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104 Lubricant Additives: Chemistry and Applications

been used in water-based formulations with good pump wear characteristics [155,156]. One of the

most recent applications has been in refrigeration compressor oils (e.g., for automobiles) that are

compatible with the more ecologically acceptable refrigerants. The reason for their selection in this

application has probably been their good hydrolytic stability in view of the need for a long  uid life

[157,158]. Other automotive industry applications for these products include use as friction modi-

 ers, for example, in automatic transmission  uids [159], or possibly as detergents in engine oils

[160–162] to keep insoluble combustion and oil oxidation products dispersed in the oil.

An alternative method for incorporating phosphorus into dispersant is exempli ed in Ref. 160.

This method involves reacting P

with a sulfurized hydrocarbon, such as sulfurized polyisobutyl-

ene, at high temperatures to form a thiophosphorus acid (see reaction 3.23, Figure 3.22). This inter-

mediate is then reacted with propylene oxide to form the hydroxypropyl esters of the phosphorus

acid (see reaction 3.24, Figure 3.22).

Aminoethane phosphonate copolymers have also been claimed to provide dispersancy, corrosion

protection, and pour point depression [163]. Among other applications mentioned in the literature

for these products or their salts in lubricating oils are the extrusion, cold rolling, and cold forging of

aluminum [164], offering improved rust inhibition [165] and antioxidant performance [166].

3.5.7 A SUMMARY OF THE PROPOSED MECHANISM FOR ANTIWEAR AND

XTREME-PRESSURE ACTIVITY OF PHOSPHORUS-BASED ADDITIVES

In attempting to produce an explanation for the activity of phosphorus-containing additives, it is

not easy, as explained earlier, to compare the results of the preceding investigations because condi-

tions vary from one investigation to another. No report evaluates all the different types of additives

with the same (high) level of purity under identical test conditions. However, it is possible to draw

together some of the more consistent “threads” running through the many papers. One parameter

highlighted in past reports (and con rmed by recent observations) is that the presence of oxygen

on the metal surface appears to be important for the activity of neutral aryl phosphates. This could

perhaps be one of the major reasons why TCP is sometimes found to be inactive. The composition

of the  lm formed on the surface is not yet completely de ned, but current work points toward the

formation of a self-regenerating polyphosphate layer in which amorphous carbon may be providing

the lubrication bene ts. The mechanism of formation of the polyphosphate layer and the role, for

example, of moisture is not yet clear but appears to be a stepwise process as follows:

• The adsorption of the material onto the surface (occurring through the

–

P=O and

–

bonds in the molecule).

• Either the hydrolysis of a –P–OR bond to form

–

OH (probably arising from water on the

surface but may also occur in solution) with the formation of acid phosphates/phosphites

PIB + P

OHPIB

PIB

OPIB

(CH

CH CH

)

(OCH

OH)

where PIB = polyisobutylene

(3.23)

(3.24)

FIGURE 3.22 An example of the preparation of a phosphorus-based detergent. (From Colyer, C.C., Gergel,

W.C., Chemistry and Technology of Lubricants, VCH Publishers, New York, 1992. With permission.)

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Ashless Phosphorus-Containing Lubricating Oil Additives 105

or, in the case of neutral phosphates, the cleavage of the C–O bond to release an aryl radical

and a residual

–

•

radical.

• Either reaction of the

–

OH or

–

(OH)

with the metal surface to form an iron salt,

possibly followed by further hydrolysis to release the remaining hydrocarbon moieties and

reaction of the new

–

OH groups with the surface to form polyphosphate, or the reaction

of the residual

–

•

radical with the iron surface to form a succession of Fe–O–P– bonds

leading to the formation of polyphosphate.

• Products that contain

–

C bonds (e.g., the phosphonates and particularly the phosphi-

nates) are less likely to operate by a mechanism involving hydrolysis, and the stability of

the P

–

C bond might be expected to prevent or delay the formation of the phosphorus-rich

surface layer with an adverse effect on EP properties. However, the same stability could

result in better friction-modi cation properties. The fact that phosphinates and phospho-

nates are active as AW/EP additives suggests that the

–

P=O bond is also involved in the

surface adsorption process, but that either the nature of the surface  lm may be different or

a polyphosphate  lm is produced as a result of the scission of a

–

C bond.

• The formation of amine salts results in an increase in activity, possibly as a result of the

stability of the ion and improved adsorption on the metal surface.

The mechanism of formation of phosphide, which is reported in many instances, has not yet been

clari ed but might possibly involve the amorphous carbon that then acts as a reducing agent on the

phosphate/phosphite layer as it forms on the surface.

These conclusions lead, as a broad generalization, to the order in activity and impact on surface

chemistry/stability as shown in Figure 3.23.

The preceding comments are, however, a simpli cation of the situation. Depending on the

length of the alkyl or alkaryl chain, if the iron salts that are formed are soluble in the oil they may

desorb from the surface, leading to poor AW/EP performance. Interaction with other surface-active

materials will inevitably in uence the performance of AW/EP additives, whereas depletion in use

due to oxidation, etc., will also affect performance.

3.6 MARKET SIZE AND COMMERCIAL AVAILABILITY

Information on the market size for ashless phosphorus-containing AW/EP additives is limited. An

approximate total market of ∼10,000 tpa is broken down, as given in Table 3.16. The data exclude

the use of phosphites as antioxidants in oil applications, which is separately estimated to be between

100 and 200 tpa.

The wide use of phosphorus-containing AW/EP additives is due, in addition to their good lubric-

ity performance, to the following features of value to formulators:

• Ashless

• Low odor, color, and volatility

• Low acidity/noncorrosive (applies to the neutral esters only)

Amine phosphites

Amine phosphates

Acid phosphites

Acid phosphates

Neutral phosphites

Neutral phosphates

Neutral phosphonates

Improvement in

EP properties

Improvement in

AW properties

Impact on

stability, etc.

FIGURE 3.23 An approximate ranking of the effect of structure on the AW, EP, and stability properties of

the base stock.

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106 Lubricant Additives: Chemistry and Applications

• Low toxicity

• Biodegradable (many but not all products)

• Compatible with most other types of additives (particularly the neutral esters)

• Soluble in a wide range of base stocks, both mineral oil and synthetic, and able to assist the

solvency of other additives

Although the physical properties of phosphorus-containing additives are not critical, the values for

the most widely used types of phosphate ester AW additives are given in Table 3.17, with TiBP as

an example of an alkyl phosphate, TCP as a natural phosphate, and ISO 32 grades of both types of

synthetic ester.

The major suppliers of phosphorus-containing lubricant additives are listed in Table 3.18, and

their current oil industry applications are summarized in Table 3.19. Undoubtedly, the most impor-

tant applications for the neutral aryl phosphates are hydraulic, turbine, and general circulatory oils,

whereas almost the entire market for the ethoxylated alkyl and aryl acid phosphates is to be found

in metalworking. The acid phosphates, acid phosphites, and amine salts of these acidic materials are

used in a mixture of metalworking, gear oils, hydraulic oils, etc., as indicated in Table 3.19.

The selection of an AW/EP additive depends on the speci c requirements for the application;

for example, whether both AW and EP performance is needed and what levels are required. When

this has been ascertained, secondary considerations may be the level of stability required (oxida-

tive or hydrolytic), potential interaction with other components of the formulation, and the effect on

surface-active properties such as foaming. Table 3.20 attempts to identify the additives that should

TABLE 3.16

A Breakdown of the Market for Ashless Phosphorus-

Containing AW/EP Additives

Product Market Size (t)

Alkyl phosphates 100

Aryl phosphates 6000

Acid phosphates, ethoxylated alkyl and aryl phosphates,

and amine salts of acid phosphates

3000

Phosphites, acid phosphites, dialkyl alkyl phosphonates,

and amine salts of acidic products

1000

TABLE 3.17

Typical Physical Properties of the Most Widely Used Grades of Phosphorus-Based

AW Additives

Property Unit TiBP TCP IPP/32 TBPP/32

Viscosity at 40°C cSt 2.9 25.0 32.3 33

Viscosity at 0°C cSt 10.0 1000 990 1500

Speci c gravity 20/20°C 0.965 1.140 1.153 1.170

Pour point °C <–90 –28 –27 –26

Acid number mg KOH/g 0.06 0.05 0.05 0.06

Water content % 0.01 0.06 0.05 0.05

Phosphorus level % 11.7 8.3 8.0 8.1

Flash point °C 155 240 245 255

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Ashless Phosphorus-Containing Lubricating Oil Additives 107

TABLE 3.18

Principal Suppliers of Ashless Phosphorus-Containing Lubricating Oil Additives

Producer

Neutral

Phosphates Alkyl/Aryl Acid Phosphates

Amine

Phosphates

Neutral

Phosphites

Acid

Phosphites

Alkyl

PhosphonatesAlkyl Aryl Nonethoxylated Ethoxylated Alkyl Aryl

Asahi Denka ××

Chemtura × × × × ×

Ciba Spec. Chem. × × ×

Croda × × ×

Daihachi × ×

Dover Chemicals ××

Johoku Chemicals × × × × ×

Krishna ××

Lanxess × ×

Libra Chemicals × × ×

Rhein Chemie × × ×

Rhodia × × × × × × × × ×

Sumitomo ××

Supresta × ×

United Phosphorus × × ×

Vanderbilt ×

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108 Lubricant Additives: Chemistry and Applications

be given prime consideration when taking these secondary requirements into account. Products that

demonstrate better AW than EP activity, and vice versa, are shown. However, the boundary between

AW and EP performance is not clear-cut and much depends on the application requirements.

3.7 TOXICITY AND ECOTOXICITY

It was mentioned earlier in this chapter that concern had been expressed in the past regarding

the toxicity of phosphorus-containing products, particularly TCP. Today, with increasing focus on the

environmental behavior of chemicals, their ecotoxicity is also under scrutiny. As a result, detailed

investigations into both the toxicity and the ecotoxicity have been carried out on alkyl and aryl phos-

phate esters. The results are summarized in recent publications [59,167], and most are available in the

safety data sheets associated with different product types. The data demonstrate a relatively low (but

variable) order of toxicity and ecotoxicity. No signi cant risks in handling are anticipated, provided

the manufacturer’s guidance, which is essentially the same as for mineral oils, is followed.

The concerns over TCP arose as a result of the o-cresol content in the feedstock as tri-ortho-

cresyl phosphate (TOCP) was found to be a signi cant neurotoxin. Initially, the level of o-cresol in

the feedstock was high (up to ∼25%) and signi cant amounts of TOCP were present in the  nished

product. Although the initial focus was on TOCP, it was later acknowledged that any isomer con-

taining the o-cresyl moiety was neurotoxic (e.g., mono-o-cresyldiphenyl phosphate was said to be

10 times more neurotoxic than TOCP [168]). For these reasons, the o-cresol content of the feedstock

used in the manufacture of TCP has been progressively reduced over time. In recent years, production

has moved to the use of 99% minimum m- and p-cresol. Levels of o-cresol in the feedstock are now

frequently <0.05%, and the TOCP content can be as low as parts per billion. Mackerer et al. [169]

TABLE 3.19

Principal Applications for Ashless Phosphorus-Containing AW/EP Additives

Application

Triaryl

Phosphate

Trialkyl

Phosphate

Amine

Phosphate

Acid

Phosphates

Alkyl/Aryl

Phosphites

Automotive

AT F 

Gear oil 

Power steering



Shock absorber 

Electric motor 

Industrial

Hydraulic oils 

Gear oil  

Turbine oils 

Compressor oils 

Gas oil 

Universal tractor 

Metalworking 

Grease 

Way oils 

Circulating oil 

Vegetable oil 

Aircraft

Piston engine 

Turbine engine 

Grease 

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Ashless Phosphorus-Containing Lubricating Oil Additives 109

estimate that the toxicity of the TCP now available commercially is ∼400 times less than that of

material available in the 1940s and 1950s, and a recent evaluation of the organophosphorus-induced

delayed neurotoxicity (OPIDN) of a commercial aviation gas turbine oil containing TCP was nega-

tive [170]. However, in view of past concerns, the use of TCP is now largely restricted to aviation gas

turbine oils. Most general industrial applications that require an aryl phosphate AW additive now use

the isopropylphenyl or, to a lesser extent, the tertiarybutylphenyl variants. In standard tests, neither

of these types display OPIDN from acute oral ingestion. There are, however, some differences in

the toxicity and the ecotoxicity behavior between the different aryl phosphates. For example, the

reproductive toxicity of the synthetic aryl phosphates, together with TXP, was recently studied in rats

(according to Organisation for Economic Co-operation and Development [OECD] method 422). Both

the isopropylphenyl phosphate and the TXP showed adverse effects at moderate to high dose levels,

but these were reversible when exposure ceased. The TBPP (produced according to reaction 3.2) did

not display any adverse effects.

Differences are also seen in ecotoxicity behavior. Owing to the high TPP content in the lower-

viscosity grades of the synthetic phosphates (particularly ISO VG 22 and 32), these products have

the worst ecotoxicity behavior. The tertiarybutylphenyl phosphates normally have a higher TPP

content than the corresponding grade of IPPP and therefore, of the synthetic phosphates, possess

relatively worse ecotoxicity. By comparison, the IPPPs generally show satisfactory behavior in these

tests. One ISO VG 46 isopropylphenyl phosphate-based AW/EP additive has, for example, been

approved by the German Environment Agency (Umweltbundesamt) for use in rapidly biodegradable

hydraulic  uids, products that are eligible for the “Blue Angel” environmental award [171].

Another difference between the product types is displayed in biodegradability tests. The tests

were carried out according to OECD method 301F (Manometric Respirometry). In this test, bio-

degradation is measured as the net oxygen uptake over that occurring in blank tests containing only

TABLE 3.20

Guidance on the Selection of AW and EP Additives

Required Characteristic

Good AW

Performance

Good EP

Performance

Combination of AW

and EP Performance

Non-phenolic additive Neutral alkyl

phosphates; dialkyl

alkyl phosphonates

Acid alkyl phosphates;

acid alkyl

phosphonates and

their salts; neutral

and acid alkyl

phosphites

Mixtures of neutral and

acid phosphates, etc.

Good hydrolytic stability TXP, dialkyl alkyl

phosphonates

Acid alkyl

phosphonates and

their salts

—

Good oxidation stability Neutral tertbutylphenyl

phosphates

Hindered aryl

phosphites

—

Low foaming/air release Neutral phosphates,

dialkyl alkyl

phosphonates

Neutral phosphites —

Good toxicity performance Neutral tertbutylphenyl

phosphates

Acid IPPPs —

Good ecotoxicity

performance

Neutral IPPPs — —

Multifunctionality, for

example, rust inhibition,

antioxidant

Neutral and acid

phosphites, acid

phosphates

Acid IPPPs

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110 Lubricant Additives: Chemistry and Applications

inoculated medium. The extent of biodegradation is calculated from the mass of test material added

to the test vessels and its theoretical oxygen demand for complete biodegradation. The test was car-

ried out in triplicate on the ISO VG 46 grades of different types of aryl phosphates manufactured

according to reaction 3.2. The results are summarized in Table 3.21.

The results are initially in the order of their hydrolytic stability, but it is interesting that TXP,

after a slow start, eventually reaches the same level as the synthetic  uids and might have exceeded

them had the test been extended.

In view of these data, the tertiarybutylatedphenyl phosphate would be regarded as readily biode-

gradable (Pw1), whereas the TXP and IPPP would be classi ed as inherently biodegradable (Pw2).

Despite of relatively benign ecotoxicity of the higher viscosity grades of aryl phosphates, all

these products are classi ed as marine pollutants because of the UN Marine Pollutant Classi cation.

However, because they are used at low concentrations, they are unlikely to contribute signi cantly to

the  nished product’s ecotoxicity.

The toxicity of other phosphorus-containing compounds is less well documented. Drake and

Calamari [172] indicate that dialkyl alkyl phosphonates generally have a low level of acute toxicity,

which decreases with increasing chain length, apparently a general observation for these classes of

compounds. As with alkyl phosphates, certain short-chain products can be skin irritants. No clues

were found to their environmental behavior, but in view of the absence of phenolics and improved

hydrolytic stability, it might be surmised that  sh toxicity could be good but biodegradability would

be inferior to that of the phosphates. Neutral phosphites, particularly the alkyl phosphites, would be

expected to have good toxicity and biodegradability behavior, but their ease of hydrolysis, which is

the factor assisting the biodegradation, would probably result in poor aquatic toxicity. The future of

the nonylphenyl phosphites is uncertain; the U.S. National Toxicology Program currently lists non-

ylphenol as an estrogen mimic and also as a thyroid disruptor. The acid phosphates, acid phosphites,

and their salts, particularly amine salts, are likely to be classi ed as irritants and, due to their ease

of hydrolysis, may again be toxic to aquatic organisms. In all cases, it is essential that reference be

made to the health and safety information provided by the manufacturer.

3.8 THE FUTURE FOR ASHLESS PHOSPHORUS-BASED

LUBRICATING OIL ADDITIVES

Although ashless phosphorus-containing additives are used in many industrial applications, there

are certain market segments where they have not, to date, been successful. These are principally

in automotive engine oils where the use of ZDDP dominates due to a combination of price and

multifunctionality and in gear oils where sulfur continues to be the EP additive of choice. However,

the use of chlorine as an EP additive, particularly in metalworking applications, is in decline for

environmental reasons and is expected to be slowly substituted by P/S combinations. The use of sul-

fur alone in applications requiring high EP performance may also move to P/S mixtures to reduce

the total sulfur level and the ability to more readily “tailor” the balance of AW and EP performance

TABLE 3.21

OECD 301F Biodegradability Data on Different

Types of Aryl Phosphates

Product (ISO VG 46

Base Stocks)

% Biodegradability After

10 Days 28 Days 68 Days

TXP 5 29 70

IPPP 18 47 65

TBPP 25 62 72

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Ashless Phosphorus-Containing Lubricating Oil Additives 111

to the application. The potential in other market segments including those traditionally using ZDDP

is discussed in greater detail in the following:

3.9 LUBRICATING OIL FORMULATIONS (GENERAL)

The current trend toward the use of groups II and III mineral oil base stocks for general industrial appli-

cations, with improved antioxidant response but inferior lubricity as a result of the removal of aromat-

ics and sulfur compounds, could encourage the wider use of phosphorus. The lack of competition for

the surface, which has previously been shown for TCP in stocks containing naphthenics and aromatics,

should result in the increased activity of phosphorus compounds. Their use may also be bene cial due

to their ability to aid the dissolution of additives that might otherwise have limited solubility.

In Europe, legislation (Directive 2000/769/EC) implemented in 2004 requires a substantial

reduction in sulfur dioxide (SO

) emissions from the combustion of waste materials including waste

oil. This may result in a move to lower sulfur levels in lubricating oils (including metalworking oils)

and a possible replacement by phosphorus to restore the level of AW/EP performance.

3.10 HYDRAULIC OILS

In recent years, there has been a move toward the use of ashless hydraulic oils. This is mainly for

two reasons. First, as a result of the sensitivity of ZDDPs toward moisture and the resulting deposi-

tion of zinc oxide/sul de. This deposit can adversely affect the  lterability of the oil and reduce oxi-

dation stability. Second, there is increasing concern regarding the environmental behavior of heavy

metals. Regulatory controls, however, are likely to extend further to cover metals such as zinc, as

in the Great Lakes Initiative between the United States and Canada. As the zinc cannot be easily

removed from waste at the ef uent plant, there has been a focus on the reduction in use levels.

Concern has also been expressed in certain countries regarding the smell of sulfur arising from

the degradation of the ZDDP when the hydraulic oil is used, for example, in elevators (Dixon, R.,

Shell Global Solutions, Private Communication, November 2007).

3.11 AUTOMOTIVE ENGINE OILS

Vehicle emissions legislation (e.g., in the United States, Europe, and Japan) now exists to control

and substantially reduce the levels of particulates, hydrocarbons, carbon monoxide, and oxides of

nitrogen in the engine exhaust. The engine manufacturers have met these requirements by a variety

of design changes that impact the composition of oils and fuels in the following ways:

• The introduction of catalytic converters to oxidize the hydrocarbon and carbon monoxide

components to carbon dioxide and water, and reduce the nitric oxide (NO) to nitrogen, has

been very successful in reducing emissions. When they operate at their normal operat-

ing temperature and optimum level of ef ciency, they are almost 100% ef cient and most

of the remaining emissions occur in the time before the catalyst reaches “light-off” tem-

perature. Many studies into reducing this period to achieve yet lower emissions have been

conducted. Although much success has been achieved, further progress may be hindered

by the formation of a deactivating  lm on the catalyst surface by the phosphorus from the

ZDDP antioxidant and AW/EP additive. As a consequence, there is pressure to reduce the

phosphorus content of engine oils to minimize catalyst fouling. Currently, oil speci ca-

tions such as ILSAC GF-4 and ACEA Cx limit the phosphorus content of both diesel and

gasoline engine oils to 0.05–0.09% with the actual level being linked to the amount of

catalyst used and the expected service interval. Further reductions below 0.05% are being

considered, but there is a concern that such a low level could adversely affect the durabil-

ity of certain engine parts, for example, the valve train and timing chain, as reducing the

ZDDP content also reduces the wear protection. However, a recent study [173] suggests that

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112 Lubricant Additives: Chemistry and Applications

the behavior of phosphorus compounds in wear and catalyst tests varies according to the

way in which phosphorus is incorporated into the molecule. Further work reports that

it is possible to achieve improvements in catalyst protection (and fuel economy) by

reducing the ZDDP content and then adding a metal-free phosphorus-containing AW addi-

tive [174].

• In an attempt to increase fuel economy, the so-called fuel-ef cient lubricants are being

developed. These are usually lower-viscosity products (since energy losses decrease with

viscosity), sometimes complemented by the use of friction modi ers. However, low-viscos-

ity oils may cause increased wear of some engine components, and the necessity for improv-

ing the wear protection is being studied. The ILSAC GF-5 speci cation, for example, will

necessitate the use of some form of friction modi er to guarantee the required level of

economy. Currently, molybdenum compounds or long-chain esters are under evaluation, but

other approaches (e.g., the use of functionalized viscosity modi ers) are also being studied

as the ability of these additives to deliver reduced friction over long periods is uncertain

(Mainwaring, R., Shell Global Solutions, U.K.). ZDDP is linked to increased friction and

therefore reduced ZDDP levels may also be required.

• To reduce the particulate (soot) levels in exhaust gas, the diesel engines in many

passenger cars and trucks need to use particulate  lters. These  lters can also remove the

ash-containing residue produced from metallic fuel and lubricant additives, and if they are

not occasionally cleaned, they will block causing a deterioration in engine performance.

The engine builders, however, are trying to preserve or even extend service intervals and

consequently are interested in reducing the ash content of the oil. Although ZDDP is not

the only source of metals in the oil, a reduction in zinc content will follow automatically

from any reduction in the phosphorus content (as long as ZDDP remains in formulations)

and will therefore help to reduce engine oil ash content (Mainwaring, R., Shell Global

Solutions, U.K.).

• One of the techniques used to remove the soot from the particulate  lters (and thereby

maintain an acceptable engine back pressure) has been to oxidize the deposited carbona-

ceous material by nitrogen dioxide (NO

). This is obtained from the exhaust gas by catalyti-

cally oxidizing the NO component. The oxidation of the soot to carbon dioxide effectively

removes carbonaceous  lter deposits, and the NO

is such a powerful oxidant that it enables

the process to be carried out at a relatively low temperature (∼250°C). Unfortunately, the

catalyst used to oxidize the NO preferentially oxidizes any SO

in the exhaust, thereby

reducing the ef ciency of the NO conversion. Additionally, the sulfur trioxide (SO

) formed

passes through the trap in the gas phase and is converted there into sulfuric acid by the

water in the exhaust. The sulfuric acid (monitored as “sulfates”) contributes, as droplets,

to the overall level of particulate emissions and is clearly undesirable if the exhaust gas

is inhaled. Any reduction in sulfur content by lowering ZDDP levels in engine oils also

reduces the phosphorus content arising from this additive. Supplemental phosphorus may

therefore be needed.

• Increased emphasis on fuel economy led some manufacturers to introduce direct injection

strati ed charge gasoline engines. Conventional catalysts cannot remove oxides of nitrogen

(NO

) in these “lean-burn” engines, and, as a result, NO

storage catalysts have been devel-

oped in which the oxides are stored as nitrates by reaction with barium sulfate contained in

the catalyst coating. When the barium-containing sites become saturated, the engine switches

to stoichiometric or slightly rich operation at which temperature the nitrates break down and

release the NO

, thus promoting its reduction through the conventional route of reaction

with hydrocarbons and carbon monoxide. Unfortunately, barium sites react preferentially

with any sulfur oxides present, reducing their ability to “store” NO

. As a consequence,

there is again pressure to reduce the fuel sulfur content. However, these levels are already

being lowered (see “Fuels” below), and at such levels, the engine oil begins to be a signi cant

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Ashless Phosphorus-Containing Lubricating Oil Additives 113

contributor to exhaust “sulfur” content.A debate has therefore arisen regarding the future

level of lubricant sulfur, and diesel engine manufacturers have already expressed interest in

lubricants with a sulfur level as low as 0.2%— considerably below the current value of ∼1%

(Mainwaring, R. Shell Global Solutions, Private Communication, January 2008).

In 1999, the European Union (EU) issued emission requirements for heavy-duty diesels that

anticipated signi cant reductions in NO

, CO, unburnt hydrocarbons, and particulates over

the period from 2001 to 2008. The greatest challenge was to lower NO

while at the same time

reducing particulates as measures to correct the former normally resulted in an increase in the

latter. In addition, in reducing NO

, the ef ciency of the diesel engine would be impaired, and

the result would be an increase in fuel consumption and CO

emissions. However, a technique

called selective catalytic reduction (SCR) has now been developed and adopted by several

engine manufacturers in the EU [175]. This involves injecting an aqueous solution of urea

(CO(NH

)

) into the exhaust stream where it degrades to carbon dioxide and ammonia (NH

The NH

then reduces the NO

to nitrogen (N

) and water on a tungsten/vanadium catalyst. It

is obviously important to avoid NH

being released into the atmosphere, and another catalyst

is required to oxidize any residual NH

while avoiding oxidation of the nitrogen.

To date, SCR (low NO

but high particulates) has generally been favored over diesel partic-

ulate  lters (high NO

but low particulates) for reducing emissions by many EU manufactur-

ers. It allows engines to operate more ef ciently—indeed suf ciently so as to more ef ciently

and more than offsets the cost of the urea. However, although it is an effective technique,

there are concerns about its size, weight, guaranteed availability throughout Europe, and its

ef cacy at the lower temperatures encountered in the exhaust of light-duty applications. These

problems have so far prevented its application to passenger cars. The effects on pollution if

urea is not used in a system designed for its use and how the solution would be made available

to the ordinary motorist are currently the subject of further investigation and debate.

3.12 FUELS

• As a result of the concern regarding the direct and indirect impacts of fuel sulfur on

engine emissions, there is pressure to reduce the sulfur content. In the EU, a limit of

50 ppm maximum sulfur in diesel fuel was introduced in 2005, with a further reduction to

10 ppm in 2008 for gasoline engines and in 2009 for diesel engines. At such low levels, it

may be necessary to restore the lubricity of the fuel by additives, and incorporating a small

amount of phosphorus has been shown to be effective [176].

• Considerable concern currently surrounds carbon dioxide emission and its connection with

global warming. Within the EU, the auto producers have reached a voluntary agreement

with the commission to achieve a carbon dioxide emission target of 140 g/km by 2008 with

a further reduction to 130 g/km by 2012. This will encourage a move toward more fuel-

ef cient vehicles, particularly those that are smaller and lighter, and perhaps also to thinner

lubricants requiring even better AW protection.

In the United States, the Senate has ruled that carbon dioxide emission from trucks are a

pollutant rather than a “by-product” of combustion. This is expected to promote reductions

in carbon dioxide levels and hence encourage the use of lower viscosity, more fuel-ef cient

diesel engine oils (Mainwaring, R., Shell Global Solutions, Private Communication, Janu-

ary 2008).

3.13 CONCLUSIONS

Ashless phosphorus-containing additives are available in a wide range of structures and perfor-

mance. Although most are used as AW and EP additives for industrial oils, they can also function as

antioxidants, rust inhibitors, metal passivators, and detergents. In some cases, the multifunctionality

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