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474 Lubricant Additives: Chemistry and Applications

The acid is then neutralized with zinc oxide to yield ZDDP at ∼70 to 90°C. There are two types of

ZDDP structure, neutral and basic, both of which can be observed by

P NMR (nuclear magnetic

resonance). The basic salt is observed at 104 ppm with the neutral salt at 101 ppm. The neutral salt

exists as an equilibrium mixture of monomer, dimer, and oligomers. The basic salt consists of a

central oxygen atom surrounded tetrahedrally by four zinc atoms and six dithiophosphate groups

attached symmetrically to the six edges of the tetrahedron. In most industrial processes, the ZDDP

is left slightly basic for providing improved stability.

2(RO)

P(S)SH + ZnO

Neutral ZDDP

−H

3[(RO)

P(S)S]

Zn +

Basic ZDDP

ZnO

19.4.4 ZDDP DEGRADATION MECHANISMS

The type of alcohol used to prepare the ZDDP will determine its thermal and oxidative stabilities.

The most reactive ZDDPs are derived from secondary alcohols and especially those that are lower

in molecular weight. Solubility is a limiting factor at carbon numbers less than  ve; therefore, most

ZDDPs will use alcohols with carbon numbers greater than  ve. Alcohols with a lower carbon num-

ber may be used if they are combined with a higher alcohol during synthesis.

The type of alcohol used in the preparation will have a signi cant effect on the stability. In most

cases, the thermal stability of the ZDDP is as follows:

Aryl > primary alkyl > secondary alkyl

The least stable ZDDPs tend to provide improved wear at lower engine oil temperatures. Therefore,

the following applies for antiwear action:

Secondary alkyl > primary alkyl > aryl

Tables 19.4 through 19.6 indicate the performance of various ZDDPs in a range of gasoline engine

wear tests.

TABLE 19.4

Comparison of ZDDP Types in Sequence VE Wear Test

Alcohol Type Zn (%) as ZDDP

Sequence VE Wear

Average (µm) Maximum (µm)

Mixed 0.127 36 203

secondary

primary

primary 0.124 121 495

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Additives for Crankcase Lubricant Applications 475

The mechanism by which secondary alcohol ZDDPs thermally degrade is shown in Figure

19.19. The degradation mechanism proceeds rapidly as the temperature rises and is made easier as a

hydrogen on the β position readily leaves to form the alkene. This mechanism may explain why the

secondary ZDDPs are much more active antiwear agents particularly at lower temperatures.

In contrast, the primary alcohol ZDDPs are more stable due to the absence a tertiary hydrogen

on the β carbon and therefore more useful for higher-temperature operation and wear such as that

found in diesel engines. The mechanism of thermal degradation is through sequential alkyl trans-

fers and relies on an intermolecular alkyl transfer (Figure 19.20).

19.4.5 SEQUENTIAL ALKYL TRANSFERS (PRIMARY ZDDP)

Other additives will affect the rate of thermal degradation of the ZDDP. For instance, it is known

that succinimide dispersants will complex with the ZDDP, making it more resistant to thermal

degradation. It is therefore important to recognize this when formulating an oil additive. Too much

dispersant may tie up the ZDDP, leaving it unable to form an effective antiwear  lm. A balance has

to be found, which is dependent on the ZDDP type and the dispersant structure.

TABLE 19.5

Comparison of ZDDP Types in Sequence VD Wear Test

Alcohol Type Zn (%) as ZDDP

Sequence VD Wear

Average (µm)

secondary 0.13 18

Mixed

secondary

primary 0.13 48

TABLE 19.6

Comparison of ZDDP Types in Sequence IIID Wear Test

Alcohol Type

Zn (%) as

ZDDP

Sequence IIID Wear

Average (µm)

Mixed 0.13 25

secondary

primary

primary 0.13 175

P=S

−

P=S

FIGURE 19.19 β-Elimination (secondary ZDDP).

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476 Lubricant Additives: Chemistry and Applications

ZDDP, also degrades at lower temperatures by oxidation (<100°C), yielding compounds that

are bene cial antiwear agents (see Figure 19.21). This leads to the mechanism of oxidative inhibi-

tion by ZDDPs, which occurs through the thiophosphoryl disul de intermediate (see Figure 19.22).

A more detailed mechanism of the antioxidant function of the thiophosphoryl disul de intermedi-

ate has been reported [18].

The effectiveness of ZDDP in decomposing hydroperoxides has been linked to wear rates in

a motored engine test [19]. Secondary ZDDPs were better at decomposing peroxides than their

primary counterparts. There is a plethora of papers in the literature with detailed analyses of the

various degradation pathways for the ZDDP molecule and its subsequent effect on the wear and

oxidation properties of the lubricant [13,14,20–22]. Although these give an excellent insight into the

ZDDP degradation mechanisms, it must be remembered that the degradation pathways are strongly

dependent on the test conditions, that is, temperature and amount and type of oxidants.

19.4.6 ANTIWEAR TESTS

The Sequence VE and Sequence IVA gasoline engine tests (for API SJ and ILSAC GF-3, respectively)

are designed to test the ability of a lubricant to prevent low-temperature sliding wear in the valve

train of a gasoline engine. Secondary ZDDPs will usually perform better than primary ZDDPs in

both tests. However, there are certain cases where it is possible to achieve a passing test result with

primary ZDDP especially with formulations in which a low level of detergent is present. High-temper-

ature gasoline wear is measured in the Sequence IIIE and Peugeot TU3 scuf ng test and responds

well to most types of ZDDP.

Wear in a diesel engine is usually accelerated by the presence of soot. There are several tests to

measure wear in diesel engines of which the four most common tests are listed in Table 19.7.

−

RCH

−O

RCH

−

Zn phosphate

(RCH

P=S

(RCH

∆

FIGURE 19.20 Sequential alkyl transfers (primary ZDDP).

Thiophosphor

l disulfide

[O]

[O]/∆

Antiwea

species

FIGURE 19.21 Thiophosphoryl disul de formation. ([O] can be almost any oxidant such as O

, ROOH,

, Cu

, and NO

+2ROO

FIGURE 19.22 Thiophosphoryl disul de decomposition of peroxy radical.

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Additives for Crankcase Lubricant Applications 477

In addition,  eld tests such as the Volvo VDS and Scania LDF speci cations also specify

limits for wear on critical engine components. Depending on the operating temperature of the oil,

primary or secondary ZDDPs will be used. For heavy-duty diesel engine oils, both primary and

secondary ZDDPs are widely used. In certain cases, an aryl ZDDP may be used for maximum

thermal stability.

19.4.7 OTHER ANTIWEAR AGENTS

Boron is widely used to provide antiwear properties in lubricants. A common method is to react

boric acid with the amine moiety of a succinimide dispersant. The boron works by reacting at the

metal surface to form boric acid, which has a layered structure with weak interlayer bonds [15]. On

its own, it is unlikely to give the same level of antiwear performance as ZDDP, but when used in

combination with other antiwear additives, it will reduce wear.

19.5 ANTIOXIDANTS

19.5.1 I

NTRODUCTION

Most of the lubricants, by virtue of being hydrocarbon based, are susceptible to oxidation [23,24].

If oxidation is not controlled, lubricant decomposition will lead to oil thickening; sludge formation;

and the formation of varnish, resin, and corrosive acids [25,26].

The oxidation reactions that occur in a lubricant at elevated temperatures in the presence

of atmospheric oxygen may lead to declined lubricant performance such as signi cant increase

in kinematic viscosity observed in severe service conditions or during extended drain intervals

(Figure 19.23).

In general, all types of base oils require addition of antioxidants depending on the amount of

unsaturation and natural inhibition present. The re ned mineral base oils contain natural inhibitors

in the form of sulfur and nitrogen compounds suf cient for many applications. The oxidative stabil-

ity of such oils shows a distinct, relative long induction period. In contrast, hydro ned oils do not

contain these natural inhibitors or contain them only in small quantities. Besides sulfur and nitrogen

compounds, other compounds such as aromatics or partially hydrogenated aromatics and phenolic

oxidation products can be of importance; their inhibiting effect is lost during various re ning pro-

cesses. Traditional mineral oils, such as group I and group II base stocks, show moderate oxidative

resistance. Synthetic oils such as polyol esters and hydrogenated polyalpha-ole ns (PAOs) exhibit

the highest oxidative stability due to their low unsaturated content. The most unstable oils include

highly unsaturated vegetable triglycerides such as corn, sun ower, canola, and peanut.

An overall scheme of lubricant oxidation starts with the conversion of hydrocarbon substrates to

carbonyl compounds (Figure 19.24). Coupling of these polar compounds through aldol and related

reactions builds molecular weight. Eventually, at very high molecular weight (i.e., >1000), the oil is

converted into insoluble sludge, which can precipitate on the engine hardware.

TABLE 19.7

Diesel Wear Tests

Engine Test Speciﬁ cation Wear Measured

GM roller follower wear test API CG-4, CH-4 Roller follower wear

Cummins M11 API CH-4 Valve bridge wear

Mercedes OM602A MB Sheet 228.X, 229.X Cam nose wear

ACEA B and E sequences Cylinder wear

Mitsubishi 4D34T Global DHD-1, JASO DH-1 Cam nose wear

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478 Lubricant Additives: Chemistry and Applications

19.5.2 MECHANISM OF OXIDATION OF LUBRICATING OILS

The oxidation of petroleum hydrocarbons proceeds according to a radical chain mechanism through

alkyl and peroxy radicals in three stages [27].

19.5.2.1 Initiation

RH + O

→ R

•

+ HOO

•

The initiation step starts by the slow abstraction of a hydrocarbon proton by molecular oxygen

to form alkyl and hydroperoxy free radicals. This process is also referred to as autooxidation

and is favored by time, higher temperature, and transition metal (i.e., iron, nickel, and copper)

catalysis.

19.5.2.2 Propagation

Propagation starts by the rapid reaction of more oxygen with an alkyl-free radical to form an

alkyl peroxy radical, which is also capable of hydrocarbon abstraction to form hydroperoxide

FIGURE 19.23 Antioxidants can prevent premature oil thickening.

Hydrocarbon

Air (with or without metal)

Primary oxidation products

(e.g., aldehydes, ketones, and carboxylic acids)

Aldol condensation

High-molecular-weight products

Polycondensation

polymerization

Slud

FIGURE 19.24 Degradation of mineral oils.

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Additives for Crankcase Lubricant Applications 479

and another alkyl radical. The alkyl radical then may react with more oxygen starting the chain

over [25].

•

+ O

→ ROO

•

ROO

•

+ RH → ROOH + R

•

19.5.2.3 Peroxide Decomposition

Alkyl hydroperoxides are quite reactive and may decompose, especially at higher temperature, to

form additional radical species. These can undergo further abstraction and chain propagation reac-

tions that increase the overall oxidation process. Alkyl peroxides and alkyl peroxy radicals also

decompose to neutral oxidation products such as alcohols, aldehydes, ketones, and carboxylic acids.

Hydroperoxide decomposition to neutral oxidation products can be viewed as a chain termination

step, since free additional radicals are not formed.

ROOH RO

2ROOH RO

+ ROO

+ H

+ ROOH Various products

Engine surface

Free radicals

or lubricant + ROOH

Alcohols

Inactive products

19.5.2.4 Termination (Self and Chain Breaking)

During termination stage, the radicals either self-terminate or terminate by reacting with oxidation

inhibitors [23].

ROO

•

+ ROO

•

→ Inactive products

ROO

•

+ IH → ROOH + I

•

Alternative oxidation pathways are listed later.

19.5.2.5 Radical Formation

RH O ROOH

ROOH RO OH

ROH NO R HNO

hydroperoxides

⫹

⫹⫹

⫹

→

••

•

••

→⫹⫹ ⫹O NO RONO RONO

nitrite and nitrite esters

Nitrogen oxides can react with alcohols (an oxidation product) to form alkyl free radicals, which

react with NO

and oxygen to form nitrate and nitrate ester oxidation products.

19.5.2.6 Decomposition and Rearrangement

ROOH + SO

→ ROH + SO

ROOH + H

→ Carbonyl compounds

•

and ROO

•

→ Oxidation products

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480 Lubricant Additives: Chemistry and Applications

The action of sulfur dioxide and H

(from SO

+ water) on alkyl hydroperoxides also leads to

neutral oxidation products such as alcohols and carbonyl compounds. Aldehydes and ketones can

react further and form polymers. Carboxylic acids can attack metallic hardware: rings, valve train,

and bearings leading to extensive wear. Furthermore, they can form metal carboxylates, which

further increase the oxidation rate. Wear metals can also enhance the rate of oxidation [26].

19.5.3 OXIDATION INHIBITORS

Oxidation inhibitors can be classi ed into the following manner:

1. Radical scavengers

a. Nitrogen-containing inhibitors—aryl amines

b. Oxygen-containing inhibitors—phenols

c. ZDDPs

2. Hydroperoxide decomposers

a. Sulfur-containing inhibitors—sul des, dithiocarbamates, and sulfurized ole ns

b. Phosphorus-containing inhibitors—phosphites and ZDDPs

Note that ZDDPs function by both antioxidant mechanisms in addition to providing antiwear

protection (covered in Section 19.4.2). This dual- (or tri-) functional ability of ZDDPs to provide

antiwear performance and antioxidancy by two different pathways explains why these additives are

by far the most effective inhibitors, especially on a cost/performance basis.

Phenols or amines of speci c structures function as radical acceptors by transfer of a hydrogen

atom from the oxygen or nitrogen atom to the hydrocarbon or peroxy radical. The inhibitor radicals

thus formed react through radical combination or electron transfer to give ionic compounds or by

addition reactions or formation of complexes that do not maintain the radical chain mechanism of

the autoxidation reaction. In further reactions, very often ethers, betones, polyaromatic systems,

etc. are formed.

19.5.4 HINDERED PHENOLS AND ARYLAMINES

Hindered phenols and arylamines are two prominent examples of inhibitors that act as radical

scavengers through hydrogen transfer. Figure 19.25 illustrates the mechanism of phenol

performance.

Among various types of phenols, polyalkylphenols have signi cant performance advantage

over nonalkylated compounds (Figure 19.26). The effect of substituents demonstrates the role of

+ ROH

Resonance

stability

FIGURE 19.25 Mechanism of inhibition by hindered phenols.

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Additives for Crankcase Lubricant Applications 481

electron density and steric hindrance at the phenolic oxygen; an increase in the number of alkyl

radicals and the introduction of electron donors in the o- and p-positions increase the ef ciency,

whereas the introduction of electron acceptors reduces it. α-branching of alkyl groups in o- position

or increasing the alkyl group to C4 in p-position has a bene cial effect [27].

A new type of antioxidant involves the base-catalyzed addition of hindered phenol to Michael

acceptors such as acrylate esters. Although more expensive than simple alkylphenols, the originators

of this chemistry claim improved upper piston deposit control in certain diesel engine tests and

better seal compatibility than arylamines.

S-coupled (X = 1,2) alkylphenols combine the antioxidant bene ts of the phenol and sul de

groups.

The mechanism of oxidation inhibition (based on generation of nitrogen radicals) by arylamines

is presented in Figure 19.27.

In general, the alkylated arylamines are more effective antioxidants than alkylphenols because

they are

Able to trap more equivalents of radicals: four versus two

Able to better stabilize nitrogen or nitroxyl radicals (by two aryl rings instead of one)

Able to operate at both low and high-temperature mechanisms (the latter of which

regenerates the alkylated arylamine)

•

−CH

−CO

(S)

FIGURE 19.26 Major types of phenolic antioxidants.

ROH+

ROO

Diarylamine

+ ROOH

FIGURE 19.27 Mechanism of inhibition by arylamines.

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482 Lubricant Additives: Chemistry and Applications

Oil-soluble amines such as diphenylamine, phenyl-α-naphthylamine are the most common type of

amine antioxidants used in lubricants. Diphenylamine can be alkylated with aluminum chloride

catalyst and a mixture of branched nonene ole ns. A mixture of mono- and dialkylate (Figure 19.28)

is intentionally targeted by the reactant charge ratio to produce a liquid product. Diphenylamine is

particularly suited for applications at elevated temperatures. Therefore, it is often used to lubricate

supersonic aircraft engines and bearings. In these applications, arylamines prevent sludge forma-

tion in synthetic ester oils.

19.5.5 SULFUR AND PHOSPHORUS CONTAINING ANTIOXIDANTS

Sulfur, phosphorus, and compounds containing both the elements decompose peroxides by reducing

the hydroperoxide in the radical chain to alcohols; the sulfur or phosphorus atoms are correspond-

ingly oxidized (Figure 19.29). Bivalent sulfur compounds (sul des, etc.) yield sulfoxides and sulfones;

trivalent phosphorus compounds (phosphates) are transformed into pentavalent ones (phosphates).

Organic compounds of tetravalent sulfur act as peroxide decomposers, but the corresponding hexava-

lent, in line with theory, do not. Destruction of hydroperoxides is important so that these intermedi-

ates do not decompose into radicals, which can continue the chain oxidation process.

Phosphates are somewhat hydrolytically unstable, and due to limits on phosphorus level in

crankcase oils, these additives are generally limited to application in gear lubricants. ZDDPs are

more ef cient phosphorus-containing antioxidants for crankcase oils, with the added bene t of

antiwear performance.

ZDDPs convert hydroperoxides to one equivalent of an alcohol and a carbonyl compound

(Figure 19.30). The ZDDP is regenerated intact and is available to convert another hydroperoxide

molecule. Many cycles of hydroperoxide decomposition are carried out by the ZDDP, before its

eventual breakdown.

DPA

(DPA)

22%

76%

FIGURE 19.28 Alkyl aromatic amine as antioxidant.

ROOH + R′

ROOH + (R′O)

ROH + R′

P=O

ROH + (R′O)

P=O

Phosphite Phosphate

ROOH + R′ S R′′

ROH + R′ S R′′

SulfoxideSulfide

FIGURE 19.29 Mechanism of S- and P-containing antioxidants.

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Additives for Crankcase Lubricant Applications 483

19.5.6 SULFUR COMPOUNDS

Elemental sulfur is an ef cient oxidation inhibitor; however, it shows a strong corrosion tendency.

From the practical approach, numerous dialkyl sul des and polysul des, diaryl sul des, modi ed

thiols, mercaptobenzimidazoles, thiophene derivatives, xanthogenates, zinc dialkyldithiocarba-

mates, thioglycols, thioaldehydes, and others have been examined as inhibitors. Dibenzyl disul de

must be mentioned among the alkyaromatic sulfur compounds.

Alkylphenol sul des that are formed in the reaction of alkylphenols such as butyl-, amyl-, or

octylphenol with sulfur chloride are more active than the compounds of the dibenzyl disul de type,

due to the position of sulfur next to the OH group.

Modern compounds of this type contain mostly tert-butyl radicals besides the methyl groups, as

for instance, 4,4′-thio-bis-(2-tert-butyl)-5-methylphenol. Sulfur–nitrogen compounds are also suited

as oxidation inhibitors for lubricating oils (2-mercaptobenzimidazole, mercaptotriazines, reaction

products of benzotriazole alkylvinyl ethers or esters, phenothiazine, and its alkyl derivatives).

Among the sulfur-containing carboxylic acid esters, 3,3′-thio-bis-(propionic acid dodecyl ester)

and bis-(3,5-di-tert-butyl-4-hydroxybenzyl)-malonic acid-bis-(3-thio-pentadecyl) ester have been

applied with success. These compounds have been replaced by the dialkyldithiophosphates due

to their broad application spectrum. Sulfoxides, sometimes in combination with aromatic amines,

have also been utilized.

19.5.7 PHOSPHORUS COMPOUNDS

Red phosphorus possesses oxidation-inhibiting properties but cannot be used because of its

corrosivity toward nonferrous metals and alloys. Triaryl and trialkyl phosphates have been proposed

as thermally stable inhibitors; however, their applications are limited. Combined phosphoric acid–

phenol derivatives such as 3,5-di-tert-butyl-4-hydroxybenzyl-phosphonic acid dialkyl esters or

phosphonic acid piperazides have a better effect.

19.5.8 SULFUR–PHOSPHORUS COMPOUNDS

Today, metal salts of thiophosphoric acids are used predominantly as oxidation inhibitors for

crankcase oils. In principle, compounds that contain sulfur and phosphorus are signi cantly more

ef cient than inhibitors that contain only sulfur or phosphorus. Most widely used are the ZDDPs

that are prepared by the reaction of P

with the respective higher alcohols (e.g., hexyl, 2-ethyhexyl,

and octyl alcohols), followed by the reaction with zinc oxide. The temperature of the exothermic salt

formation is kept at 20°C by successive addition of zinc oxide and cooling and is limited even at the

end of the reaction to 80°C because of the thermal ability of the free dialkyl dithiophosphoric acids.

They react very corrosively with metals and are toxic. Metal dialkyldithiophosphates are prepared

in mineral oil solution. Their solubility in hydrocarbon oils increases with the increasing number

of carbon atoms of the alkyl residues and is satisfactory with the diamyl compounds and higher.

Longer-chain derivatives act as solubilizers for short-chain products. Metal dialkyldithiophosphates

act not only as antioxidants but also as corrosion inhibitors and EP additives.

The ef ciency of Zn dialkyldithiophosphates (with octyl or cetyl and, respectively, propyl,

butyl, or octyl radicals in various combinations) decreases with increasing molecular mass of the

alcohol substituents. The best results have been obtained with isopropyl and isoamyl radicals.

ZDDP+HOOCR

ZDDP + ROH+

FIGURE 19.30 ZDDPs hydroperoxide destruction.

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