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Several effective antioxidant classes have been developed over the years and have seen use in engine oils, automatic transmission fl uids, gear oils, turbine oils, compressor oils, greas

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1 Lubrication and lubricants Additives I Rudnick, Leslie R., 1947- II Title III Series.

Jun Dong and Cyril A Migdal

Chapter 2 Zinc Dithiophosphates 51

PART 2 Film-Forming Additives

Chapter 6 Selection and Application of Solid Lubricants as Friction Modifi ers 173

Gino Mariani

Chapter 7 Organic Friction Modifi ers 195

Dick Kenbeck and Thomas F Bunemann

PART 3 Antiwear Additives and Extreme-Pressure Additives

Chapter 8 Ashless Antiwear and Extreme-Pressure Additives 213

Liehpao Oscar Farng

Chapter 9 Sulfur Carriers 251

Thomas Rossrucker and Achim Fessenbecker

PART 4 Viscosity Control Additives

Chapter 10 Olefi n Copolymer Viscosity Modifi ers 283

PART 5 Miscellaneous Additives

Chapter 13 Tackifi ers and Antimisting Additives 357

Victor J Levin, Robert J Stepan, and Arkady I Leonov

Chapter 14 Seal Swell Additives 377

Ronald E Zielinski and Christa M A Chilson

Chapter 15 Antimicrobial Additives for Metalworking Lubricants 383

William R Schwingel and Alan C Eachus

Chapter 16 Surfactants in Lubrication 399

Chapter 19 Additives for Crankcase Lubricant Applications 457

Ewa A Bardasz and Gordon D Lamb

Chapter 20 Additives for Industrial Lubricant Applications 493

Chapter 23 Additives for Grease Applications 585

Robert Silverstein and Leslie R Rudnick

PART 7 Trends

Chapter 24 Long-Term Trends in Industrial Lubricant Additives 609

Fay Linn Lee and John W Harris

Chapter 25 Long-Term Additive Trends in Aerospace Applications 637

Carl E Snyder, Lois J Gschwender, and Shashi K Sharma

Chapter 26 Eco Requirements for Lubricant Additives 647

Tassilo Habereder, Danielle Moore, and Matthias Lang

PART 8 Methods and Resources

Chapter 27 Testing Methods for Additive/Lubricant Performance 669

Preface

Lubricant additives continue to be developed to provide improved properties and performance to

modern lubricants

Environmental issues and applications that require lubricants to operate under severe conditions

will cause an increase in the use of synthetics Owing to performance and maintenance reasons,

many applications that have historically relied on petroleum-derived lubricants are shifting to

synthetic lubricant-based products Cost issues, on the contrary, tend to shift the market toward

group II and III base oils where hydrocarbons can be used Shifts to renewable and biodegradable

fl uids are also needed, and this will require a greater need for new effective additives to meet the

challenges of formulating for various applications

There are several indications that the lubricant additive industry will grow and change

Legislation is driving changes to fuel composition and lubricant components, and therefore, future

lubricant developments will be constrained compared to what has been done in the past Registration,

Evaluation, Authorisation and Restriction of Chemicals (REACh) in the European Union (EU) is

placing constraints on the incentive to develop new molecules that will serve as additives The cost

of introduction of new proprietary materials will be the burden of the company that develops the new

material For many common additives that are produced by several manufacturers, they will share

costs to generate any needed data on the toxicology or biodegradability of the materials

Continued progress toward new engine oil requirements will require oils to provide improved

fuel economy and to have additive chemistry that does not degrade emission system components

This will require a new test to evaluate the volatility of phosphorus in engine oils and to improve

the oil properties in terms of protecting the engine Future developments and requirements will

undoubtedly require new, more severe testing protocols

The market for lubricant additives is expected to grow China and India, for example, represent

highly populated markets that are expected to see growth in infrastructure, and therefore a growth

in industrial equipment and number of vehicles Many U.S and EU companies continue to develop

a presence in Pacifi c and Southeast Asia through either new manufacturing in that region or sales

and distribution offi ces

More advanced technologies will require application of new types of lubricants, containing

new additive chemistries required for exploration of space and oceans Since these remote locations

and extremes of environment require low maintenance, they will place new demands on lubricant

properties and performance

This book would not have developed the way it has without the invaluable help and

encour-agement of many of my colleagues I want to thank all of the authors of the chapters contained

herein for responding to the deadlines There is always a balance between job responsibilities and

publishing projects like this one My heartfelt thanks to each of you It is your contributions that

have created this resource for our industry

I especially want to thank Barbara Glunn, at Taylor & Francis Group, with whom I have worked

earlier on Synthetics, Mineral Oils and Bio-Based Lubricants, for her support to this project from

its early stages through its completion I also want to thank Kari Budyk, project coordinator,

who has been invaluable in every way in the progress of this project and has been a tremendous

asset to me as an editor and helpful to the many contributors of this book I also want to thank

Jennifer Derima, Jennifer Smith, and the team at Macmillan Publishing Solutions for their efforts,

patience, and understanding during the time I have been working on this book

I also thank Paula, Eric, and Rachel for all of their support during this project

Les Rudnick

PolyMod Technologies Inc

Fort Wayne, Indiana

Villa Park, Illinois

Liehpao Oscar Farng

ExxonMobil Research and Engineering

Arkady I Leonov

University of AkronAkron, Ohio

Victor J Levin

Functional Products Inc

Macedonia, Ohio

Gino Mariani

Acheson Colloids Company

Port Huron, Michigan

Ciba Specialty Chemicals plc

Process & Lubricant Additives

Macclesfi eld, Cheshire,

United Kingdom

W David Phillips

W David Phillips and Associates

Stockport, Cheshire, United Kingdom

Robert Silverstein

Orelube CorporationBellport, New York

Carl E Snyder

AFRL/RXBTWright-Patterson Air Force Base, Ohio

Joan Souchik

EvonikRohMax USA, Inc

PolyMod Technologies Inc

Fort Wayne, Indiana

Part 1

Deposit Control Additives

Jun Dong and Cyril A Migdal

CONTENTS

1.1 Introduction 4

1.2 Sulfur Compounds 5

1.3 Sulfur–Nitrogen Compounds 6

1.4 Phosphorus Compounds 7

1.5 Sulfur–Phosphorus Compounds 8

1.6 Amine and Phenol Derivatives 10

1.6.1 Amine Derivatives 10

1.6.2 Phenol Derivatives 13

1.6.3 Amine and Phenol-Bearing Compounds 13

1.6.4 Multifunctional Amine and Phenol Derivatives 13

1.7 Copper Antioxidants 16

1.8 Boron Antioxidants 17

1.9 Miscellaneous Organometallic Antioxidants 18

1.10 Mechanisms of Hydrocarbon Oxidation and Antioxidant Action 18

1.10.1 Autoxidation of Lubricating Oil 19

1.10.1.1 Initiation 19

1.10.1.2 Chain Propagation 19

1.10.1.3 Chain Branching 19

1.10.1.4 Chain Termination 20

1.10.2 Metal-Catalyzed Lubricant Degradation 20

1.10.2.1 Metal Catalysis 21

1.10.3 High-Temperature Lubricant Degradation 21

1.10.4 Effect of Base Stock Composition on Oxidative Stability 21

1.10.5 Oxidation Inhibition 23

1.10.6 Mechanisms of Primary Antioxidants 24

1.10.6.1 Hindered Phenolics 24

1.10.6.2 Aromatic Amines 26

1.10.7 Mechanisms of Secondary Antioxidants 28

1.10.7.1 Organosulfur Compounds 28

1.10.7.2 Organophosphorus Compounds 28

1.10.8 Antioxidant Synergism 29

1.11 Oxidation Bench Tests 30

1.11.1 Thin-Film Oxidation Test 31

1.11.1.1 Pressurized Differential Scanning Calorimetry 31

1.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097) 31

1.11.1.3 Thin-Film Oxidation Uptake Test (ASTM D 4742) 33

1.11.2 Bulk Oil Oxidation Test 33

1.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310) 33

1.11.2.2 IP 48 Method 34

1.11.2.3 IP 280/CIGRE 34

1.11.3 Oxygen Update Test 34

1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) 34

1.12 Experimental Observations 34

1.13 Antioxidant Performance with Base Stock Selection 37

1.14 Future Requirements 38

1.15 Commercial Antioxidants 39

1.16 Commercial Metal Deactivators 41

References 41

1.1 INTRODUCTION

Well before the mechanism of hydrocarbon oxidation was thoroughly investigated, researchers had

come to understand that some oils provided greater resistance to oxidation than others The

differ-ence was eventually identifi ed as naturally occurring antioxidants, which varied depending on crude

source or refi ning techniques Some of these natural antioxidants were found to contain sulfur- or

nitrogen-bearing functional groups Therefore, it is not surprising that, certain additives that are

used to impart special properties to the oil, such as sulfur-bearing chemicals, were found to provide

additional antioxidant stability The discovery of sulfurized additives providing oxidation stability

was followed by the identifi cation of similar properties with phenols, which led to the development

of sulfurized phenols Next, certain amines and metal salts of phosphorus- or sulfur-containing

acids were identifi ed as imparting oxidation stability By now numerous antioxidants for lubricating

oils have been patented and described in the literature Today, nearly all lubricants contain at least

one antioxidant for stabilization and other performance-enhancing purposes Since oxidation has

been identifi ed as the primary cause of oil degradation, it is the most important aspect for lubricants

that the oxidation stability be maximized

Oxidation produces harmful species, which eventually compromises the designated

functiona-lities of a lubricant, shortens its service life, and to a more extreme extent, damages the machinery it

lubricates The oxidation is initiated upon exposure of hydrocarbons to oxygen and heat and can be

greatly accelerated by transitional metals such as copper, iron, nickel, and so on when present The

internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation

with heat and engine metal parts acting as effective oxidation catalysts Thus, in-service engine oils

are probably more susceptible to oxidation than most other lubricant applications For the

preven-tion of lubricant oxidapreven-tion, antioxidants are the key additive that protects the lubricant from

oxida-tive degradation, allowing the fl uid to meet the demanding requirements for use in engines and

industrial applications

Several effective antioxidant classes have been developed over the years and have seen use in

engine oils, automatic transmission fl uids, gear oils, turbine oils, compressor oils, greases, hydraulic

fl uids, and metal working fl uids The main classes include oil-soluble organic and organometallic

antioxidants of the following types:

1 Sulfur compounds

2 Sulfur–nitrogen compounds

3 Phosphorus compounds

4 Sulfur–phosphorus compounds

5 Aromatic amine compounds

6 Hindered phenolic (HP) compounds

7 Organo–copper compounds

8 Boron compounds

9 Other organometallic compounds

1.2 SULFUR COMPOUNDS

The initial concepts of using antioxidants to inhibit oil oxidation date back to the 1800s One of the

earliest inventions described in the literature [1] is the heating of a mineral oil with elemental sulfur

to produce a nonoxidizing oil However, the major drawback to this approach is the high corrosivity

of the sulfurized oil toward copper Aliphatic sulfi de with a combined antioxidant and corrosion

inhibition characteristics was developed by sulfurizing sperm oil [2] Additives with similar

func-tionalities could also be obtained from sulfurizing terpenes and polybutene [3–5] Paraffi n wax has

also been employed to prepare sulfur compounds [6–9] Theoretical structures of several sulfur

compounds are illustrated in Figure 1.1 Actual compounds can be chemically complex in nature

Aromatic sulfi des represent another class of sulfur additives used as oxidation and corrosion

inhibitors Examples of simple sulfi des are dibenzyl sulfi de and dixylyl disulfi de More complex

compounds of a similar type are the alkyl phenol sulfi des [10–15] Alkyl phenols, such as mono- or

di-butyl, -amyl, or -octyl phenol, have been reacted with sulfur mono- or dichloride to form either

mono- or disulfi des As shown in Figure 1.1, the aromatic sulfi des such as benzyl sulfi de have the

sulfur attached to carbon atoms in the alkyl side groups, whereas the alkyl phenol sulfi des have the

sulfur attached to carbon atoms in the aromatic rings In general, the alkyl phenol sulfi de chemistry

appears to have superior antioxidant properties in many types of lubricants Mono- and

dialkyl-diphenyl sulfi des obtained by reacting dialkyl-diphenyl sulfi de with C10–C18 alpha-olefi ns in the presence

of aluminum chloride have been demonstrated to be powerful antioxidants for high-temperature

lubricants especially those utilizing synthetic base stocks such as hydrogenated poly-alpha-olefi ns,

diesters, and polyol esters [15] The hydroxyl groups of the alkyl phenol sulfi des may also be treated

CH2

CH2

C CH2

CH C

CH3S

C

CH3

CH2S

Sulfurized dipentene

Sulfurized ester

CH3 (CH2)x CH CH (CH

2 )xS

CH CH (CH2)xS

Dialkylphenol sulfide

S

O S

CH2

FIGURE 1.1 Examples of sulfur-bearing antioxidants.

with metals to form oil-soluble metal phenates These metal phenates play the dual role of detergent

and antioxidant

Multifunctional antioxidant and extreme pressure (EP) additives with heterocyclic structures

were prepared by sulfurizing norbornene, 5-vinylnorbornene dicyclopentadiene, or methyl

cyclo-pentadiene dimer [16] Heterocyclic compounds such as n-alkyl 2-thiazoline disulfi de in

combi-nation with zinc dialkyldithiophosphate (ZDDP) exhibited excellent antioxidant performance in

laboratory engine tests [17] Heterocyclic sulfur- and oxygen-containing compositions derived from

mercaptobenzthiazole and beta-thiodialkanol have been found to be excellent antioxidants in

auto-matic transmission fl uids [18] Novel antioxidant and antiwear additives based on

dihydrobenzothio-phenes have been prepared through condensation of low-cost arylthiols and carbonyl compounds in

a one-step high-yield process [19]

1.3 SULFUR–NITROGEN COMPOUNDS

The dithiocarbamates were fi rst introduced in the early 1940s as fungicides and pesticides [20] Their

potential use as antioxidants for lubricants was not realized until the mid-1960s [21], and since then,

there have been continuous interests in this type of chemistry for lubricant applications [22] Today,

dithiocarbamates represent a main class of sulfur–nitrogen-bearing compounds being used as

antioxi-dants, antiwear, and anticorrosion additives for lubricants

Depending on the type of adduct to the dithiocarbamate core, ashless and metal-containing

dithiocarbamate derivatives can be formed Typical examples of ashless materials are methylene

bis(dialkyldithiocarbamate) and dithiocarbamate esters with general structures being illustrated in

Figure 1.2 Both are synergistic with alkylated diphenylamine (ADPA) and organomolybdenum

compounds in high-temperature deposit control [23] In particular, methylene bis(dialkyldithiocar

bamate) in combination with primary antioxidants such as arylamines or HPs and triazole

deriva-tives is known to provide synergistic action in stabilizing mineral oils and synthetic lubricating oils

[24–26] This material has been used to improve antioxidation characteristics of internal

combus-tion engine oils containing low levels (<0.1 wt%) of phosphorus [27] In another effort to reduce

phosphorus content in aviation gas turbine lubricants, methylene-bridged bis(dialkyl) or bis(alkylar

yldithiocarbamate) was used as high-temperature antioxidant and antiwear agent to replace tricresyl

phosphates that are of a concern to produce neurotoxic ortho-cresol isomers in trimethylolpropane

triester base oil under high-temperature service conditions [28]

It has been known that metal dithiocarbamates such as zinc, copper, lead, antimony, bismuth,

and molybdenum dithiocarbamates (MoDTCs) possess desirable lubricating characteristics

includ-ing antiwear and antioxidant properties The associated metal ions affect the antioxidancy of the

additives Within the group, MoDTCs are of greater interest particularly for engine crankcase

lubri-cants Certain molybdenum additives posses good oxidation resistance and acceptable corrosion

characteristics, when prepared by reacting water, an acidic molybdenum compound, a basic

nitro-gen complex, and a sulfur source [29,30] Oil-soluble trinuclear MoDTCs prepared by reacting

FIGURE 1.2 Ashless dithiocarbamates for lubricants.

N C S

S R

R

S

N R

R

S R R

C C

C R C O O

O Bis(disubstituted dithiocarbamate) Dithiocarbamate ester

R

ammonium polythiomolybdate with appropriate tetralkylthiuram disulfi des were found to be

supe-rior to dinuclear molybdenum compounds in terms of providing lubricants antioxidant, antiwear,

and friction-reducing properties [31]

When combined with an appropriate aromatic amine, MoDTCs can exhibit synergistic antioxidant

effects in oxidation tests [32] As a result, molybdenum dialkyldithiocarbamates (C7–24) and ADPAs

are claimed broadly for lubricating oils [33] More restrictive are claims for molybdenum

dialkyl-dithiocarbamates (C8–23 and C3–18) and ADPAs in lubricating oils that contain <3 wt% of aromatic

content and <50 ppm of sulfur and nitrogen [34] Molybdenum dialkyldithiocarbamates and HP

anti-oxidants are jointly claimed for lubricating oils that contain 45 wt% or more one or two ring

naph-thenes and <50 ppm sulfur and nitrogen [35] MoDTC was used to top-treat engine oils formulated

with group I base stocks (>300 ppm S) and an additive package designed for group II base stocks

The oils passed the sequence IIIF oxidation test, in which the oils would otherwise fail without the

molybdenum top-treatment [36] Further demonstrated is a combination of ADPAs, sulfurized olefi n,

or HP and oil-soluble molybdenum compounds including MoDTC The mixture is highly effective in

stabilizing lubricants, especially those formulated with highly saturated, low-sulfur base oils [37]

Thiadiazole derivatives, particularly the monomers and dimers, represent another class of

sulfur- and nitrogen-bearing multifunctional additives with antioxidant potency For example, the

monomeric 2-alkylesterthio-5-mercapto-1,3,4-thiadiazole has been reported to increase oxidative

stability of engine oils under thin-fi lm oxidation conditions by using thin-fi lm oxygen uptake test

(TFOUT) [38] Lithium 12-hydroxystearate grease containing

2,5-dithiobis(1,3,4-thiadiazole-2-thiol), a dimer, exhibited superior oxidative stability in the American Society for Testing and

Materials (ASTM) D 942 pressure bomb oxidation test [39] When used in conjunction with ADPA

and organomolybdenum compound, the thiadiazole derivative improved the thermal-oxidation

engine oil simulation test (TEOST) deposition (ASTM D 7097) characteristic of an engine oil from

the control oil containing sulfurized isobutylene instead [40] In addition to providing antioxidant

benefi t, the thiadiazole derivatives have been widely used as ashless antiwear and EP additives

Some of them can also provide corrosion inhibition and metal deactivation properties to nonferrous

metals such as copper

Phenothiazines are also well-known sulfur- and nitrogen-bearing antioxidants and have been

used to stabilize aviation fl uids Recent advances have lead to N-substituted thio alkyl phenothiazines,

having improved antioxidant activities and oil solubility [41]as well as

N-aminopropylpheno-thiazine that can be used for further derivatization of the N-amino group [42] For example, alkyl

phenothiazines together with aromatic amines can be attached to olefi n copolymers to result in a

multifunctional antioxidant, antiwear agent, and Viscosity index (VI) improver for lubricants [43]

Diamine sulfi des, including diamine polysulfi des, can also provide effective oxidation control

when used in conjunction with oil-soluble copper In demonstration, dimorpholine disulfi de and

di(dimethyl morpholine) disulfi de were compared to primary alkyl ZDDP and found to be superior

in controlling oil viscosity increase of engine crankcase lubricants at elevated temperatures [44]

1.4 PHOSPHORUS COMPOUNDS

The good performance of phosphorus as an oxidation inhibitor in oils was identifi ed early on in

lubrication science The use of elemental phosphorus to reduce sludge formation in oils has been

described [45] However, elemental phosphorus, like elemental sulfur, may have corrosive side effects

to many nonferrous metals and alloys, so it is rarely incorporated in oils in this form, rather oil- soluble

organic compounds of phosphorus are preferred Naturally occurring phosphorus compounds such

as lecithin have been utilized as antioxidants and many patents have been issued on these materials

for single use or in combination with other additives [46–49] Lecithin is a phosphatide that has been

produced commercially as a by-product from the processing of crude soybean oil

The antioxidant property of synthetic neutral and acid phosphite esters has been known for

sometime Alkyl and aryl phosphites such as tributyl phosphite and triphenyl phosphite are effi cient

antioxidants in some petroleum base oils, and many patents have been issued on such compositions

[50,51] Table 1.1 summarizes the patenting activities of the past three decades on the stabilization

of various lubricants with organophosphites For optimum antioxidant performance, phosphites are

customarily blended with aminic or HP antioxidants that can lead to synergistic effect For better

hydrolytic stability, tri-substituted phosphites with sterically hindered structures such as

tris-(2,4-di-tert-butylphenyl) phosphite and those based on pentaerythritol as described in the U.S Patent

5,124,057 [52] are preferred The aluminum, calcium, or barium salts of alkyl phosphoric acids are

another type of phosphorus compound that displays antioxidant properties [53,54]

1.5 SULFUR–PHOSPHORUS COMPOUNDS

The identifi cation of sulfur and phosphorus compounds as powerful antioxidants for protection

of hydrocarbons has led to the development of oil-soluble antioxidants, having both elements

in one molecule Numerous patents have been issued on such compositions, and a considerable

number have been used commercially [60–67] In fact, antioxidants containing both sulfur and

phosphorus are usually more effective and effi cient in a wider variety of base stocks than those

containing only phosphorus or sulfur Many commercial oils have employed one kind or other of

these sulfur–phosphorus-type additives

One widely used class of sulfur–phosphorus additive is the metal dialkyldithiophosphates,

which are typically prepared by the reaction of phosphorus pentasulfi de with alcohols to form

dithio-phosphoric acids, followed by neutralization of the acids with an appropriate metal

com-pound Many types of alcohols such as the aliphatic, cyclic [62], and phenolic derivatives have

been used, and those of relatively high molecular weight (such as lauryl, octyl, cyclohexyl, methyl

cyclohexyl alcohols, and amyl [65] or butyl phenols) are preferred to give suffi cient thermal stability

to the fi nal products while rendering suffi cient solubility in oils For the second-step reaction, zinc,

TABLE 1.1

Applications of Organophosphites as Antioxidants for Lubricants

Compressor oils Trinonylphenyl phosphite, tributyl

phosphite, tridecylphosphite, triphenylphosphite, trioctylphosphite, dilaurylphosphite

Secondary aminic and hindered phenolic

Secondary aminic and hindered phenolic

Secondary aminic and hindered phenolic

57

Hydraulic fl uids, steam

turbine oils, compressor oils,

and heat-transfer oil

Steric hindered tributyl phosphite, bis(butylphenyl pentaerythritol) diphosphite

58

Hydraulic fl uids, Automatic

transmission fl uids

Trialkyl phosphites Secondary aminic and hindered

phenolic including bis-phenol

59

barium, molybdenum, or calcium oxides are usually chosen For more than 60 years, zinc salts

of dialkylthiophosphoric acids (ZDDP) have been one of the most cost-effective antioxidants and

therefore have been included as a key component in many oxidation inhibitor packages for engine

oils and transmission fl uids In addition, ZDDPs show good antiwear properties, especially in the

valve train area owing to the formation of sulfi de and phosphate fi lms through corrosive reactions

on metal surfaces These fi lms can also provide protection against corrosive attack from the organic

acids formed during the oxidation process The salts of C4/C5 dialkyldithiophosphoric acid are

the most common, but a broad range of other alkyl and aryl derivatives have been developed to

meet special needs, for instance, protection at higher temperatures The reaction scheme of making

ZDDP is shown in Figure 1.3

A number of patents describe modifi cations to the fi rst step of the reactions shown in Figure 1.3; by

conducting preliminary condensation reaction of phosphorus pentasulfi de with unsaturated organic

compounds such as terpenes, polybutenes, wax olefi ns, fatty acids, fatty esters, sperm oil, and so

on to form high-molecular-weight intermediate products [68–89] During these reactions, hydrogen

sulfi de is liberated, and the intermediates are usually acidic The mechanism of the P2S5 reaction

with olefi ns in these cases may be one of substitution (replacement of reactive hydrogen atoms) as

well as of addition In preparing the fi nal additives, these acidic intermediates were neutralized by

the treatment with alkaline earth oxides or hydroxides to form metal salts The calcium, barium, or

potassium salts are the most preferred products Some additives may also display detergency

char-acteristics The concept of conducting preliminary condensation reactions provides a facile route

to the synthesis of a wide variety of products from the reaction of phosphorus pentasulfi de and an

unsaturated organic moiety Several of these, particularly the terpene and polybutene reaction

prod-ucts, have been used extensively in commercial applications

To reduce the staining effect of ZDDP on metal parts (especially copper), addition of alkyl or

aryl phosphites during the synthesis has been attempted [90] For example, triphenyl phosphite is

added to the dialkyldithiophosphoric acid and heated at 110ºC for an hour before the addition of

zinc oxide In another patent, a novel dithiophosphate with improved oxidation stability is described

[91] An acid is reacted with a glycol, to give a monoester having a hydroxyl group, which is then

reacted with P2S5 to give the dialkyl dithiophosphoric acid Zinc oxide is subsequently added to give

the novel dithiophosphates To improve solubility, the salts can be made of lower dialkyl

dithiophos-phates by utilizing both primary and secondary alcohols, including butyl alcohols in the process

[92] Mixed metal salts of dialkyl dithiophosphoric acids and carboxylic acids are claimed to have

higher thermal stability [93]

Many descriptions have recently appeared of organomolybdenum phosphorodithioate

com-plexes that impart excellent oxidation stability to lubricants In certain circumstances, oil-soluble

molybdenum compounds are preferred additives owing to their multifunctional characteristics

such as antiwear, EP, antioxidant, antipitting, and antifriction properties For instance, several

molybdenum dialkylphosphorodithioate complexes with varying alkyl chain length of amyl,

octyl, 2-ethylhexyl, and isodecyl were reported to exhibit appreciable antioxidation, antiwear,

FIGURE 1.3 Synthesis of ZDDP.

S

SH + H2S RO

2 RO

P

S

SH + ZnO RO

RO

S S

2

Zn + H2O

and antifriction properties [94] Novel trinuclear molybdenum dialkyldithiophosphates prepared

by reacting an ammonium polythiomolybdate and an appropriate bis(alkyldithiophosphoric) acid

possess excellent antioxidant as well as antiwear and friction-reducing properties [31] Some

molybdenum compounds have been used commercially in engine oils and metal working fl uids

as well as in various industrial and automotive lubricating oils, greases, and specialties [95]

The combination of ZDDP with a molybdenum-containing adduct, prepared by reacting a

phos-phosulfurized polyisoalkylene or alpha olefi n with a molybdenum salt, has been described [96]

In this case, the molybdenum adduct alone gave poor performance in oxidation tests, but the

mixture with ZDDP provided good oxidation stability Novel organomolybdenum complexes

pre-pared with vegetable oil have been identifi ed as synergist with ADPAs and ZDDPs in lubricating

oils [97]

Owing to increasing concerns on the use of metal dithiophosphates that are related to toxicity,

waste disposal, fi lter clogging, pollution, etc., there have been extensive research activities on the

use of ashless technologies for both industrial and automotive applications A number of ashless

compounds based on derivatives of dialkylphorphorodithioic acids had been reported as

multifunc-tional additives Upon reacting diisoamylphosphorodithioic acid with various primary and

second-ary amines, eight alkylamino phosphorodithioates with vsecond-arying chain length from C5 to C18 were

obtained and found to possess excellent antiwear and antioxidant properties as compared to ZDDP

[98] Alkylamino phosphorodithioates obtained from reacting heptylated or octylated or nonylated

phosphorodithioic acids with ethylene diamine, morpholine, or tert-alkyl (C12–C14) amines have

been demonstrated to impart similar antioxidant and antiwear effi cacy and superior hydrolytic

sta-bility over ZDDP [99] Phosphorodithioate ester derivatives containing a HP moiety are also known

to have antioxidant potency This type of chemistry can be obtained by reacting metal salts of

phosphorodithioic acids with HP halides [100] or with HP aldehydes [101] Substituting the phenol

aldehydes with hindered cyclic aldehydes, in which the carbon atom attached to the carbonyl carbon

contains no hydrogen atoms, may also result in products having excellent antioxidant and thermal

stability characteristics [102]

1.6 AMINE AND PHENOL DERIVATIVES

Oil-soluble organic amines and phenol derivatives such as pyrogallol, gallic acid, dibutylresorcinol,

hydroquinone, diphenylamine, phenyl-alpha-naphthylamine, and beta-naphthol are early examples

of antioxidants used in turbine oils and lubricating greases [103,104] In engine oils, these types of

compounds showed only limited effectiveness Other amines and phenol derivatives such as

tetra-methyldiaminodiphenylmethane and alizarin were used to some degree, rarely alone, but more often

in combination with other types of antioxidants For example, a mixture of a complex amine with

a phosphorus pentasulfi depolybutene reaction product has been reported [105] Another reported

mixture is a complex phenol derivative such as alizarin in combination with an alkyl phenol sulfi de

and a detergent additive [106] As technology advances, numerous amine and phenol antioxidants

have been invented, and many of them have become the most widely used antioxidants in the

lubri-cant industry

1.6.1 A MINE D ERIVATIVES

ADPAs are one of the most important classes of amine antioxidants being used today Owing to their

higher reactivity over the unsubstituted diphenylamine, ADPAs have been workhorse antioxidants

for engine oils and various industrial lubricants for more than two decades Figures 1.3 and 1.4

illustrate the typical synthesis routes of some commonly used ADPAs The reactions start with

benzene, which is fi rst converted into nitrobenzene [107], followed by a high-temperature

reduc-tion to aniline [108] Under very high-temperature (400–500°C) and high-pressure (50–150 psi)

conditions, aniline can undergo a catalytic vapor-phase conversion to form diphenylamine [109]

To make ADPAs, diphenylamine is reacted with an appropriate alkylating agent such as alcohol,

alkyl halide, aliphatic carbonyl compound, or an olefi n The olefi ns are preferred for economic

rea-son The most commonly used are isobutylene (C4), diisobutylene (C8), nonenes (C9), styrene, and

propylene tetramer (C12) Depending on the acidic catalyst, olefi n, and other reaction conditions, for

instance, the temperature, the degree of alkylation will vary from mono- to di-alkylation

Mono-ADPA is generally more effective than the corresponding disubstituted on a weight

basis because additional alkylation substantially reduces the number of moles of diphenylamine

per weight unit However, in practice, obtaining monosubstituted diphenylamine in relatively

pure format is diffi cult because as soon as the diphenylamine is monoalkylated, it quickly

pro-ceeds to dialkylation Attempt in the preparation of high content of mono-ADPAs has led to

the use of novel clay catalyst with greater selectivity in alkylation reactions and C6–C18 linear

olefi ns to produce high levels (at least 50 wt%) of mono-ADPAs with lower levels of dialkyl

diphenylamines and undesirable unsubstituted diphenylamine [110] Alkyl groups of six or more

carbon of mono-ADPA tend to render the material lower yellow color and higher resistance to

discoloration [111]

It was found that monosubstituted diphenylamines more readily oligomerize under various

conditions to produce higher-molecular weight, linear oligomers Oligomers with 2–10 degrees of

polymerization are desirable antioxidants especially for high-temperature applications

Disubsti-tuted and polysubstiDisubsti-tuted diphenylamines, however, are more restricted from forming oligomers

higher than dimers Oligomeric versions of monosubstituted diphenylamine prepared from reacting

diphenylmine with C4–C16 olefi ns have been described for use in ester lubricants [112] The

prod-ucts are claimed to be more effective than simple diphenylamines for extremely high-temperature

applications Homo-oligomers of alkylated (C4–C8) diphenylamines, styryenated diphenylamines,

FIGURE 1.4 Synthesis routes of ADPA antioxidants.

N H

Catalyst

or cross-oligomers of the ADPAs with substituted N-phenyl-α(β)-naphthylamine (PNA) are claimed

to possess superior antioxidant effi cacy in synthetic ester lubricants for high-temperature

applica-tions [113] Oligomeric products derived from thermal and chemical condensation of ADPA and

alkylated PNA in the presence of aldehyde can provide high performance and nonsludging

attri-butes, as evident in the rotating pressure vessel oxidation test (RPVOT, ASTM D 2272) and the

ASTM D 4310 sludging tendency test designed for turbine oils [114]

There appears to be a great number of patenting activities on the process of using isobutylene

derivatives as alkylating agents Under certain mole ratio range, diphenylamine can be reacted

with diisobutylene at a temperature of 160°C or higher to facilitate chain scission of

diisobutyl-ene [115] In the presence of an acid clay catalyst, the resulting product has <25% of 4,4′-dioctyl

diphenylamine, which yields a liquid at room temperatures In another process that involves

two-step reactions [116], a light-colored, liquid product is obtained by fi rst reacting diphenylamine with

diisobutene, followed by reaction with a second olefi n, preferably isobutene Specifi c mole ratio,

reaction temperature, and reaction duration are critical to obtain the desired ADPAs To obtain

higher levels (>50 wt%) of monosubstituted diphenylamine content in the fi nal product,

diisobutyl-ene is allowed to react at a lower temperature range of 105–157°C in the presence of a clay catalyst

By carefully controlling mole ratio of the reactants together with reaction duration, the process,

as disclosed, selectively results in a higher proportion of mono-ADPA and a lower proportion of

unsubstituted diphenylamine and disubstituted or polysubstituted diphenylamines [90,117] U.S

Patent 6,355,839 [118] discloses a one-step process using highly reactive polyisobutylene oligomers

having an average molecular weight of ~160 to 280 and at least 25% of 2-methylvinylidene isomers

as the alkylating agents to make ADPAs and other types of alkylated diarylamine The resulting

products are liquid at ambient temperatures

Several antioxidant patents based on alkylation of benzotriazole compounds have been issued

One particular benefi t of using this class of antioxidant over the ADPAs is their additional activity in

the reduction of copper corrosion Examples are N-t-alkylated benzotriazoles obtained by reacting a

benzotriazole with an olefi n such as diisobutylene [119], and the reaction products of a benzotriazole

with an alkyl vinyl ether or a vinyl ester of a carboxylic acid such as vinyl acetate [120] Antioxidant

and antiwear properties were reported for benzotriazole adducts of an amine phosphate [121] or an

organophosphorodithioate [122] The former type also exhibited rust prevention characteristics in

the ASTM D 665 corrosion test

Aromatic diamines are a broad group of aminic antioxidants suitable for lubricants

diethyltoluenediamines with the amino moieties being located on the 2,4 and 2,6 positions

rela-tive to the methyl group have been claimed to be effecrela-tive in the prevention of oil viscosity

increase and acid buildup [123] The additives are relatively noncorrosive to copper and lead

bearings and are compatible with seals at high temperatures and pressures Substituted

ben-zylamines or substituted 1-amino-1,2,3,4-tetrahydronaphthalene is particularly useful for

syn-thetic lubricants such as polyalphaolefi ns (PAOs) or polyol esters Oils bearing these additives

demonstrate very low metal corrosion, low viscosity increase, and low sludge buildup [124]

N,N ′- diphenyl-p- phenylenediamines in which the phenyl groups may be substituted with methyl,

ethyl, or methoxy have been claimed as effective antioxidants [125].A broader range of

substi-tuted p-phenylenediamines has been claimed for crankcase lubricating oils for use in

environ-ments where iron- catalyzed oxidation reactions can take place [126] 2,3- Dihydroperimidines that

are prepared from the condensation of 1,8-diaminonaphthalenes with ketones or aldehydes show

good oxidation inhibition in the RPVOT (ASTM D 2272) Synergistic behavior of the amines was

also observed when an appropriate phenolic antioxidant is present [127] Oils containing N,N

′-disubstituted-2,4-diaminodiphenyl ethers and imines of the same ethers have shown low viscosity

increase, low acid buildup, and reduced metal corrosion in bench tests [128,129] The reaction

product of a hydrocarbyl succinic anhydride and 5aminotriazole demonstrated antioxidant effi

-cacy in a railway diesel oil composition [130]

1.6.2 P HENOL D ERIVATIVES

Phenols, especially the sterically hindered phenols are another class of antioxidants being extensively

used in industrial and automotive lubricating oils and greases Based on the chemical structure,

phenols may be customarily categorized into simple phenols such as 2,6-di-tert-4- methylphenol

(also known as BHT) and complex phenols that are typically in polymeric forms having molecular

weights of 1000 or higher The structures, important physical properties, and typical applications of

some commonly used HPs are given in Table 1.2

Similar to the alkyl phenol sulfi des discussed earlier, the combinations of HPs and sulfur

chem-istry have been widely reported For example, the reaction products of simple phenols such as the

2,6-di-tert-butylphenol listed in Table 1.2 with selected thioalkenes have shown effectiveness in the

prevention of acid buildup and oil viscosity increase, without causing lead corrosion [131] Another

patent describes a process for preparing hydrocarbylthio-HPs by reacting substituted phenols with

hydrocarbyl disulfi des using an aluminum phenoxide catalyst [132] Using a 4,4′-methylene

bis(2,6-di-tert-butylphenol) as reference, the thiophenols were found to be superior in bulk oil oxidation

tests and bench corrosion test on bearings High oligomeric phenolic antioxidants in the form of

hindered and sulfur bridged have been developed [133] These compounds have lower volatility,

bet-ter thermal stability, and improved seal compatibility and corrosion properties In general,

sulfur-bridged HPs are more effective than the conventional phenolics under high-temperature oxidation

conditions and are considered particularly suitable for the lubricants formulated with highly refi ned

base stocks [134] Figure 1.5 shows structures of some commercial sulfur-bridged HPs that have

found use in various lubricant formulations Thioalkene-bridged hemi-HPs prepared from catalytic

reaction of HP with thioalkene have also been reported to be active in the stabilization of mineral

oils and synthetic oils [135]

1.6.3 A MINE AND P HENOL -B EARING C OMPOUNDS

Given the high popularity and effectiveness of amine and phenol derivatives as lubricant

anti-oxidants, the combination of amine and phenolic moieties in one molecule represents a logic

approach to enhance performance In a prior art [136], fusing amine with a long carbon chain

3,5-di-tert-butyl-4-hydroxyphenalkyl group that separates the phenol group from the amino nitrogen

leads to novel products with lower volatility, better thermal stability, and higher solubility in oils

Nelson and Rudnick [137] reacted an ethyoxylated alkyl phenol with an alkyl arylamine in the

presence of an aldehyde The resulting product had improved antioxidant potency owing to a

syn-ergistic action between the phenolic moiety and the amine, and also showed enhanced solubility

in oils owing to the presence of alkylated aromatic moiety in the molecule Phenolic

imidazo-lines have been prepared from polyaminophenols and carbonyl compounds [138] In addition to

providing antioxidant activity, the products also have corrosion inhibition and metal deactivation

properties owing to the cyclic imidazoline moiety

Multifunctional additives containing sulfur, nitrogen, and phenolic moieties in one molecule

have been reported In this instance, mercaptobenzothiazoles or thiadiazoles are Mannich reacted

with HP antioxidants to yield oil-soluble compounds with antioxidant and antiwear properties [139]

More complex product having similar functionalities was obtained by reacting a sulfur-containing

HP ester with an ADPA [140]

1.6.4 M ULTIFUNCTIONAL A MINE AND P HENOL D ERIVATIVES

The industry-wide trend in the reduction of phosphorus and sulfur, in particular, ZDDP in fi nished

lubricants has led to increasing activities in the development of novel multifunctional additives

that have combined properties of antioxidancy, antiwear, and to some extent dispersancy, while

having low-to no-sulfur and phosphorus contents It has been shown that products obtained from

2-Propenoic acid, 3-[3,5-bis(1,1- dimethylethyl)-4-hydroxyphen

reacting alkyl or alkenyl succinic acid anhydride with an appropriate amine may impart such

mul-tifunctionalities Product made by reacting a polyalkenylsuccinic acid or anhydride fi rst with an

aromatic secondary amine, then with an alkanol amine, was found to provide appreciable

antioxid-ancy, dispersantioxid-ancy, and anticorrosion effects to engine oils as tested in a Caterpillar engine test [141]

A more recent U.S Patent literature [142] discloses materials made from the reaction of alkyl or

alkenyl succinic acid derivative with a diamino naphthyl compound for use as antioxidant, antiwear,

and soot dispersing agents for lubricating oils By fusing a HP moiety to an alkenyl succinimide

domain, a novel dispersant having antioxidant property was obtained [143] The product improved

the performance of engine oils in the sequence VG, an industry recognized sludge test to evaluate

the ability of a lubricant in preventing the formation of sludge and varnish deposits in a fi red engine

U.S Patent 5,075,383 [144] describes novel antioxidant–dispersant additives obtained by reacting

amino-aromatic polyamine compound, including aromatic secondary amines, with

ethylene–pro-pylene copolymer grafted with maleic anhydride Engine oils containing the additives displayed

improved performance characteristics in laboratory oxidation and sludge dispersancy tests, as well

as in the sequence VE and the MWM-B engine tests

1.7 COPPER ANTIOXIDANTS

The ability of copper compounds to function as oxidation inhibitors has been of interest to the

lubricant industry for years Copper is usually considered to be an oxidation promoter, and its

presence is of a concern in lubricants such as power transmission oils, where fl uid contact with

copper-containing bearings and sintered bronze clutch plates takes place [145] It has been

sug-gested that copper corrosion products, originating from surface attack of copper metal, are generally

catalysts that accelerate the rate of oxidation [146], whereas oil-soluble copper salts are antioxidants

[147] To maximize the full antioxidant strength of a copper compound, the initial concentration

needs to be maintained at an optimum range, normally from 100 to 200 ppm [145,147] Below this

range, the antioxidant effect of the copper compounds will not be fully realized, whereas above the

range, interference with antiwear additives may occur, leading to pronounced increase in wear on

high-stress contact points [148]

Examples of oil-soluble copper antioxidants developed in early years were a group of copper–

sulfur complexes, obtained by sulfurizing certain types of unsaturated hydrocarbons in the presence

of copper [149–151] A more recent patent describes lubricant compositions that are stabilized with

a zinc hydrocarbyl dithiophosphate (ZDDP) and 60–200 ppm of copper derived from

oil-solu-ble copper compounds such as copper dihydrocarbyldithiophosphate or copper dithiocarbamates

[148] Oxidation data are given for fully formulated engine oils containing the ZDDP and various

supplemental antioxidants including amines, phenolics, a second ZDDP, and copper salts Only the

blends with copper salts passed the oxidation test With the other additives, the viscosity increase

was excessive Organo-copper compounds including copper naphthenates, oleates, stearates, and

polyisobutylene succinic anhydrides have been reported to be synergistic with multiring aromatic

compounds in controlling high-temperature deposit formation in synthetic base stocks [147]

FIGURE 1.5 Examples of commercial sulfur-bridged phenolic antioxidants.

More complex compounds obtained from further reactions of copper salts have also been

reported to be effective antioxidants in various lubrication applications For example, copper

car-boxylate or copper thiocyanate was reacted with a mono-oxazoline, bis-oxazoline, or lactone

oxa-zoline dispersant to form coordination complexes, wherein the nitrogen contained in the oxaoxa-zoline

moiety is the ligand that complexes with copper The resulting products exhibitimproved varnish

control and oxidation inhibition capabilties [152] Reaction products of a copper salt (acetate,

car-bonate, or hydroxide) with a substituted succinic anhydride derivative containing at least one free

carboxylic acid group are effective high-temperature antioxidants and friction modifi ers When

incorporated in an engine oil formulation, the oil passed rust, oxidation, and bearing corrosion

engine tests [153] In another patent [154], a HP carboxylic acid was used as the coupling reagent

The resulting copper compounds are reported to be effective in the controls of high-temperature

sludge formation and oil viscosity increase when used alone or in synergistic mixtures with a

con-ventional aminic or phenolic antioxidant

1.8 BORON ANTIOXIDANTS

The search for more eco-friendly additives to replace ZDDP has led to renewed interest in boron

esters owing to their ability to improve antioxidation, antiwear, and antifriction properties of

lubri-cants when used alone or in combination with other additives The complex tribological behavior of

boron compounds in formulated lubricants depends on their particular chemical structures and the

interactions between boron and other active elements such as sulfur, phosphorus, nitrogen, or their

combinations when present [155,156]

A number of boron–oxygen-bearing compounds have been reported to be effective oxidation

inhibitors in terms of prevention of oil viscosity increase and acid formation at elevated

tempera-ture (163°C) [157–161] Representatives are boron epoxides (especially 1,2-epoxyhexadecane) [157],

borated single and mixed alkanediols [158], mixed hydroquinone-hydroxyester borates [159],

phe-nol esters of hindered phenyl borates [160], and reaction products of boric acid with the condensates

of phenols with aromatic or aliphatic aldehydes [161]

Borate esters with nitrogen are known for their antioxidant activity and improved antiwear

properties probably due to the formation of additional boron nitride fi lm on rubbing surface [162]

Borated adducts of alkyl diamines with long-chain hydrocarbylene alkoxides and

low-molecular-weight carboxylic acids have been reported to have antifriction properties and high inhibition

abil-ity especially at elevated temperatures [163] Appreciable oxidation inhibition effect has also been

reported for borate esters of hydrocarbyl imidazolines [164], borates of mixed ethyoxyamines and

ethoxyamides [165], and borates of etherdiamines [166]

Synergistic antioxidant effect of borate esters with ADPAs or with zinc dithiophosphates has

been established When tested at 180°C in a PAO using a pressurized differential scanning

calo-rimetry (PDSC), strong synergistic antioxidant action was observed between borate esters and a

dioctyl diphenylamine at a 1:1 (w/w) blending ratio [167] Similar effect was observed in the

mix-tures of borate esters and a ZDDP [155] The synergism with ZDDP is of practical importance as

it allows reduced phosphorus level in a fi nished lubricant without sacrifi ce of oxidative stability

The catalytic effect of boron in enhancing antioxidant performance has led to the development of

phenolic-phosphorodithioate borates, obtained from coborating HP and alkyl

phosphorodithioate-derived alcohol The borates were found to possess exceptional antioxidant and antiwear

proper-ties Both the HP moiety and the phosphorodithioate alcohol moiety were believed to provide the

basis for the synergy each of which are subsequently enhanced by the integral boron coupling

moiety [168]

Despite many tribological and antioxidation benefi ts that borate esters can offer, large use of

the chemistry for lubricant applications has not taken place One serious drawback with most borate

esters has been their high susceptibility to hydrolysis, a process that liberates oil-insoluble and

abrasive boric acid Following attempts have been made to address the issue with varying degrees

of success:

1 Incorporation of HP moiety to sterically inhibit the boron–oxygen bonds from hydrolytic

attack Commonly used HPs are 2,6-dialkyl phenols [169], 2,2′-thiobis(alkylphenols) and thiobis(alkylnaphthols) [170]

2 Incorporation of amines that have nonbonding pairs of electrons The amines coordinate

with the electron-defi cient boron atom, thus preventing hydrolysis U.S Patents 4,975,211 [171] and 5,061,390 [172] disclose the stabilization of borated alkyl catechol against hydrolysis by complexing with diethylamine Signifi cant improvement in hydrolytic sta-

bility was reported for borate esters incorporated with a N,N′-dialkylamino-ethyl ety [156] It was hypothesized that the formation of a stable fi ve-member ring structure

moi-in molecules moi-involvmoi-ing coordmoi-ination of nitrogen with boron substantially moi-inhibited the hydrolytic attack from water

3 Use of certain hydrocarbon diols or tertiary amine diols to react with boric acid to form

stable fi ve-member ring structures [173]

1.9 MISCELLANEOUS ORGANOMETALLIC ANTIOXIDANTS

More recently, a number of oil-soluble organometallic compounds, for example, organic acid

salts, amine salts, oxygenates, phenates and sulfonates of titanium, zirconium, and manganese

have been claimed to be effective stabilizers for lubricants [174,175] Some of the compounds are

essentially devoid of sulfur and phosphorus, therefore, suitable for modern automotive engine

oils where lower contents of the two elements are desired In one example [174], lubricating oils

having 25 to ~100 ppm of titanium derived from titanium (IV) isopropoxide exhibited excellent

oxidative stability in the high-temperature (280°C) Komatsu hot tube test and ASTM D 6618

test evaluate engine oils for ring sticking, ring and cylinder wear, and the accumulation of piston

deposits in a four-stroke cycle diesel engine In another example [175], titanium (IV) isopropoxide

was used to react with neodecanoic acid, glycerol mono-oleate, or polyisobutenyl bis-succinimide

to form respective titanated compounds These compounds, when top-treated in a SAE 5W30

engine oil to result in 50 to ~800 ppm of titanium in oil, improved the deposit control capability

of the oil as tested by using the TEOST (ASTM D 7097) Similar antioxidant effect was observed

for neodecanoates of zirconium and manganese in the same oil

Oil-soluble or dispersible tungsten compounds, more specifi cally, amine tungstates and tungsten

dithiocarbamates, have been attempted as antioxidants for lubricants and found to be synergistic

with secondary diarylamine and alkylated phenothiazines The mixtures, when added to an engine

crankcase lubricant to result in ~20 to 1000 ppm of tungsten, were highly effective in controlling oil

oxidation and deposit formation [176]

Sulfur-free molybdenum salts such as molybdenum carboxylates have been attempted as

anti-oxidants and found to be synergistic with ADPAs in lubricating oils [177,178] The synergistic

mixtures improved oxidation stability of crankcase lubricants while providing additional friction

modifi cation characteristics

1.10 MECHANISMS OF HYDROCARBON OXIDATION

AND ANTIOXIDANT ACTION

It is now understood that oxidation of hydrocarbon-based lubricants undergoes autoxidation, a process

that leads to the formation of acids and oil thickening To a more severe extent, oil-insoluble sludge

and varnish may be formed, causing poor lubrication, reduced fuel economy, and increased wear

Antioxidants are essential additives incorporated in lubricant formulations to delay the onset of

autoxidation and minimize its impact The mechanisms of lubricant degradation and its

stabiliza-tion by antioxidants are discussed in the following secstabiliza-tions

1.10.1 A UTOXIDATION OF L UBRICATING O IL

The well-documented autoxidation mechanism involves a free-radical chain reaction [179–181] It

consists of four distinct reaction steps: chain initiation, propagation, branching, and termination

1.10.1.1 Initiation

R⫺REnergy→Ri⫹Ri (1.2)

The initiation step is characterized as the formation of free alkyl radicals (R•) from the breakdown

of hydrocarbon bonds by hydrogen abstraction and dissociation of carbon–carbon bonds These

reactions take place when hydrocarbons are exposed to oxygen and energy in the form of heat, UV

light, or mechanical shear stress [182] The ease of homolytic cleavage of an R–H bond follows this

order, as determined by the C–H bond strength and the stability of the resulting radical [183]: phenyl

< primary < secondary < tertiary < allylic < benzylic Thus, hydrocarbons containing tertiary

hydrogen or hydrogen in an alpha position to a carbon–carbon double bond or aromatic ring are

most susceptible to oxidation The reaction rate of chain initiation is generally slow under ambient

conditions but can be greatly accelerated with temperature and the presence of catalytic

transition-ing metal ions (copper, iron, nickel, vanadium, manganese, cobalt, etc.)

1.10.1.2 Chain Propagation

The fi rst propagation step involves an alkyl radical reacting irreversibly with oxygen to form an

alkyl peroxy radical (ROO•) This reaction is extremely fast, and the specifi c rate is dependent on

the radical’s substituents [179] Once formed, the peroxy radical can randomly abstract hydrogen

from another hydrocarbon molecule to form hydroperoxide (ROOH) and a new alkyl radical (R•)

Based on this mechanism, each time a free alkyl radial is formed, a large number of hydrocarbon

molecules may be oxidized to hydroperoxides

1.10.1.3.2 Aldehyde or Ketone Formation

The chain-branching steps begin with the cleavage of hydroperoxide into an alkoxy radical (RO•)

and a hydroxy radical (HO•) This reaction has high activation energy and is only signifi cant at

tem-peratures >150°C Catalytic metal ions accelerate the process The resulting radicals will undergo a

number of possible reactions: (a) the alkoxyl radical abstracts hydrogen from a hydrocarbon to form

a molecule of alcohol and a new alkyl radical according to reaction 1.6, (b) the hydroxyl radical

fol-lows the pathway of reaction 1.7 to abstract hydrogen from a hydrocarbon molecule to form water

and a new alkyl radical, (c) a secondary alkoxyl radical (RR′HCO•) may decompose through

reac-tion pathway 1.8 to form an aldehyde, and (d) a tertiary alkoxy radical (RR′R″CO•) may decompose

to form a ketone (reaction 1.9)

The chain-branching reaction is a very important step to the subsequent oxidation state of

the oil as not only will a large number of alkyl radicals be formed that expedites the oxidation

process, but also the lower-molecular-weight aldehydes and ketones generated will immediately

affect the physical properties of the lubricant by decreasing oil viscosity and increasing oil

vola-tility and polarity Under high-temperature oxidation conditions, the aldehydes and ketones can

undergo further reactions to form acids and high-molecular-weight species that thicken the oil

and contribute to the formation of sludge and varnish deposits Detailed mechanisms will be

discussed in Section 1.10.3

1.10.1.4 Chain Termination

As oxidation proceeds, oil viscosity will increase due to the formation of high-molecular-weight

hydrocarbons When oil viscosity has reached a level that diffusion of oxygen in oil is signifi cantly

limited, chain termination reactions will dominate As indicated by reactions 1.10 and 1.11, two

alkyl radicals can combine to form a hydrocarbon molecule Alternatively, an alkyl radical can

combine with an alkyl peroxy radical to form a peroxide This peroxide, however, is not stable and

can easily breakdown to generate more alkyl peroxy radicals During the chain-termination

pro-cesses, formation of carbonyl compounds and alcohols may also take place on the peroxy radicals

that contain an extractable α-hydrogen atom:

1.10.2 M ETAL -C ATALYZED L UBRICANT D EGRADATION

Metal ions are able to catalyze the initiation step as well as the hydroperoxide decomposition in

the chain-branching step [184] through a redox mechanism illustrated in the following section The

required activation energy is lowered for this mechanism, and thus, the initiation and propagation

steps can commence at much lower temperatures

1.10.3 H IGH -T EMPERATURE L UBRICANT D EGRADATION

The preceding discussion provides the basis for the autoxidation stage of lubricant degradation

under both low and high-temperature conditions The end result of low-temperature oxidation is the

formation of peroxides, alcohols, aldehydes, ketones, and water [185,186] Under high-temperature

oxidation conditions (>120°C), breakdown of peroxides including hydroperoxides becomes

pre-dominant, and the resulting carbonyl compounds (e.g., reactions 1.8 and 1.9) will fi rst be oxidized

to carboxylic acids as shown in Figure 1.6 As an immediate result, the oil acidity will increase As

oxidation proceeds, acid or base-catalyzed Aldol reactions take place The reaction mechanism is

illustrated in Figure 1.7 [187] Initially, α,β-unsaturated aldehydes or ketones are formed, and

fur-ther reaction of these species leads to high-molecular-weight products These products contribute to

oil viscosity increase and eventually can combine with each other to form oil-insoluble polymeric

products that manifest as sludge in a bulk oil oxidation environment or as varnish deposits on hot

metal surface Oil viscosity increase and deposit formation have been identifi ed to be the principal

oil-related factors to engine damages [188]

1.10.4 E FFECT OF B ASE S TOCK C OMPOSITION ON O XIDATIVE S TABILITY

Mineral base stocks used to formulate lubricants are hydrocarbons that are originated from crude

oils and essentially contain mixtures of n-paraffi ns along with isoparaffi ns, cycloparaffi ns (also

called naphthenes), and aromatics having about 15 or more carbon atoms [189] In addition, small

amounts of sulfur-, nitrogen-, and oxygen-containing species may be present depending on the

refi nery techniques employed In the American Petroleum Institute (API) base oil classifi cation

system, mineral oils largely fall into the groups I, II, III, and V, with some distinctions shown in

Table 1.3 in terms of saturates, sulfur contents, and viscosity index Group I base oils still dominate

the base oil market, accounting for more than 50% of global capacity Groups II and III base stocks

O C R

O C R

are on the horizon, and their use is expected to grow in large scale in the coming future, especially

after the completion of nearly a dozen new group II/III oil refi nery plants worldwide [190]

It has been widely recognized that base oil composition, for example, linear and branched

hydrocarbons, saturates, unsaturates, monoaromatics, polyaromatics, together with traces of

nitrogen-, sulfur-, and oxygen-containing heterocycles, etc., plays an important role in the

oxida-tive stability of the oil There have been quite extensive research activities attempting to establish

correlations between base stock composition and oxidative stability [191–195] However, owing

to the large variations in the origin of the oil samples, the test methods, test conditions, and the

performance criteria employed, the conclusions are not always consistent and in some cases

contradictory to each other In general, it has been agreed that saturated hydrocarbons are more

stable than the unsaturated toward oxidation Of the different saturated hydrocarbons found in

mineral oils, paraffi ns are more stable than cycloparaffi ns Aromatic compounds, due to their

complex and large variation in the chemical makeup, play a more profound role Monocyclic

aromatics are relatively stable and resistant to oxidation, whereas bi and polycyclic aromatics

are unstable and susceptible to oxidation [196] Alkylated aromatics oxidize more readily due to

API Category Percent Saturates Percent Sulfur Viscosity Index

the presence of highly reactive benzylic hydrogen atoms Kramer et al [193] demonstrated that

the oxidative rate of a hydrocracked 500N base oil doubled when the aromatic content increased

from 1 to 8.5 wt% Naturally occurring sulfur compounds are known antioxidants for the

inhi-bition of the early stage of oil oxidation Laboratory experiments have shown that mineral oils

containing as little as 0.03% of sulfur had good resistance to oxidation at 165°C over sulfur-free

white oils and PAOs [145] In hydrocracked oils that are essentially low in aromatics, better

oxi-dative stability was found with elevated sulfur concentration (>80 ppm) versus a level at 20 ppm

or lower [192] It has been proposed that sulfur compounds act as antioxidants by generating

strong acids that catalyze the decomposition of peroxides through a nonradical route or by

pro-moting the acid-catalyzed rearrangement of arylalkyl hydroperoxides to form phenols that are

antioxidants [145,179] Contrary to sulfur, nitrogen-bearing compounds, especially the

hetero-cyclic components (also called “basic nitrogen”), accelerate oil oxidation even at relatively low

concentrations [197] In highly refi ned groups II and III base stocks that are essentially devoid

of heteroatom-containing molecules, aromatic and sulfur contents are considered as the main

factors which infl uence the base oil oxidative stability [192,193] It has been shown that

oxida-tive stability of a given base stock can be enhanced when the combinations and concentrations

of base stock sulfur and aromatics are optimized [194]

1.10.5 O XIDATION I NHIBITION

The proceeding mechanistic discussion makes clear several possible counter measures to control

lubricant oxidation Blocking the energy source is one path However, this is only effective for

lubricants used in low shear and temperature situations A more practical approach for most

lubri-cant applications is the trapping of catalytic impurities and the destruction of alkyl radicals, alkyl

peroxy radicals, and hydroperoxides This can be achieved through the use of a metal

deactiva-tor and an appropriate antioxidant with radical scavenging or peroxide decomposing functionality,

respectively

The radical scavengers are known as primary antioxidants They function by donating hydrogen

atoms to terminate alkoxy and alkyl peroxy radicals, thus interrupting the radical chain mechanism

of the auto-oxidation process The basis for a compound to become a successful antioxidant is that

peroxy and alkoxyl radicals abstract hydrogen from the compound much more readily than they do

from hydrocarbons [198] After hydrogen abstraction, the antioxidant becomes a stable radical, the

alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide

HPs and aromatic amines are two main classes of primary antioxidants for lubricants

The peroxide decomposers are also called secondary antioxidants [180] They function by

reducing alkyl hydroperoxides in the radical chain to nonradical, less-reactive alcohols

Organo-sulfur and organophosphorus compounds and those containing both elements, such as ZDDPs, are

well-known secondary antioxidants

Since transitional metals are present in most lubrication system, metal deactivators are usually

added to lubricants to suppress the catalytic activities of the metals Based on the functioning

mech-anisms, metal deactivators for petroleum products can be classifi ed into two major types: chelators

[180] and surface passivators [199] The surface passivators act by attaching to metal surface to form

a protective layer, thereby preventing metal–hydrocarbon interaction They can also minimize

cor-rosive attack of metal surface by physically restricting access of the corcor-rosive species to the metal

surface The chelators, however, function in bulk of the lubricant by trapping metal ions to form an

inactive or much less-active complex With either mechanism, metal deactivators can effectively

slow the oxidation process catalyzed by those transitional metals, which in turn lends metal

deacti-vators an antioxidant effect Table 1.4 lists examples of metal deactideacti-vators that are commonly found

in lubricant formulations