The load-carrying properties of diester disulphides

Wear -

THE

Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

LOAD-CARRYING

E. S. FORBES,

K. G. ALLUM,

The British Petroleum (Ct. B&a&) (Received

PROPERTIES

Company,

OF DIESTER

E. L. NEUSTADTER Limited,

AND

341

DISULPHIDES

24. J. D. REID

BP Reseavch Cmtre, Sunbury-on-Thames,

Middlesex

March 13, rg7o)

SUMMARY

A range of diester disulphides was synthesised and their extreme-pressure and antiwear properties assessed using the Four-Ball machine. The diester disulphides had better antiwear properties, and poorer extreme-pressure properties than the corresponding unsubstituted disulphides. Dialkoxy substituted disulphides had similar extreme-pressure and antiwear properties to unsubstituted disulphides. Maximum antiwear protection at low additive concentration, followed by subsequent increase in wear with increasing concentration, was obtained for diester disulphides [-S(-CHZ)~COOC~H~]~ where n= I, z and 3, but not when n=5, IO. Explanations for these results are given and these are supported by heats of adsorption and electron probe microanalysis studies. It is suggested that the prime requirement for improving the antiwear properties of organo-sulphur compounds is the presence of strongly adsorbing groups at both ends of the molecule. These results also highlight the fact that the load-carrying properties of additives for one applicationcanbeimprovedonly at the expense of its possible use in other fields. INTRODUCTION

A study of the load-carrying properties of a series of simple organic disulphides has shown that the relative load-carrying ability depends markedly on the nature of the attached group 1~~.Thus the order of decreasing antiwear* activity2 is : phenyl> benzyl > ally1 > n-butyl and n-hexadecyl

> n-dodecyl> n-octyl> n-butyl> ethyl

It was suggested that disulphides function in the antiwear (a.w.) region by formation of an iron mercaptide layer. The differences in activity of the disulphides can be rationalized in terms of the rate of formation of this layer, as governed by the * Load-carrying additives generally fall into two classes, namely antiwear (a.w.) and extremepressure additives. An effective antiwear additive functions by resisting penetration of the oil film in the mixed lubrication region. In contrast, extreme-pressure additives function by reacting with the metal surface when the oil film collapses, to form an inorganic surface layer. The term “antiwear activity” is used throughout this report to define the relative load-carrying ability of an oil blend in the mixed lubrication region. It is not intended to imply that the oil is necessarily superior to the base oil. Wear, r5 (1970) 341-352

E. S. FORBES

34”

et ait.

ease of scission of the sulphur-sulpllur bond, and its physical properties once formed. GROSZEK~ has shown that the rates and heats of adsorption are important factors in determining the a.w. activity of amines and carboxylic acids. For this reason, it was considered that the antiwear activity of disulphides might be improved by incorporation of a polar group in the molecule. Therefore a number of polar disulphides were synthesised and studied as antiwear additives using the 4-ball machine. EN 31 steel balls were used in all this work. The extreme pressure properties of selected compounds were also determined as were heats of adsorption on ironoxide. Electron probe microanalysis ,techniques were also used. This paper describes the work carried out on these polar group substituted disulphides.

I.

Synthesis

ofadd&es

Starting materials, intermediate compounds and products were purified by distillation (generally up an 8 in. glass column) or by recrystallization. A list of the starting materials used is given in the Appendix. The products were examined, where possible, by nuclear magnetic resonance, infra-red spectroscopy and gas-liquid chromatography. Except where specifically stated, it may be assumed that no impurities were detected by the above techniques. Microanalysis figures were satisfactory. (a,I I,I’ d~[ca~boethoxy)d~~lethyl dim&hide, [-S-CHFCOOC&~~~ Ethyl x-mercapto acetate was added to a sodium acetate/acetic acid buffer solution at zo*C and then oxidized by ethanolic iodine to yield the disulphide ho.15 = IOI "C. (Oxidation using either sodiumllydroxideandKI~salution ordimethylsulphoxide results in an impure product. It is believed that this is due to the high activity of the,%-methylenic hydrogen.) Infra-red detected a possible impurity band at 8.9 pm. GLC detected 0.3 wt.?/, of a low boiling impurity. The disulphide had a zero acidity value. (b) 2,2' di(carboethoxy)disthyl disulphide, [-S-(CHX)FCOOCZH~]Z g-SIercapto propionic acid was esterified in ethanolic toluene using para-toluene sulphonic acid as a catalyst. The resulting ethyl 3-mercapto propionate (bro=63”C) was then oxidized1 and the disulphide washed, dried and distilled (bo.2 = x22”--3%). The product had a zero acidity value. (c) 2,2' fZi(carbodeco~~J)ddiethyZ d~s~$~~h~de, ~-S-(CH~)~-COOClo~~~ 1~ 3-Xercapto propionic acid was esterified with decanol in toluene using paratoluene sulphonic acid as a catalyst. The resulting ester was distilled (bo.1 = ~oii”C) and then oxidizedr. The product had a zero acidity value. (d) 3,3’ di(carboetho?cy)di-n-@+opyl disul$hide, [-S-(CH&-COOGHZ’s Ethyl 4-bromo butyrate was refluxed with thiourea in ethanol for 40 h. The isothiouronium salt was isolated and then hydrolysed with aqueous sodium hydroxide at 30°C. The mercaptan (bo.e=76.5Y) was then oxidized1 and the resulting disulphide distilled (b0.3= 150°C). The product had a zero acidity value. (e) 5,5’ di(carboethoxy)di-n-pentyl disul$hide, [-S-(CH2) T-COOCzH 512 G-Bromo hexanoic acid was esterified in ethanolic toluene using para-toluene sulphonic acid as catalyst. The bromo ester (b3=94’C) was refluxed with thiourea in ethanol for 24 11 and the isothiouronium salt isolated and hydrolysed with aqueous sodium hydroxide at 35°C. The mercaptan was then oxidizedl. Some hydrolysis of Il%ar. r.5 (1970) 34x-352

LOAD-CARRYINGPROPERTIES OF DIESTER DISULPHIDES

343

the ester to free acid occurred and therefore the disulphide was re-esterified. The resulting disulphide (bo.05=173’-4’ C) had a zero acidity value. (f) IOJO di(carboethoxy)di-n-decyl disulphide, [-S-(CH&-COOCzH5]2 II-Bromo undecanoic acid was esterified in ethanolic toluene using para-toluene sulphonic acid as a catalyst. The resulting bromo ester (bo.05 = 120~--2~C) was refluxed with thiourea in ethanol for 13 h. The isothiouronium salt was isolated and then The mercaptan (bo.l= 114”-5°C) was hydrolysed with aqueous sodium hydroxide. oxidizedl. Some hydrolysis of the ester group occurred during the last reaction and therefore the disulphide was re-esterified. The final product had a zero acidity value. (g) 2,2’ diethoxy diethyl disulphide, [-S-(CH&OCzH& I-Bromo-a-ethoxy ethane was refluxed with thiourea in ethanol for 40 h. The isothiouronium salt was isolated and then hydrolysedwithaqueoussodiumhydroxide. The mercaptan (b760= 124’-6”C) was oxidized1 to the disulphide (bo.6=84%). (h) 3,3’ diethoxy di-n-propyl disulphide, [-S-(CH&OCzH5]z Ally1 bromide was added to sodium ethoxide in dioxan (purified) at 5o”C.The solution was refluxed for three hours and ally1 ethyl ether (b 760= 57”-9°C) distilled off. Thiolacetic acid was then added to the ether and an exothermic reaction ensued to produce 3-ethoxy s-propyl thiolacetate (bls=85.5”-87.5”C). The thiolacetate was hydrolysed to 3-ethoxy-propyl mercaptan (b30 =6z” C) using a 10% aqueous-alcoholic solution of potassium hydroxide. The mercaptan was then oxidized’ to the disulphide (b0.4=98’-g°C). GLC detected seven per cent of an impurity and small impurity bands were detected in the NMRspectrum. Itislikelythattheimpurityis2,2’diethoxy di-iso-propyl disulphide. 2. Base oil The base oil was a liquid paraffin (ex British Drug Houses Limited) which was purified by percolation through an alumina columnlv2 and stored under nitrogen. This liquid paraffin had kV 21o’F of 3.38 cSt, KV 100°F of 16.25 cSt, VI of 103 and sulphur content of 6 p.p.m. 3. Equipment and method of test of load-carrying properties The a.w. properties of an oil were assessed using the 4-ball machine. The following test conditions were used: sliding speed 115 ft./min (25 rev./set) applied load I5 kg bulk oil temperature 50°C test duration 0.5,0.75 or I h The balls were made of EN 31 steel and had a surface finish in the range 2-4 pin. c.1.a. The 4-ball machine and the method of test have been discussed elsewherelp2. 4. Electron probe microanalysis

The application of this technique has already been fully described5. Briefly, by this technique, the surface topography, element content and element distribution in a lubricated contact are obtained by first bombarding the surface with a stream of electrons. These electrons penetrate the surface to a depth of two to four microns and cause X-rays, characteristic of the elements of the surface, to be emitted. Quantitative we’J+‘,15 (1970) 341-352

314

et al.

E. 5. FORBES

estimates of element content can be carried out using spot determinations with a static beam or average determinations with a scanning beam. The quantitative analysis is achieved by comparing the intensity of the sulphur radiation with the intensity for pure l;eSz and iron under standard

conditions.

5. Heats of adsorption measuremmts h flow microcalorimeter was used to determine

the heats of adsorption

of a

series of compounds on iron oxide from n-heptane solution. This method has been fully described”23. The iron oxide used was lie203 (Johnson Matthey “Specpure”) and had a surface area of 2.1 m”/g. RESVLTS

Antiwear

results for the base oil used are shown in Table I.

ANTIWEAR TEST RESlJI_TS FOR LIQUID PARAFFIN Mean wear scar diameter Mean wear scar diameter Mean wear scar diameter

Antiwear

after 0.5 h after 0.75 h after I h

vs. concentration

periods are given in Table

ANTIWEAR

RESULTS FOR

0.604 mm 0.703 mm 0.7 j9 mill

results

for additives

after 0.5,

0.75

and

test

I h

II and in Figs. r-7.

ADDITIVES

AT

A SERIES

OF

IN LIQUID

CONCENTRATIONS

PARAFFIN

Weav SCLZY diametev (nm) after :

I

1, r ‘di(carboethoxy) [-~S-CH-COOCaH5]2

dimethyl disulphide (see Fig. I)

IF;..j.? 9.26 4.63

__Wear,

____ I.5 (1970) 341-352

disulphide (set Fig. 3)

0,710

0.61.3

Ih 0.790 0.763 0.690

I.IOL

0.600

0.7’5 0.590

0.531 0.270

0.5’7 0.532 0.578

0.590 0.553

0.702

0.785

0.1.38 o.oog

0.635

0.628

rS.5.z

4.926

0.787

0.835

0.8YX

9.26 4.63 2.32

2.463 1.231

0.680

0.617

0.555 0.492

0.753 0.043 0.547

0,755 0.690 O.OOL

I.10

0.309

0.405 0.4’2 0.433 0.610

o.457 0.440 0.445 0.630

0.507 0.453 0.463 0.060

0.538 0.438 0.577 0.640

0.610 0.503 0.595 0.080

0.680 0.578 0.010 0.720

0.575 0.720 0.690

0.750 0.810

0.950 0.890 0.890

0.1

j4

0.2c)

0.077 0.039

8.98 4.49 2.25

4.406

J .‘.3

0.57

0.551 0.276

0.29

O.I@

o.r5

0.069

o.15

2, r’di(carbodecoxy)diethyl [-S-(CHz)~-COOC~~&]~

O.(ijj

2.20.+

0.58

0.58

3

4.408

0.29

1.10

2, z’di(carboethoxy) dicthyl disulphidc [-S-(CHZ)Z-COOCZH~]Z (see Fig. L)

o.75h

0.453 0.497 0.522

r.jr

2

0.5 h

2.203 I.102

0.800

LOAD-CARRYING TABLE hTo.

II

PROPERTIES

OF DIESTER

DISULPHIDES

345

(Continued)

mM/roo g base oil

Additive

wt. %

Wear SC~Ydiameter fmm I after :

h

o.75h

0.667 0.568

0.687

0.5

4

3,3’di(carboethoxy)diin_propyl disulphide [-S-(CHZ)~-COOC~H~]~ (see Fig. 4)

18.52 9.26 4.63 2.32 1.16 0.58 0.29 0.15

5

5,5’di(carboethoxy)di-n-pentyl disulphide [-S-(CHZ) s-COOCzHs]2 (see Fig. 5)

18.52 9.26 4.63 2.32 1.16 0.58 0.29

6

~o,~o’di(carboethoxy)di-n-decyl disulphide [-SS(CH2)1o-COOCzHs]a (see Fig. 6)

17.42 8.71 ;::; 1.09 0.55 0.28 0.14

7

8

2,2’di(ethoxy)dGethyl [CzH5O(CHz)z-S-12

3,3’diethoxy di-n-propyl [CzHenO(CH2)3-S-12

disulphide

18.52 4.63 2.32 1.16

disulphide

18.52 9.26 4.63 2.32 1.16

9

IO

ethyl 4-mercaptobutyrate HS(CHZ)&OOC~H~ (see Fig. 7)

diethyl sebacate [-(CHz)aCOOGH&

18.52 9.26

5.445 2.722 1.361 0.681 0.340 0.170 0.085

0.527 0.432 0.412

0.593 0.538 0.468 0.475 0.670

rh 0.747 0.618 0.568 0.492 0.501

0.043

0.545 0.620 0.670

6.483 3.242 1.621 0.811 0.406 0.203 0.102

0.539 0.538 0.447 0.455 0.450 0.571 0.651

0.551 0.569 0.523 0.504 0.531 0.543 0.691

0.566

8.540 4.270 2.135 1.068

0.530 0.488 0.568 0.582 0.640 0.660 0.720 0.690

0.538 0.530 0.598 0.680 0.680 0.690

0.134 0.067

0.449 0.455 0.532 0.535 0.593 0.620 0.650 0.660

3.889 0.972 0.486

0.845 0.887 0.830

0.828 0.892 0.928

0.243

0.793

0.875

4.410 2.205 1.103

0.855 0.877 0.852 0.815

0.872

0.534 0.267

0.552 0.276

2.740 1.370 0.685

4.63 2.32 1.16 0.58 0.29 0.15

0.043 0.022

18.52 9.26

4.775 2.388

4.63 2.32 1.16 0.58 0.29

1.194 0.597 0.299 0.149 0.075

0.343 0.172 0.086

0.743

0.541 0.495 0.498 0.457 0.498 0.456 0.574 0.685

0.585 0.539 0.524 0.536 0.570 0.551 0.565

0.740 0.740

0.897 0.892 0.910 0.852

0.589 0.550 0.545 0.547 0.501 0.471 0.574 0.675 0.654 0.642 0.664 0.633 0.634 0.646 0.620

0.750 0.860 0.830

0.599 0.573 0.545 0.551 0.581 0.709

0.770 0.720

0.800 0.927 0.938 0.950

0.945 0.930 0.915 0.922 0.943 0.611 0.566 0.597 0.572 0.523 0.545 0.611 0.645 0.704 0.696 0.674 0.647 0.698 0.660 0.649

Weaf’, =5 (1970) 341-352

\/,

>

2

4

t

6

-+-iij--e

CONCENTAATtON

Fig.

I.

Q’ear vs. concentration

curves for

Fig. 2. Wear w. concentration

I, i’di(carboethoxy)dimethyl

/

Fig. 3. Wear US. concentration Fig. 4. Wear paraffin.

US. concentration

5, , 2

I

I

,

disulphide

I

t

i

in liquid paraffin.

I

14 16 8 10 12 6 CONCENTRATION IM~LLIMOLES ‘1. J

curves for z,z’di(csrhoethoxy)diethyl curves

%I

disulphide in liquid paraffin.

curves for r,r’tii(carhoethoxy)rliethyl

tJ

14

[MILLIMOLES

18

‘O-4 20

disulphide in liquid paraffin.

for 3,j’di(car~oethoxy)di-n-pmpyl

disulphidc

in liquid

The a.~. results for selected di-n-alkyl di~ulph~des are given in Table III whilst e.p. results for selected di-rc-alkyl disulphides and diester disulphides are given in Fig. 5. The wear scars produced in the presence of three blends containing different concentrations of z,z’ di(carboethoxy)diethyl disulphide have been examined by electron probe microanalysis. The average sulphurcontents determined by this method are shown in Table IV.

LOAD-CARRYING

PROPERTIES

OF DIESTER

347

DISULPHIDES

r

t

Fig. 6

Fig. 5

- 0.5

2 $! 4 i

4

2

8

6

I2

10

CONCENTRATION

Fig. 5. Wear paraffin.

vs. concentration

Fig. 6. Wear paraffin.

vs. concentration

curves curves

for

I4

16

(MILLMOLES

20

18

0.L

*,.I

5,5’di(carboethoxy)di-n-pentyl

disulphide

in liquid

for ~o,ro’di(carboethoxy)di-n-decyl

disulphide

in liquid 3.L

, C,4’OifCAi?EOETHOXY~ Oi-@=WFfL DISULPHIU P 3,3’Di~CARBOETHOXY) DIETHYL DISULPHIOE > Di-6-DCTYL DISULPHIOE , cii-n_-Bum DISUL~~OE < DIETHYL ~SULPHIDE

Fig.

- 3.2 - 3.0

Fig. 8 -+

7

- 2.0 -2.6

-0,7

Is

- 2.2

5 1 HOUR

TESTS

HOUR

-0.6

TESTS

-0.5

I_O14 2

L

6

8

IO

CONCENTRATION

Fig.

7. Wear

vs. concentration

Fig. 8. Extreme-pressure TABLE

I.?

16

14

(MILLIMOLES

16

20

- 2.0 3

5 ::

-1.8

5 5

- 1.6

test results

I

80

61

/

100

/

120

i

I40

I

8

160

160

LOAD

for A-mercaptoethylbutyrate for di-n-alkyl

and

L

220

I

240

11.2 260

(kg)

in liquid

disulphides

I

200

diester

paraffin. disulphides.

III

ANTIWEAR RESULTS Additive

FOR

DIALKYL

DISULPHIDES

mM/roo base oil

g

IN

LIOUID

wt.%

-.-.Diethyl Di-n-butyl disulphide disulphide I?i-n-octyl disulphide

18.52 18.52 18.52

5 &

i

%)

curves

% x

2 0

- 1.4

0

& L I

-2.‘%

-

z

0.75

? g

2.259 3.296 z;.c17o

PARAFFIN

(REF. 2)

Wear scar diameter (mm) after : rh 0.75 -.h 0.5 k 0.978 0.908 0.845 0.855 0.807 0.777 0.724 o.‘iro 0.658 Wear,

15 (1970) 34r-352

348

HEATS

E. S. FORBES

OF

ADSORPTION

ON

IRON

OXIDE

OF

A

SERIES

OF

COMPOVINDS

AT

5

Wdif/100

g

OF

et al.

FL-IIEI’TANE

The heats of adsorption of three ~sulphides, a diester disulphide and methyl sebacate are given in Table V.

In this discussion, the main points of the results are presented together with an evaluation of two possible explanations for the results. A study of these results shows: (I) Carboethoxy substituteddisulphides of formula [-S-(CH2)&O&Hs]z where N= I, 2 and 3 all show a marked “dip effect” with relation to the effect of concentration on the wear scar diameter, i.e., the wear scar diameter decreases to a minimum and then increases with increasing concentration of the additive. A slight dip effect is observed when 9~=5 but for ~L==IO, the wear scar diameter decreases to a certain value and remains there with further increase in concentration of the additive (this is the usual type of curve obtained with conventional load-carrying additives}. z,z’Di(carbodecoxy)diethyl disulphide is also found to give this dip effect (Table II, Fig. 3). (2) The a.w. properties of these diester disulphides are much superior to the corresponding diester or disulphide (cf Table II, Pt. 1-4 with Table II, Pt IO and Table III). (3) Ethoxy-substituted disulphides of formula [-S-(CHZ),OC~H~]Z where ?Z= 2, 3 do not show the dip effect and furthermore are poor antiwear additives similar to the corresponding disulphides (see Table II, Pt. 7 and 8) (4) Diester disulphides have po0rere.p. propertiesthanthecorresponding~ns~bstituted disulphide (Fig. 8). Two possible explanations for these results were initially suggested, viz., (i) since a.w. protection can be regarded as a controlled corrosion of the surface, these Wear, 15 (1970) 347-352

LOAD-CARRYING

PROPERTIES

OF DIESTER

DISULPHIDES

349

diester d&ulphides could be corrosive materials which are effective at low concentrations but corrode the surface rapidly at high concentrations and (ii) since these diester disulphides are trifunctional compounds, their adsorption on metal surfaces and its variation with concentration could explain the dip effect. Considering (i) there is some evidence to support the feasibility of this corrosion explanation. For example, in the series [-S-(CHB).CO~C~HS]Z, when ‘IZ=I, 2 or3,the carboethoxy group is near enough to the S-S group so that the proton a to the carboethoxy group is highly activated”, whilst no activation of the proton would occur with compounds where B = 5 and IO. Thus it could be expected that the diester disulphides showing the dip effect would be more reactive chemically. However, the corrosion explanation is not considered to be correct for the following reasons: (I) The corresponding mercaptan to z,z’di(carboethoxy) ethyl disulpllide i.e. ethyl-4-mercaptobutyrate HSCH&H&O&Hj would be expected to have similar chemical properties to the disulphide. Therefore if corrosion was the explanation, a dip effect would be expected for the mercaptan. However, no significant dip effect was observed. The result also excludes the possibility of corrosion being due to the carboxylit acid formed by hydrolysis of the carboethoxy group, since this group is just as likely to be hydrolysed in the mercaptan as in the disulphide. (2) Previous work has shown that the more corrosive a load-carrying additive, the poorer its performance in the antiwear region and the betteritsperformancein the e.p. region. However, these diester disuIphides have poorer e.p. properties than the corresponding disulphides. (3) If corrosive action of the d~esierdisulphide wasthe reason for the increase in wear scar diameter with increasing additive concentration, a higher concentration of additive would be expected in the larger wear scars and only a low concentration in the small wear scar. In fact, as shown by the electron probemicroanalysisresults for z,a’di(carboethoxy) diethyl disulphide (Table IV) the reverse occurs. Thus maximum S content (1.6r.8%) is found in the smallest wear scar whilst little sulphur (0.2-o.4 “,{,) is found in the large wear scar obtained when high concentrations of the additive are used. The second possible explanation of these results involving an adsorption mechanism could be as follows. When the diester disulphide approaches the surface, it is initially adsorbed at the ester groups and then at the S-S group. Thus at low concentrations of the additive, a type of adsorbed layer exists where the S-S group is bonded on the surface as well as the carboethoxy groups. However, at high concentrations it is postulated that the S-S group is either squeezed or lifted off the surface or does not have much chance of getting on the surface in the first place. This postulation is supported by the following evidence. (a) Heats of adsorption measurements (Table V) show that unsubstituted disulphides are weakly adsorbed on iron oxide, the diesters considerably more strongly and the diester disulphides even more strongly. The heat of adsorption for the diester disulphide is approximately equal to the sum of the heats of adsorption of the corresponding disulphide and diester, thus supporting the possibility that these compounds adsorb on the surface at both the ester and S-S groups. (b) The electron probe microanalysis resuhs show that at high concentrations of &ester disulphide, little sulphur is found in the wear scar whilst considerably larger quantities are found at lower additive concentrations. This supports the idea of the

E. S. FORBES

35”

et al.

S--S group being on the surface at low additive concentrations but not being there at high concentrations. (c) Since substances with anchoring groups at opposite ends of the molecule, in general, disulphides 2

form very expanded interfacial films, it is to be expected that diester form surface films of this type on the metal surface. Therefore when 1% = I,

or 3, at low additive concentrations

the ester groups are firmly bonded on the surface

whilst the S-S group is more weakly held. However, as the concentration of the additive is increased, the expanded film becomes compressed and even a slight compression causes the S-S group to be lifted out of the surface, whilst the ester groups remain in contact with the metal. Studies with Courtaulds models have confirmed that only slight compression is required to lift the S-S group off the surface and that, in this position, it is sterically prevented from approaching the surface. The positions of the diester disulphide molecules in the expanded state and in the compressed state are illustrated in Fig. o(a) and (b), respectively. The next question is why do we not get a dip effect for diester disulphides when

Fig. 9. (a). Diester

disulphide

in expanded

state;

(1,) diester disulphide

in compressed

state

LOAD-CARRYING

PROPERTIES

OF DIESTER

12= 5,1o since these compounds ever, Courtaulds

351

DISULPHIDES

will also form an expanded

film at the surface.

model studies have shown that, due tothelengthof

How-

methylenechain,

it is possible for the S-S group to maintain contact with the surface as the film is compressed. The film would have to be very compressed, i.e., the ester groups about touching, before the S-S group is removed from the surface and it is most probable that the film would rupture before it was compressed to this stage. Thus, for these diester disulphides where n = 5, IO, increases in additive concentration after the minimum wear scar, do not allow film pressure to build up sufficiently to form a close packed layer lifting the S-S group out of the surface to give the dip effect. (d) The fact that the diester disulphide is considerably more strongly adsorbed on the surface probably explains why superior antiwear results to those of the corresponding diester or disulphide are obtained. It is also well known that ethoxy groups are weak polar groups unlikely to be strongly adsorbed like estergroupsonthesurface. This explains why the diethoxy-substituted disulphides do not give a dip effect and also have antiwear properties similar to the unsubstituted disulphide. The dip effects for [-S-(CH~)ZCOOC~H~]Z (Fig. 2) and [-S-(CH&COOC1oHzl]z (Fig. 3) are different in that a poorer result is obtained for the latter additive and at much higher concentrations. This maybeexplainedintermsofinterfacialfilmcharacteristics. It is known that stability of palmitate esters at an air-water interface decreases as the chain length of the esterifying chain is increased7. This effect is due to decreased “anchoring power” of the ester group. The decyl chain will have a similar effect on the diester film at the oil-metal interface. Thus compared with the ethyl diester, a high concentration of decyl diester is required to furnish a relatively weakly held surface film. Thus, it would appear that the antiwear results obtained for this series of polar group-substituted disulphides can be explained on the basis of their adsorption at the metal surface. The main requirement for both the dip effect and improved antiwear properties appears to be the presence of strongly molecule with the S-S group in the middle. The poorer by the interfering the surface. Under of sulphur on the

adsorbing

groups at both ends of the

e.p. properties of these diester disulphides can readily be explained effect of the ester groups preventing the sulphur atom getting to e.p. conditions, it is well known that the higher the concentration surface, the better the e.p. performances.

Thus, this work shows that these polar group-substituted disulphides couldgive improved or decreased load-carrying properties in various applications, compared to the corresponding unsubstituted disulphides, depending on whether a.w. or e.p. lubrication properties are required. It also highlights the importance of carrying out detailed studies of the films formed by these additives at oil-metal interfaces. CONCLUSIONS

I. A study of the load-carrying properties of terminal polar group-substituted disulphides (using the 4-ball machine) has shown that when the polar substituent is an estergroup, markedly improved antiwear properties with slightly decreased extremepressure properties were obtained compared to the unsubstituted disulphides. However, when the substituent was a less polar group like alkoxy, similar results to the unsubstituted disulphides were obtained. Wear, 15

(1970)

341-352

E. 5. I;ORBES et d.

352

2. Maximum antiwear activity at low additive concentrations followed by subsequent decrease in activity with increasing additive concentration was observed for diester disulphides [--S-(CHz) n-C0~C2H 51s where n = I, 2 and 3 but not when ‘~6 = 5 or IO. An explanation has been suggested for these results based on the adsorption

propelties of these compounds; this explanation is supported by heats of adsorptinrl and electron probe microanalysis studies and by studies of possible adsorbed layers using molecular (Courtaulds) models. 3. These results suggest that the prime requirement for improvement of the antiwear properties of organo-sulphur compounds is the presence of strongly adsorbing polar groups attached at both ends of the molecule. These results also lli~l~li~llt the inll~ortan~e of more detailed studies of the nature of the adsorbed film at the metal surface.

Permission Company,

APPENDIX

to publish

this paper has been given by The British

Limited.

: STARTIKG MXTERI:\LS

(I) Ethyl (2)

r-mercapto acetate (Fluka “purum”) 3-PIZercapto propionic acid (Fluka “purum”)

(3) Ethyl (4) (5) (0) (7) (8)

3-bromo-propionate

bzo = 62°C

b12 = 79.5”~8o”C

Ethyl 4-bromohutyrate (Fluka “purum”) boo = 82”-3°C h-Bromo hexannic acid (Fluka “purum”) I I-Rromo undecanoic acid (Fluka “purum”) Ally1 bromide (British Drug Houses Limited) b:eo = 70-C I-~ronio-z-et~~ox~~ethane (AIdri~l~)

(9) Thiolacetic acid (British Drug Houses Limited) b760 = KY-9n”C (IO) Decanol-1 (Fluka “puriss”) bo.5 = 73”--4°C (II) Thiourea (British Drug Houses Limited) (12) Sodium acetate (Hopkins and Williams “Analar”) (13) Acetic acid (Hopkins and Williams “Analar”) (14) Toluene (Hopkins and Williams “GPR” “Sulphur-free”) (IS) Sodium bicarbonate (Hopkins and Williams “Analar”) (16) Sodium hydroxide (British Drug Houses Limited “Analar”) (17) Potassium hydroxide (British Drug Houses Limited “Analar”) (IS) Potassium iodide (British Drug Houses Limited “Analar”) (19) Iodine (British Drug Houses Limited “Analar”) (zo) para-Toluene sulphonic arid (Hopkins and Williams Limited)

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