extreme pressure lubrication

7/24/2019 Extreme Pressure Lubrication

1/16

EXTREME PRESSURE LUBRICATION.

By E.

A.

EVANS*

(MEMBERO F COUNCIL).

October,

rg42.

KESEARCH on chemical addition agents to lubricating oils has

expanded to such an extent that any paper which summarizes in a

concise manner their chemical properties and correlates these pro-

perties with their physical behaviour, should be of great service. If

investigations

on

film strength improvers had been co-ordinated, then

it is conceivable that some such correlation would be possible. The

disconnected work which has been in progress in different laboratories

has given unrelated results, valuable in themselves, but difficult t o

piece together in an ordered pattern . Doubtless the commercial value

of the discovery of suitable chemical compounds to fortify a lubricant

with some specific characteristic has encouraged the winning of

a

prize

in the minimum of time. When the initial successes have been won

and competition hardens, a more systematic scientific inquiry will

necessarily follow. Whilst the time is not ye t ripe to summarize in

orderly fashion the relationship between chemical constitution and

film rupture strength, there is

a

visible emergence from the kaleido-

scopic views to a more geometrical aspect which

is

capable of expression.

The uses of mineral oils, and the advantages of added fa tty oils,

have been so often retold that repetition can

be

excused. During th e

past few years conditions have arisen which demand lubricants of

newly-possessed characteristics. Provision has had to be made for

lowering the pour point, raising the viscosity index, giving greater

resistance to oxidation, and increasing the film rupture strength.

Much of th is has been done by the introduction of chemical com-

pounds into the oils. Specific addition agents in lubricants have been

likened to th e control of the physical characteristics of steel by alloying

with tungsten for hardness, nickel for toughness, and chromium for

resistance t o corrosion. Newer methods of refining have, in part,

conferred some of these desiderata, but to intensify them recourse

must be made to synthetic compounds. Lubricants are thus changing

*

Chief Chemist

to

Messrs. C.

C Wakefield

and

Co.,Ltd.

I


2/16

2 THE INSTITCJTION

01

AIITOMOBILE ENGINEERS.

to such an extent tha t the old methods of viewing then1 must of neces-

sity change, new standards will be built, and new systems of analysis

constructed.

Lubrication under conditions of fluctuating load and unevenness of

opposing rubbing surfaces is complicated by their effects upon the

thickness of the oil film. Thick film lubrication may thereby be trans-

formed into thin film or boundary lubrication-a transition from

viscosity effect into chemical effect. The high spots on the metal

surface may even puncture t he oil film, and result in local seizures.

The welding together of the metal indicates that a local temperature

of

1 000~. can occur. This is a long and complex story,

so

i t is well

to pass on to consider how to deal with the situation which arises

when boundary lubrication, as

at

present accepted, is past.

It

may

well be argued that

as

long as the lubricant is still passing between

the surfaces boundary lubrication can never be succeeded. Suppose,

however, tha t the load is sufficient to rupture any conventional oil film,

then the zone of extreme pressure will have been reached. Then, if it

is possible to continue the lubrication, it is legitimate to consider a

phase beyond boundary lubrication, now described a s extreme pressure

lubrication.

For extreme pressure conditions a mineral oil containing

one

or more

chemical additives

is

used. The function of th e additive is not to com-

bine or react with the oil, nor to be adsorbed on the metal surface,

analogously to a fa tt y oil. Exactly how it behaves is still unsolved.

A

working hypothesis has been advanced that its active atoms combine

with the metal, producing a protective coating.

As

the popular agents

contain sulphur, chlorine, or phcsphorus, the hypothesis suggests

that the film is either a sulphide, chloride, or phosphide. This supposi-

tion has greatly assisted in the research programmes, and should

therefore not be discredited lightly. If there were ready and con-

vincing means

at

our disposal t o examine micro-films, there would be

no need to resort to hypothesis. True, X-ray a nd electron-diffraction

methods have been used extensively for film structure investigations,

but it may be that they are too discerning to differentiate between

absorbed and adsorbed atoms. Electron-diffraction has, however,

disclosed the one major defect in the hypothesis that an amorphous

oxide film is formed. Should t h s discovery be constantly confirmed,

then a new line of reasoning becomes instantly necessary. Two lead-

ing questions remain to be answered. Why should selected organic

compounds containing sulphur, chlorine, or phosphorus give rise to

amorphous oxide

?

Why should these three elements differ in their

film rup ture strength abil ity ? The protagonists of the hypothesis

face a danger when they are called upon to explain why a sulphur-

containing compound, when mixed with a chlorine-containing com-

pound, can be more t@cient than either, or a compound containing both

sulphur and chlorine can be better than one containing either. For the

immediate future, contentment must be found in observed facts and

practical applications.

The di'fiiculty of accurately defining the expression extreme presT

sure has been a great nuisance. Several attempts have been made

without much success, mainly because the expression is purely relative,

and prone to sophistication.

It is

being progressively realized that

extreme pressure lubricants are having greater usefulness than was


3/16

EXTREME PRESSURE LUBRICATION. 3

originally foreseen, consequently extreme pressure

is

slowly diverging

from its original meaning. Almost any job which is overloaded can be

lubricated with advantage with a selected E.P. lubricant. To-day

they are being used for such widely different things as a hypoid gear

and the pinion bearing of a timepiece.

A s

a matter of convenience it is usual to consider the bir th of E .P

lubricantsas having taken place immediately af ter the conception

of

the

hypoid gear, about the year 1929.

This postulate is not strictly true,

because oils with enriched film strength were used previously, although

their special characteristics were unrecognized as such. Oils con-

taining flowers of sulphur, colloidal sulphur, or chemically combined

surphur had well-known merits

;

therefore they formed a starting point

after it had been established that all other gear oils were not good

enough for the hypoid gear.

Even castor oil, which was considered to

be th e best gear oil, failed to prevent tooth distress. Lead soap in an

oil was alleged to produce a protective film, in time, on a bearing surface,

without the addition of any other reactive body, bu t it failed to ensure

a sufficiently thick film to give the needed protection. The idea of

producing a protective film very quickly focussed attention upon oils

containing chemically-combined sulphur. Experiment established the

soundness of the thought and gave rise to the working hypothesis.

SULPHURIZEDILS.

Many organic sulphur compounds were known a t that time to

chemists, b u t nobody thought of using them, possibly because they had

never been used in lubricants, but probably because experience had

already been obtained with oils containing sulphur combined with

fa tty oils. Sulphurized oils enabled the pioneers to reach success with

their initial work. The sulphurizing of fa tty oils by heating together

sulphur and oil was extended to fa tty acids, and further developed by

the action of sulphur chlorides on fatty oils and fat ty acids. Sulphurized

oil, although empirical in make-up and unadorned by scientific back-

ground, has stood the test of time, and has not yet been completely

overshadowed. Time has shown that modifications in the method of

manufacture have been necessary, but the basic principle has remained

unaltered. The increase of tooth loading and shearing forces have led

to higher operational temperatures, which in turn have led to sulphur

corrosion. To combat this lead soaps were incorporated into the

lubricant. Then came criticisms from the ball-bearing makers, who

proclaimed that the lead soaps damaged their bearings. The probable

explanation was tha t when the soaps were made by treat ing lead oxide

with f at ty acids some oxide remained unconverted to abrade the balls.

This little problem was faced successfully.

Another difficulty which

was not so easily solved was the precipitation of lead sulphide through

the inter-action

of

the sulphurized oil and the lead soap. Although the

precipitation in any one lubricant may have been cured, it is by no

means certain that when two such lubricants are mixed freedom from

trouble

is

assured. For this reason manufacturers recommend tha t

mixing shall be avoided. The comparative ease with which these lubri-

cants at tack metal has earned for them the description

active sulphur

lubricants. The description is probably more frightening than the

actual activity. When used with ferrous metals a darkening of the

2 )


4/16

4

THE INSTITUTIOE: O F AUTOMOBILE ENGINEERS.

metal is all tha t happens, but with cuprous alloys black flakes may be

formed. For this reason

it

is prudent to make a preliminary investi-

gation into the activity if phosphor bronze parts or cuprous alloy rings

in the ball bearings are in use. The eye of an experienced observer may,

however, be required to interpret the results.

Exactly what happens when sulphur is heated with fatty oils is

difficult to forecast, but when the sulphurizing is done with sulphur

chlorides it is almost

a

hopeless task to sort out the reactions.

I t

is

surprising, therefore, that so much time was allocated to sulphurizing

oils and organic acids

on a

commercial scale.

Doubtless flexible dis-

placements have served a useful purpose, particularly when standardiza-

tion was a t a lower ebb than i t is to-day. The time has come when we

must turn from undefined compounds obtained from sulphurizing

complex mixtures, and direct our attention to sulphur compounds of

more exactly defined constitution.

TESTING.

In the pioneer days of thc hypoid gear oil all testing was made upon

a complete hypoid gear unit. The method was costly, sometimes

lengthy, and not always repeatable. Rapidly there developed a natural

desire to design a laboratory testing unit in spite of the obvious diffi-

culties. The early dcsigns look to us cumbersome, but

it

cannot be

denied that most, if not all, of the testing machines which have been

recommended suffer from defects. To-day an apparatus which would

depict the peculiarities more accurately would be welcomed. The

designers of the machines have all aimed a t producing something which

will exert extreme pressure. How far they succeeded can be gauged by

the fact that not one of the machines, under any possible set of con-

ditions, would ra te the lubricants in accordance with their true service

values. The rating of the lubricants differed from machine to machine

and in some cases even the order was reversed. The conclusion

is

tha t

extreme pressure is only one factor. Another factor

of

supreme

importance is the surface finish of the tes t pieces. In

1936

the

S.A.E.

adopted a tester which had been designed at the Bureau of Standards.

I ts essential parts consist of two ball races, with their perimeters in

contact, running under load, and a t different speeds of rotation. This

machine therefore provides both extreme pressure and slip. Unless the

ball races are carefully standardized, including surface finish, repro-

ducibility is poor. The outcome of these disabilities is that laboratory

tes ts without road tests can only be signposts, .b ut signposts can be

directional and helpful.

Mineval Oils .

mately the same film rupture strength (F.R.S.).

varies with the several laboratory machines.

the limits are

3,000

lb. to

5,000

lb. per sq. in.

Fatty Oi l s .

Fatty oils are usually somewhat higher,

so

there is some advantage in

blending fa tty oils with mineral oils for special high-loaded jobs. When

a low coefficient of friction is demanded castor oil is often chosen, but

Mineral oils, and all pure hydrocarbons already tested, have approxi-

The numerical value

In the Almen machine


5/16

EXTREME PR E S S U R E L U B R I C A T I ON .

i f film strength is required rape oil would be selected.

film rup ture strengths are shown in Table I.

TABLE I.

Oil.

Castor .........................................................

Lard ...................................................

Neatsfoot

......................................................

Olive. ......................................

...................

Esters.

5

The respective

F.R.S.

9,000

8 000

9 000

11,000

9 000

15 000

15,ooo

Fatty oils being composed

of

fatty acids and

a

trihydric alcohol

(glycerol),

it

was reasonable to suppose that esters, which

are

fa t ty

acids combined with alcohols, might be useful. The film ru ptu re

strengths of these are given in Table

11.

TABLE 11.

3 per cent solution in mineral

oil.

F.R.S.

Benzyl oleate ................................................

3,000

Amyl acetate ................................................ 7,000

Amyl oxalate ................

Ethyl benzoate

.............

Ethyl phthalate .............

Methyl salicylate.. ..........

MISCELLANEOUS.

If it were legitimate to base an opinion

upon

patent specifications it

would appear th at, even subsequent to

1934,

oiliness and film strength

were considered to be very closely related. The supporting evidence

is t h a t polar groups were introduced into various molecular arrange-

ments. The esterification of castor oil an d other hydroxy derivatives'

is a clear indication of the line of thought. Added substantiation is

found in the suggested higher fa tty acid esters of monohydric alcohols,*

mono-esters of a dibasic organic acid, such as octyl acid ~ h t h a l a t e , ~

polycarboxylic acids, particularly alkenyl succinic acid,* an d an

aromatic compound containing hydroxyl and /or carboxyl ~ ~ o u D S ~

which may be represented by the formula

Aldehydes,s ketones, nitriles and zmines' also figured in the list.

SULPHUR.

In

1936

a patent appeared claiming the sulphurization of light mineral

oil,* by free su lphur a t

a

temperature up to

315

deg. under substantial

pressure. This was followed by the sulphurization of organic acids,s

esters10 and semi-drying

oils.

Sulphurization

is

carried a stage further

by th e trea tment of methyl oleate with sulphur.'* A specific compound

Note.-Numerical references throughout the paper are to the .4ppendix


6/16

6

THE

INSTITUTION O F AUTOMOBILE ENGINEERS.

was claimed which ccintains 20-24 per cent sulphur, o f which

1 3

per cent

is active.

s s s

I

CH,.

(CH , ) ,

.

CH-CH

.

CH,),

.

COOCH,

In view of a remark previously made in the paper, it is interesting to

observe th at a direct effort was made

to

remove the corrosive sulphur

from a sulphurized oil by precipitation with a lead soap.13

Even during the advent of sulphurized oils i t was becoming increas-

ingly clear that attention must be directed towards pure organic com-

pounds which would compete in price and effect with the more simply

made materials. The tas k was not a simple one, because there was

practically nothing to indicate the course to pursue. It was therefore

remarkable that two somewhat unusual chemical structures should be

chosen from the vas t store for independent study almost simultaneously

in several laboratories. One set of investigators confined their at tent ion

to aliphatic compounds, and took the thiocarbonate radical a s the

C \ S -

O-

sr / s=c

0-

\S

0-

monothiocarbonate xantha te trithiocarboiiate

starting point.

aromatic field using the thioether structure for the foundation.

Another school of t hought centred i ts energies i n the

/\,-s-,\

u l

Starting with potassium ethyl xanthate, a series of compounds can be

synthesized having

E.P.

properties.

O.C,H, O C,H, I4

s-c/ s=c( + KBr

\SK

+

BrC,H,

S.C,H,

diethyl xanthate

ethylene ethylene dixanthate

dichloride

16

O.C,H, 0 . C,H,

Brdichlorodiethyl diethyl xanthyl ethyl

ether. ether.


7/16

EXTREME P R E S S U R E L U B R I C A T I O N . 7

Before advancing it is necessary to deflect slightly to a patent which

claims alkyl mono- and disulphides, and aryl mono- and d i~ ul ph ides .1 ~

This

invention of using an alkyl disulphide reveals th e possibility of

combining two thiocarbonate radicals to produce xanthic disulphide.

Although this compound

is

mentioned in the pa tent specification, i t

does not appear in the claims.

The rearrangement

of

the grouping in the formula is given because

th e use of bis-alkyl xanthogen has been patented.'*

R .O .

C-S-S-C .O

II

RR'=alkyl or aralkyl radicals.

S

I :

s

Having succeeded in introducing two atoms of sulphur into

a

molecule, an d th en four,

it

caused no surprise t o see the announcement

of xanthogen tetrasulphide,lo which contains six.

S

R.O.C-S--S-S-C.O*

R'

Rli '=alkyl, aryl, aralkyl or

cycloalkyl radicals.

i I

S

The energy necessary for the activation was presumably assumed to

lie within the sulphur atoms. If this were not so it would be difficult

to

indicate why it appeared necessary to multiply the sulphur atoms.

The subject was sufficiently important

to

warrant further probing ;

consequently, a series of aromatic compounds was synthesized con-

taining successively larger quantities of sulphur and tested in oil.

Results ar e given in Table 111.

TABLE

111.

F.R.S.

B-Naphthyl sidphide

trisulphide /=cr.Ps-'.o(: per cent 5,000

tetrasulphide

02-j-s-OJ

per cent

io,ooo

\

/

The conclusion is that even with . hree atoms of sulphur in line in

this structure no E.P. value is developed, but when a fourth atom of

sulphur is added in the iso-position an increase is apparent.


8/16

8 THE INSTITUTION, O F

AUTOMOBILE

ENGINEERS.

From these two sets of observations it is difficult t o deduce whether

the sulphur behaves differently in purely aliphatic compounds from

aromatic compounds. The degree of bonding of the ato m within the

molecule

is

doubtless of serious importance, but i t is also conceivable

th a t the direct attachment of sulphur to an aliphatic carbon atom may

also be of importance. To te st this la tter possibility aromatic com-

pounds were made in which the sulphur atoms were linked directly

to aliphatic carbon.

TABLE

IV.

F.K.S.

Dibenzyl disulphide (

z-s-s-cHz()

I

per cent 8 000

Dibenzoyl

,,

A number of di'fhculties remain, but th e impression is on the whole

th a t the theory is correct.

Having shown th at two sulphur atoms, when suitably bonded, can

be superior to three,

it

will now be demonstrated th at even one sulphur

atom can impart useful E.P. qualities if th e surrounding groups are

carefully chosen. The simple thioether framework was take n and

various extensions made, which led finally to bis(3-carbomethoxy-4-

hydroxyphenyl) thioether.20

F.K.S.

I per cent

15.000

The exact functions of th e substituent groups has not been elucidated,

bu t it should be kept in mind tha t much enlightening knowledge has

already been accumulated on the influence of carboxyl ( .COOH), ester

(.COOR),

and hydroxyl

( .OH)

groups on lubrication, particularly in

measurements on coefficient

of

friction.

Attempts to simplify the procedure have at times been handi-

capped by the lack

of

reproducibility. At first it was difficult

to

account

for thc inconsistency of the film rupture strength determinations, and

it was not unti l the influence of small amounts

of

impurity had been

discovered that the value of some sulphur compounds

was

revealed.

Judging from th e literature, th is fact does not secm t o have beemtaken

into account, possibly because there has not been the need to prepare

very pure compounds for lubrication.

C H L O R ~ N I - .

The assessment of chlorine compounds has been complicated by the

difference in the results obtained

on

the Alnien and Timken machines.

The Timken machine seems to pick out differences between certain

classes of chlorine compounds which are no t manifest on t he Almen

machine. Unfortunately, there is not available sufficient data from field

tests using hypoid gears to justify the conclusion th a t the Timken

results do indicate a true picture. This point, however, is not suffi-

ciently important in this review, which is dealing in the main with

laboratory tests.


9/16

EXTREME PRESSURE L U B R I C A T I O N . 9

The multiplication of chlorine atoms in simple aliphatic hydro-

carbons indicates, by the Timken machine, that an increase

of film

strength can be obtained (see Table V), b u t successive chlorination does

TABLE

V.

F.R.S.

Timken).

Ethylene dichloride CH,CI CH,CI

23

Tetrachlorethane CHCI, * CHCl 23

Pentachlorethane CHCl,. CCl

38

not necessarily lead t o the same conclusion, as will be seen in Table VI.

TABLE VI.

F.R.S. (Timken).

Dichlorobutane

CH,. CH,.

CHCl.

CFI,Cl

34

Trichloro-tertiary butyl

CH CI\

chloride CH Cl-CCI

CH,CI/

33

Trichlorethylene CCl

:

CHCl 33

In those compounds whose

F.R.S.

is

23

the chlorine is fairly firmly

held, but still active, whereas in dichlorobutane and trichloro-tertiary

butyl chloride some explanation must be found to account for the e xtr a

activity. If the formula for dichlorobutane be re-written

C H a

H,Cl

cHaxl

it

will be seen that there is a correlation with trichloro-tertiary butyl

chloride. I n these two configurations there

is

a chlorine atom which

is

more loosely held than the other chlorine atoms. Added support t o

the idea that the loosely held chlorine is important

is

obtained in

Table V II following.

TABLE

VII

F.R S

49

l i

Diphenylmethyl chloride

\

/c

BenzoDhenone dichloride

84

Successive chlorination

of

benzene offers no encouragement when the

tests are made o n the Almen-machine, but differences are found when

the Timken machine is used, as shown in Table VIII.


10/16

I0 THE INSTITUTION

OF AUTOMOBILE

ENGINEERS.

TABLE V I I I .

F.R.S.

Almen.

Timken.

Chlorobenzene

A

4,000 8 8

J

o-Dichlorobenzcne

C1

P-Dichlorobenzene { )

x

c1

2

Trichlorobenzene

CIQ

c1

6 000 -

G

: 2 : 4 : 5-Tetra-

chlorobenzene

When one atom of chlorine is directly linked to an aliphatic carbon

which is attached to the benzene nucleus a distinct improvement is

observed,

but

when two chlorine atoms are likewise introduced a very

considerable increase in film strength is found (see Table IX).

TABLE IX.

F.R.S. (Alrnen).

CH,Cl

Be n z y l chloride 7,000

-

3

39

Benzal chloride

CHCI,

0

It

is

interesting to compare two substances having the same number

of

carbon atoms and each possessing one atom of chlorine.

The com-

parison in Table

X

reveals th e difference due t o structure.

TABLE

X.

F.R.S. (Timken)

2-Chlorohcxane CH8 CH,-CH,.CH,.CHC1.CH,

8 8


11/16

EXTREME PRESSURIC 1.UBRICATION. I1

In chlorobenzene the chlorine is substituted in the ring and firmly

held, consequently the Timken value is only 8 . 8 , whereas in chloro-

cyclohexane it is added,

so

presumably

it is

less firmly attached.

Added

s

not scientifically correct, but is used to indicate the type

of structure exemplified in which chlorine is attached to

a

saturated

carbon atom. Support to this supposition is afforded by an examina-

tion of naphthalene containing chlorine, as shown in Table XI.

TABLE

XI.

F.R.S.

(Almen).

H

,c1

Tetrachlorotetralin

(diagrammatic)

FI

.CI

Naphthalene tetrachloride

I

5,000

CI \H

Although tetrachlorotetralin has the same number of chlorine atoms as

naphthalene tetrachloride,21 t has approximately half the film strength.

This is due, no doubt, to the added and substituted chlorine in tetra-

chlorotetralin, and to the four added atoms in naphthalene tetra-

chloride.

Even when chlorine is firmly held in the benzene nucleus its activity

can be augmented by substituted groups (see Table

XII).

TABLE

XII.

F.R.S.

(Timken).

c1

Chlorobenzene

CI

p-Chlorophenetole

p-Chlorophenol

p-Chloroaniline

0

.C ,H,

c1

0

H,

18.8

38

38


12/16

I THE INSTITUTION O F AUTOMOBILE ENGINEERS.

F.R.S. (Timken)

c1

Trichlorobenzene

2 :

4

: 6-Trichlorophenol

Trichlororesorcinol

C * O C I

39

49

At this juncture i t would not be inappropriate to deflect from

E.P.

values to give momentary consideration

to

wear values. Although

wear

is

not necessarily associated with E.P. values, yet under load some

compounds reduce wear more th an others. The instrument which was

used

to

measure wear was designed by Brownsdon. It consists in

principle of a wheel rota ting under applied load on a metal strip. The

amount of wear is measured by the length of th e impression.

The activation of chlorine by substituted nitro-groups permits the

conclusion tha t the introduction of one nitro-group into the benzene

ring induces an improvement, which is not further increased by the

addition of

a

second ;

a

third, however, results in

a

surprising decreasc,

as

uill be

rioted in Table

XIII.

Chlorobenzene

o-Nitrochlorobenzene

TABLE

XIII.

CI

F.R.S Wear

4 000 0430

\

c1

Picryl chloride

This is a eature

common

t o other compounds

in

which two nitro-groups

are in the o-position to the same chlorine atom.


13/16

EXTREME PRESSURE

LUBRICATION.

13

TABLE

XIV.

I7.R.S. Wear.

Nitro+-dichlorobenzene

02

7,000 0210

cl

CI

Tetrachlorobenzene

Trichlorotoluene

A

methyl group also

has

a n influence on the properties

of

chlorinated

benzenes (see Tab le XV).

TABLE

XV.

F.R.S. Wear.

c/3 ,000

0.280

c1

CH,

Tetrachloro-3-nitrobenzene

CI

c1

9,000

0.215

8,000 0.240

Now it will be shown in Tabl e XVI t h a t the nitro-group can react when

i t is

in a

sepa rat e molecule from t h e chloro-compound.

TABLE XVI.

z per cent Nitrobenzene

2 ,, Chlorobenzene

2

,, Nitrobenzene

I ,

p-Dichlorobenzene

I

,, m-Dinitrobenzene

0.400

2 ,, Nitrobenzene

0.400

:: :::}

:: :}

I

,

p-Dichlorobenzene 0.410)

I

,,

Benzal chloride 0.215)

1 - 0 ,,

Nitrobenzene

0.5 ,, Trichlorobenzene

0 - 5

,, Nitroform

I

-o ,

Chlorobenzene

0.430

0.270

0.280

0.285

0 . 1 7 0

0210

0.240


14/16

14

THE

INSTITUTION

O F AUTOMOBILE

ENGINEERS.

Dichlorophenanthrene tetrachloride presents a curious phenomenon.

Although containing four added an d two substituted chlorine atoms in

the molecule, it will be noted in Table XVII th at this substance does

not reduce wear.

TABLE XVII.

Phenanthrene dibromide

Br Br

Dichlorophenanthrene Formula

tetrachloride uncertain

0 . 3 5 0

0 . 4 2 0

As it

is

scarcely likely that

it

will have commercial value, i t was not given

specialized study. To those who are interested in a n academic study,

an investigation with a view to ascertaining why an excess of chlorine

can increase the wear may be recommended.

A catalogue of all the chlorine compounds which have been patented

would serve very little purpose except to show th a t chlorine has been

pu t into almost every type of compound into which i t will conveniently

fit with the hope that something of commercial advantage will result.

SULPHUR

AND

;CHLORINE.

The different intensity between a mixture and the individual com-

ponents is not peculiar t o nitro and chloro compounds. In the search

for new associations which would lead t o an enrichment sulphur

compounds with chlorine compounds have been studied. A mixture of

chlorinated wax and dibenzyl disulphideza is

a

well-established example

of a n inactive sulphur constituent. Halogenated aromatic and hetero-

cyclic compounds with active sulphur compounds have also been

suggested.23

The construction of compounds in which both sulphur and chlorine

occur in the same molecule must be mentioned. Perhaps the simplest

is thionyl chloride, SOCl,. I t s other properties, however, would militate

against its use. Perchloromethyl mercaptan, CSCl,, can only be used

under cold conditions. To extend its use i t has been necessary t o react

it with unsaturated c0mpounds.~4 The thiocarbonates reappear in this

category as chlorinated alkyl thiocarbonates.26

PHOSPHORUS.

The third and last element which may seriously be included in the

ambit of E.P. compounds is phosphorus. The early publications aroused

comparatively little interest, because the phosphorus compounds had

insufficientE.P. properties for gear lubrication.

Credit, however, must

be accorded to the pioneers for their zeal in adhering to organic chemistry

instead of endeavouring to push atoms of phosphorus into a hetero-


15/16

E X T R E M E P R E S S U R E L U B R I C s 4 T I O N . 15

geneous collection of substances. In

1935

tri-(3-naphthyl phosphateZe

made an appearance in the patent literature,

(C10H7)3P04~

followed by tricresyl phosphate,27

(CH, * C,H4)3PO4.

A

further contribution to the series of phosphorus esters containing an

alkylated aryl group is tri-p-ethyl phenyl phosphate,28

In an endeavour to obtain an aliphatic compound the temptation to

attack the hydroxyl group in castor oil with phosphorus pentoxide,

P20,.

or phosphorus oxychloride, POCl,, proved too great t o resist,so

The use of phosphites has assumed unexpected proportions in internal

combustion engines, particularly for prolonging the life of cadmium

bearihgs. The two which

are

worthy of mentioning are tributy l phos-

phite

and triphenyl phosphite,

(CH,

CH,

-CH 2 C H,),PO,

Even phosphines have been re~onimended.~O

l p

R

=

aryl

radicals.

RY

PHOSPHORUS

ITH

S U L P H U R R CHLORINE.

Future development will probably lie in this category, now that

internal combustion engines are becoming

so

highly stressed that

mineral oils afford insufficient protection. The additives which are

now being used for Diesel engine lubricants are chiefly detergents to

prevent the accumulation of deposits within the engine, and should not,

therefore, be confused with the subject now under review. Phosphorus

compounds are already in use to protect bearing metals from corrosion,

but they have also mild

E.P.

properties.

It

is reasonable to conjecture

that if they can be suitably blended with sulphur and/or chlorine new

uses will be found.

A start has been made with thiophosphoryl

chloride, PSCl,. As far back as

1935

triaryl trithiophosphite~~lere

patented, and in succession followed alkyl th iopho~phi tes.3~ hio-

phosphite esters generally open the way t o many possibilities,33although

i t

must be realized that the halogenated esters of phosphorusJ4 do not

appear to have been exploited. Very litt le work has been published on

phosphines, possibly because many of them have

a

very disagreeable

odour. That they have not been overlooked is manifested in propyl

benzene dichlorophosphine.

In view of the great interest which has been taken in metal soaps35

in

1935,

t should be recorded that

a

mixture of metal soaps and organic

phosphitesas was patented about th at time. Organo-metallic com-

pounds will no doubt come into prominence in due time, performing

the dual

rBle

of oxidation inhibitors and

E.P.

lubricants.

The ability

of

any technique to play a substantial pa rt in the advance


16/16

16 THE INSTITUTION O F AUTOMOBILE ENGINEERS.

of engineering can only be gauged b y i ts practical results. It may

therefore be asked whether there is sufficient evidence that the long

list of chemical formuk are capable of being translated into term s

of

utility. A branch of science which emerged from a little-explored side-

line

of

chemistry less th an te n years ago cannot claim

to

have crystallized

into definite shape. The intensive and specialized stu dy brough t

to

light a number of compounds which are, at least, usable, and a few which

are eminently suitable for the lubrication of hypoid gears. Without

wishing to give undue prominence to any particular substances, the

engineer would be glad to know th at , amongst others, the chlorinated

alkyl thiocarbonates, dibenzyl disulphide, bis(3-carbomethoxy-4-

hydroxyphenyl) thioether, and chlorinated wax have been proved

in

hypoid gears. Not only are th ey of use in hypoid gears, bu t the y ha ve

been used successfully in m any conventional rear axles under severe

overload. I n addit ion the life of heavily stressed bearings has been

prolonged b y the ir introduction.

The author is aware th at E.P. additives have a wide application in

engines, and perhaps an opportunity may occur after th e war to discuss

them.

Thanks are due t o Messrs. C . C. Wakefield and Company, Ltd.,

for

permission to publish th e results, and t o Mr. J. S. Elliott, M.A. ,

for

valuable help.

.%PPENDIX

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extreme pressure lubrication

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