialkyldithiophosphoric acid becomes viable. At temperatures >125°C, the dialkyldithiophos-

phoryl disul de decomposes into the dialkyldithiophosphoryl radicals, which further react with

hydroperoxide to produce more dialkyldithiophosphoric acid [6]. Thus, many pathways are avail-

able to form the active dialkyldithiophosphoric acid.

The neutral ZDDP also reacts with alkyl peroxy radicals. This is an electron-transfer mechanism

that involves the stabilization of a peroxy intermediate. An attack by a second peroxy radical leads

to the intramolecular dimerization of the resulting dithiophosphate radical forming the inactive

dialkyldithiophosphoryl disul de as seen in the following reaction:

•

2 RO

−

(2.24)

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58 Lubricant Additives: Chemistry and Applications

The zinc metal atom provides an easy route for heterolysis of the radical intermediate; thus, the

disul de, by itself, has little antioxidant functionality [15]. ZDDP acts as an oxidation inhibitor

not only by trapping the alkyl radicals, thus slowing the chain reaction mechanism, but also by

destroying alkyl hyperoxides and inhibiting the formulation of alkyl radicals. Empirical determi-

nation of the relative antioxidant capability of the three main classes of ZDDP shows secondary

ZDDP > primary > aryl ZDDP. The relative performance of each ZDDP type may correlate with

the stabilization of the dialkyl(aryl)dithiophosphoryl radical and its subsequent reactivity with

alkyl hydroperoxide to produce the catalyzing acid.

Commercial ZDPs are a mixture of both neutral and basic salts. It has recently been

determined that neutral and basic ZDDPs give essentially equivalent performance with respect to

antioxidant behavior. This can be explained by the equilibrium shown in Reaction 2.8. At elevated

temperatures, as would occur in an oxidation test, the basic ZDDP is converted into the neutral

ZDDP. As the temperature is lowered, the equilibrium shifts back toward the formation of the

basic ZDDP, indicating that the concentration of basic ZDDP as a function of temperature. The

solvent used and the presence of other additives also play a role in this equilibrium. Thus, the exact

composition of neutral versus basic salts at any time in an actual formulation is a complex function

of many variables.

2.6 ANTIWEAR AND EXTREME-PRESSURE FILM FORMATION

ZDDPs operate mainly as antiwear agents but exhibit mild EP characteristics. As an antiwear

agent, ZDDP operates under mixed lubrication conditions with a thin oil  lm separating the metal

parts. Surface asperities, however, intermittently penetrate the liquid  lm, giving rise to metal-on-

metal contact. The ZDDP reacts with these asperities to reduce the contact. Likewise, when the load

is high enough to collapse the oil  lm, the ZDDP reacts with the entire metal surface to prevent

welding and to reduce wear. A great deal of study has been done to determine the nature of this

protective  lm and the mechanism of deposition, where the thermal degradation products of the

ZDDP are the active antiwear agents.

The antiwear  lm thickness and composition are directly related to temperature and the

extent of surface rubbing. Initially, ZDDP is reversibly absorbed onto the metal surface at low

temperatures. As the temperature increases, catalytic decomposition of ZDDP to dialkyldithio-

phosphoryl disul de occurs, with the disul de absorbed onto the metal surface. From here, the

thermal degradation products (as described in Section 2.3) are formed with increasing tempera-

ture and pressure until a  lm is formed on the surface [16]. The thickness and composition of this

 lm have been studied using many different analytical techniques, but no analysis gives a concise

description of the  lm size and composition for the various kinds of metal-to-metal contact found

in industrial and automotive lubrication regimes. In general, the antiwear/EP ZDDP  lm can be

said to be composed of various layers of ZDDP degradation products. Some of these degradation

products are reacted with the metal making up the lubricated surface. The composition of the lay-

ers is temperature-dependent.

The  rst process that takes place is the reaction of sulfur (from the ZDDP thermal degrada-

tion products) with the exposed metal leading to the formation of a thin iron sul de layer [17].

Next, phosphate reacts to produce an amorphous layer of short-chain ortho- and metaphosphates

with minor sulfur incorporation. The phosphate chains become longer toward the surface, with

the minimum chain length approaching 20 phosphate units. Some studies have indicated that this

region is best described as a phosphate “glass” region in which zinc and iron cations act to stabilize

the glass structure. At the outermost region of the antiwear  lm, the phosphate chains contain

more and more organic ligands, eventually giving way to a region composed of organic ZDDP

decomposition products and undegraded ZDDP itself. The thickness of the  lm has been analyzed

to be as small as 20 nm using ultra thin  lm interferometry and as large as 1 μm using electrical

capacitance [18–21].

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Zinc Dithiophosphates 59

Recent work has concluded that, although the rate of  lm formation is directly proportional

to temperature, a stronger correlation exists between  lm formation and the extent of metal-to-

metal rubbing as quanti ed by the actual distance that the metal slides during a given test period.

The  lm reaches a maximum thickness at which point a steady state between formation and

removal exists, the rate of formation being more temperature-dependent than the rate of removal.

It was also found that the ZDDP reaction  lm has a “solid-like” nature (as opposed to be a highly

viscous liquid) due to the lack of reduction of  lm thickness observed with time on a static test

ball [22].

Another mechanism of wear found to be inhibited by ZDDP is wear produced from the reaction

of alkyl hydroperoxides with metal surfaces. It was found that the wear rate of automobile engine

cam lobes is directly proportional to alkyl hydroperoxide concentration. The mechanism proposes

the direct attack of hydroperoxide (generally through fuel combustion and oil oxidation) on fresh

metal, causing the oxidation of an iron atom from a neutral charge state to Fe

by reaction with 3

mol of alkyl hydroperoxide as described in the following reactions:

222

ROOH Fe RO OH Fe⫹⫹⫹

⫺⫹

→

∗

(2.25)

ROOH Fe RO OH Fe⫹⫹⫹

⫹⫺⫹23

→

∗

(2.26)

The ZDDP and its thermal degradation products neutralize the effect of the hydroperoxides by the

mechanism described in Reactions 2.20 through 2.23 in Section 2.5. It was also shown that peroxy

and alkoxy radicals were far less aggressive toward metal surfaces than hydroperoxides, indicating

that free-radical scavengers such as hindered phenols would be ineffective in controlling this kind

of engine wear. This may explain why the antiwear performance of ZDDP is directly related to its

antioxidation performance in the order of secondary ZDDP > primary ZDDP > aryl ZDDP rather

than correlating with the order of thermal stability (aryl > primary > secondary) [23].

A recent study has been conducted to investigate the difference in wear performance between

neutral and basic ZDDPs in the sequence VE engine test. The neutral ZDDP performed better in

value train wear protection than the basic ZDDP. The basic salt actually failed the sequence VE

engine test, indicating that using commercial ZDDPs with lower basic salt content may be preferred

when limited to 0.1% maximum phosphorus content (as mandated by the International Lubricant

Standardization and Approval Committee [ILSAC] GF-3 motor oil speci cation). It was suggested

that the increased wear protection by neutral ZDDP could be explained by the superior adsorption

of the oligomeric structure of the neutral salt, leading to the formation of longer polyphosphate

chains relative to the basic salt [5].

2.7 APPLICATIONS

ZDDPs are used in engine oils as antiwear and antioxidant agents. Primary and secondary ZDDPs

are both used in engine oil formulations, but it has been determined that secondary ZDDPs perform

better in cam lobe wear protection than primary ZDDPs. Secondary ZDDPs are generally used

when increased EP activity is required (i.e., during run-in to protect heavily loaded contacts such as

valve trains). ZDDPs are generally used in combination with detergents and dispersants (alkaline

earth sulfonate or phenate salts, polyalkenyl succine amides or Mannich-type dispersants), viscosity

index improvers, additional organic antioxidants (hindered phenols, alkyl diphenyl amines), and

pour point depressants. A typical lubricant additive package for engine oils can run in high at

25% in treatment level. The ILSAC has designated its GF-3 engine oil speci cation to include a

maximum limit of 0.1% phosphorus to minimize the engine oil’s negative impact on the emissions

catalyst. For the GF-4 speci cation, the limit in phosphorus was reduced even further. As a result

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60 Lubricant Additives: Chemistry and Applications

of the minimum phosphorus requirement, the treatment level for ZDDP in organic oils is limited to

∼0.5 to 1.5%, depending on the alkyl chain length used.

The new challenge to motor oil formulators is in passing the required ILSAC tests while keeping

the ZDDP level low. Yamaguchi et al. have shown that the antioxidant effect of ZDDP is signi -

cantly enhanced in API group II base stocks with as much as 50% increase noted for a basic ZDDP.

An increase in antioxidancy was also noted when using ZDDPs in polyol ester [24]. Several studies

have also shown that ZDDP oxidation by-products are in effective antiwear agents. The use of these

base-stock effects to extend the oxidation life of the ZDDP may be a suitable method for the formu-

lator to reduce the level of ZDDP needed to accommodate the GF-3 limits.

The synergistic effect between organic molybdenum compounds and ZDDP in wear reduction

is currently being studied as a means of lowering phosphorus content in engine oils. In U.S. patent

5,736,491, molybdenum carboxylate is used with ZDDP to give a synergistic reduction in friction

coef cient by as much as 30%, thus allowing a reduction in the total phosphorus content and an

improvement in fuel economy [25]. The patent literature has sited other organic molybdenum com-

pounds such as molybdenum dithiocarbamates (MoDTC) and dialkyldithiophosphates (MoDTP) as

being useful, synergistic secondary antiwear agents [26].

ZDDPs are also used in hydraulic  uids as antiwear agents and antioxidants. The treatment level

for ZDDP in hydraulic  uids is lower than that used for engine oils, typically running between 0.2

and 0.7% by weight. They are used in combustion with detergents, dispersants, additional organic

antioxidants, viscosity index improvers, pour point depressants, corrosion inhibitors, defoarmers,

and demulsi ers for a total treatment level of between 0.5 and 1.25% [27]. Primary ZDDPs are

preferred over secondary ZDDPs due to their better thermal and hydrolytic stability. One problem

faced by hydraulic  uid formulators is the need for a  uid that will service both high-pressure

rotary vane pumps and axial piston pumps, preferably out of the same sump. High-pressure vane

pumps require a hydraulic  uid with antiwear properties and oxidative stability commonly achieved

through the use of ZDDPs. High-pressure piston pumps need only rust and oxidation protection and

do not require ZDDPs. ZDDPs can cause catastrophic failure to axial piston systems by adversely

affecting the sliding steel–copper alloy interfaces. The patent literature has several examples of

formulators trying to overcome this problem with the use of additional wear-moderating chemis-

tries such as sulfurized ole ns, polyol esters or borates of them, fatty acid imidazolines, aliphatic

amines, and polyamines. Another problem faced by hydraulic  uid formulators is the interaction

of ZDDPs with overbased alkaline earth detergent salts (as well as the interaction of carboxylic

acid and alkenyl succinic anhydride rust preventatives with these detergents) in the presence of

water to give  lter-clogging by-products. Formulators have tried to overcome this problem of poor

“wet”  lterability by using nonreactive rust inhibitors (i.e., alkenyl succinimides) and improving the

hydrolytic stability of ZDDP antiwear agent [28].

ZDDPs are used in EP applications such as gear oils, greases, and metalworking  uids.

Secondary ZDDPs are preferred due to their thermal instability resulting in quick  lm formation

under high loads. In automotive gear oils, ZDDPs are used at 1.5–4% in combination with EP agents

(such as sulfurized ole ns), corrosion inhibitors, foam inhibitors, demulsi ers, and detergents. Total

multifunctional additive package treatment levels for automotive gear lubricant additives are from

5 to 12% by weight. Industrial gear oil formulators have generally gone to ashless systems using

sulfur–phosphorus-based EP antiwear chemistries at total additive package treatment levels of

1.5–3%. In general, the recent focus in gear oil technology improvement has centered on increased

thermal stability and EP properties.

ZDDPs are used in greases in chemical systems that closely resemble gear oil formulations.

Many gear oil lubricant additives are used in EP greases. In general, the ZDDP treatment level

for greases is in the same range as that used for gear oils. ZDDP, usually secondary or a mixture

of secondary and primary, is used in combination with sulfurized ole ns, corrosion inhibitors,

ashless antioxidants, and additional friction modi ers. A recent advancement in grease technology

is the use of ZDDP/sulfurized ole n synergy to replace antimony and lead in high-EP grease

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Zinc Dithiophosphates 61

formulations. This has generally been limited to the European market, having been pioneered in

Germany.

ZDDPs, in combination with sulfurized ole ns, are also used to replace chlorinated paraf ns

in medium- to heavy-duty metalworking  uids. This is due to the possible carcinogenicity of the

low-molecular-weight analogs of chlorinated paraf n. European formulators, and to a certain extent

Japanese formulators, use ZDDPs in this way. The use of ZDDPs in metalworking  uids in the

United States is limited due to environmental concerns. The U.S. Environmental Protection Agency

classi es them as marine pollutants.

In conclusion, after 50 years, ZDDPs still enjoy a wide variety of uses in the lubrication indus-

try, with production volumes remaining at high levels. The majority of ZDDP production is used

in automobile engine oil. The impact of the GF-2 and GF-3 phosphorus-level speci cation of 0.1%,

however, was reduction of ZDDP production in the past 10 years. The Ford Motor Company is

currently evaluating engine oils with 0–0.6% phosphorus levels in  eet tests in preparation for the

looming GF-4 standard in 2004, which will require engine oils to have minimal impact on emis-

sion system deterioration. This could further negatively impact ZDDP production. The need to

understand clearly how ZDDPs function in terms of wear and oxidation protection is reinforced by

the need to develop satisfactory phosphorus-free alternatives to ZDDP. The development of such

chemistries, within the economic and functional limits that ZDDPs impose, will be a daunting task

for future researchers. Until that time, the elimination of ZDDPs from various industrial lubricants

will mandate either higher costs or less performance.

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Ashless Phosphorus–

Containing Lubricating

Oil Additives

W. David Phillips

CONTENTS

3.1 Introduction and Scope ...........................................................................................................64

3.2 Historical Background ............................................................................................................65

3.3 Manufacture of Phosphorus-Containing Lubricating Oil Additives ......................................68

3.3.1 Neutral Alkyl and Aryl Phosphate Esters ....................................................................68

3.3.1.1 Natural Phosphates..........................................................................................69

3.3.1.2 Synthetic Phosphates from Isopropylphenols ................................................. 70

3.3.1.3 Synthetic Phosphates from Tertiarybutylphenols ........................................... 70

3.3.2 Acid Phosphate Esters ..................................................................................................71

3.3.2.1 Alkyl and Aryl Acid Phosphates (Non-ethoxylated) ...................................... 71

3.3.2.2 Alkyl- and Alkylarylpolyethleneoxy Acid Phosphates ...................................72

3.3.3 Amine Salts of Acid Phosphates and of Polyethyleneoxy Acid Phosphates ................72

3.3.4 Neutral Phosphite Esters ...............................................................................................73

3.3.5 Alkyl and Aryl Acid Phosphites ................................................................................... 73

3.3.6 Dialkyl Alkyl Phosphonates ......................................................................................... 74

3.4 The Function of Lubricity Additives ....................................................................................... 74

3.4.1 The Basic Mechanism of Lubrication and Wear and the In uence of Additives ......... 75

3.5 Investigations into the Mechanism and Activity of Phosphorus-Containing Additives ......... 79

3.5.1 Early Investigations into Antiwear and Extreme Additives .........................................80

3.5.2 Neutral Alkyl and Aryl Phosphates ..............................................................................80

3.5.2.1 Historical Background ....................................................................................80

3.5.2.2 Recent Technical Developments .....................................................................86

3.5.2.3 Recent Commercial Developments .................................................................89

3.5.3 Alkyl and Aryl Acid Phosphates .................................................................................. 91

3.5.3.1 Non-ethoxylated .............................................................................................. 91

3.5.3.2 Alkyl and Alkarylpolyethleneoxy Acid Phosphates .......................................92

3.5.3.3 Amine Salts of Acid Phosphates .....................................................................95

3.5.4 Neutral Alkyl and Aryl Phosphites ..............................................................................98

3.5.4.1 Use as Antiwear/Extreme-Pressure Additives ................................................98

3.5.4.2 Use as Antioxidants for Lubricating Oils .......................................................99

3.5.5 Alkyl and Aryl Acid Phosphites ................................................................................. 100

3.5.5.1 Amine Salts of Acid Phosphites.................................................................... 102

3.5.6 Phosphonate and Phosphinate Esters.......................................................................... 103

3.5.7 A Summary of the Proposed Mechanism for Antiwear and Extreme-Pressure

Activity of Phosphorus-Based Additives ....................................................................104

3.6 Market Size and Commercial Availability............................................................................ 105

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