Diesters as high-temperature lubricants

~eav -

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

DIESTERS

AS HIGH-TEMPERATURE

97

LUBRICANTS*

B. T. FOWLER Esso Keseavch Centr~, Abingdon, (Received

November

I.+‘

Berks.

(Gt. Brifain)

1969)

INTRODUCTION Interest in the alphatic esters as main engine lubricants may be traced back to the mid-rgso’s and the development of the high-performance aviation piston engine. A lubrication performance beyond that offered by conventional mineral oils was required for this application and castor oil blends were developed and used despite an inherent instability present in the castor oil component which created some problems due to gum and varnish formation within the engine lubrication system. The attractiveness of castor-based oils, from the lubrication viewpoint, of prompted an investigation by ZORN et ad.1 in the late 1930’s of the instability natural fats, These studies indicated that the instability was due to the presence of glycerol, a trihydric alcohol containing two primary and one secondary hydroxyl groups. When heated in the presence of acids or metal catalysts, water is readily removed from glycerol to form the hfi unsaturated aldehyde, acrolein, which exhibits a marked tendency to polymerise-a prerequisite to gum and varnish formation.

CH20H CHOH

I

CHpOH

Glycerol

CH*CJH

I

r-rrCH

: Ii AOC”

-

-

F”’

CX

I

CHO Acrolein

An additional feature of these studies was the fact that glycerol esters were most readily hydrolysed at the secondary hydroxyl position. This in itself is a detrimental factor for glycerol-containing products since ready hydrolysis leading to the liberation of an acid will again accelerate decomposition. These factors led ZORN to the preparation of synthetic fats based on the trihydric alcohol trimethylol ethane, a polyol containing all primary hydroxyl groupings. Evaluation of esters based on trimethylol ethane showed that the structural weakness * Paper presented at the Symposium on “The Effect of Temperature on Lubrication Paisley College of Technology, Paisley, Scotland, October x4-17, 1969. Wear,

15

Systems”,

(1970)97-IQ4

1:. ‘I’. I~o\vl~lil;

98

had been eliminated, product thermal stabilit?; being grcatl!. imprmwl. I~lowe\:~~r,it was also noted that the branched structure of the poI\-ol-based ester imposctl iirnitations on the \‘.I. ceiling possible compared with what could he obtained using t11(* linear diester-molecule (Table I}. As a result of a survey of some 3=joo structurall~~ different

esters

aliphatic

diesters

in the period

1937744,

%OI
offered the best combination

of properties

that

tlie sirnph

for use as main cnginc

lubricants. As will be mentioned below, the more stable esters based on polyols, the starting point of ZOXN’S investigations, waited a further 25-30 years before being c-ommerciallv developed as aviation turbine lubricants.

stability

Only in the last 5-7 years have the polyol esters, by virtue of their volatilitycharacteristics, begun to take over the role played so successfully by diester-

based lubricants. ALIPHATIC

LfIESTEHS

Structural guides based on studies of pure and synthetic hydrocarbons suggested that the simple diester offered one of the best routes available for the development of maximum temperature range high V.I. lubricants. The rjo-160 V.I. of a diester such as di-z-etllylhexyIsebacate offered a shear-stable lubricant of acceptable working

Kinetic

viscosity

(cSt)

2 10°F 40°F

-6,j”F v. I.

I’our point

(“F)

Vapour p*essure (p.s.i.)

rgo”F 350°F

O.Ij

400°F

?.S

550°F Ryder gear machine failure load lb./in. tooth face width Flash point (open cup, OF)

rvenr,

rg (1970) 97-‘“4

2.5

3,ooo r5,ooo 75 C-75 1 ..?

GO0

295

DIESTERS AS HIGH-TEMPERATURE LUBRICANTS

99

viscosity coupled to a loo-temperature fluidity suitable for use in the temperature For comparable ZIO’F viscosity, the diester has a higher range -40°F to -65°F. flash point and lower volatility than the corresponding mineral oil (Table II). Also, maximum viscosity limitations placed on diesters by the non-availability of suitable raw materials may be removed by use of complex ester structures, based on glycols, diacids and alcohols. In this way the linearity (high V.I.) of the diester structure may be preserved, i.e. alcohol-diacid-(polyglycol-diacid)-alcohol. Alternatively, the camplex ester may be diacid-centred and terminated with monobasic acid, either structure lending itself readily to a single or two-stage esterification process. Extension of the diester viscosity range in this manner proved particularly valuable in the development of the aviation turboprop engine where a high loadcarrying capacity lubricant was required for satisfactory operation of the propellor reduction gearing. In this respect, esters showed no particular advantage compared with mineral oils (Table III). The difference in load-carrying capacities of mineral oils and esters is usually attributed to the somewhat more effective boundary-lubrication characteristics of the polar ester structure. TABLE III LOAD-CARRYING

CAPACITY

Jet engine mineral oil Di-ethylbutyl adipste Di-nonyl sebacate Aviation mineral oil Aviation mineral oil Complex ester

____

.-.-l__.-l”-

OF DIESTERS

AND

MINERAL

A.5 2.3 4-7 9.0

30 45 50 75 75

20.1 10.2

OILS

90

__~

Despite the advantages enumerated above, ATKINS et al.2 considered that the simple diesters had an oxidation/thermal stability comparable to that of a wellrefined mineral oil. As such they were considered to be useful as lubricants only up to temperatures of 150°F. Certainly, studies in the mid-rg3o’s by HURD AND BIXNK~ of the thermal stability of simple esters in no way suggested that they might possess an outstanding stability suitable for high-temperature operation. HURD AND BLUNK’S work, based on the analysis of the products of pyrolysis of several esters of acetic acid, led to the belief that esters which possessed a p-hydrogen atom in the alkyl group could undergo a chelate-type ring closure by hydrogen bridging. As illustrated below, readjustment of the electrons would then give rise to the acid and olefins which were the primary degradation products detected in their experiments. :o:

II

HI,

p-

R-C \6,CRz ..

-

100

Ii.

I.

l~O\Vl,l
it \vas assunlc~d tht

(b) with oxygen being more negative than carbon, the hydrogen ~~lcl remain with the oxygen when the bridge was broken. The formation of this lo\v-energy cyclic intermediate structure in the pyrolysis of non-hindered, i.e. $-hydrogen containing, esters is generally accepted a~ the esplanation for the relatively low, 500-525’Y, tlrermal de~orn~~)siti(~n temperature recorded by the isoteniscope. This level is comparable to that of a well-refined paraffinis mineral oil, though vet-v heavily extracted mineral oils (WI ac+trieve thermal ~le~o~n~~ositio~l temperatures in the range hoo-6~02’l~. Study of ester oxidation stability, however, proved much more rewarding and, in fact, provided the key to ester exploitation as synthetic lubricants. Major effort by k~\‘IUWPH\;et al.3 and others in the late ~c+p’s showed that, by acting as chain stop pers, oxidation inl~ibitors could exert a marked influence on the oxygen absor~)t~on by esters. Of numerous antioxidants studied, phenothiazine proved to be about the most effective compound for the aliphatic ester series and most if not all of the proprietarylubricantsmarketedin the r<)jo’s had 0.5-1.0 wt.‘;,; IArenothiazine present a5 the oxidation inhibitor (Table Ii:). ft gave satisfactory ~~erf(~r~nan~e in ali e:u’Iv formS of aviation turbo-prop and turbo-jet engines.

The intr(~duction of turbo-fan engines and larger turb(~-promo units, however, saw a general rise in aircraft performance, and there was a corresponding rise in engine lubricant operating temperature. Bulk oil temperatures of 3oo”lT with peak bearing running temperatures of 500°F were not uncornrn~~~. in addition, there was a move at this time away from the American practice of fixed oil drain periods, the interval being dependent on the engine model and lubricant employed. U.K. engine builders had never adopted this procedure, relying on (a) oil drains at major engine overhauls and (b) normal engine consumption during service to maintain a satisfactory oil quality within the lubricant system. Oil monitoring programmer allowed this procedure to continue as engine overhaul life moved from 125 h (in the case of the first Dart engines) to 6-10,000 h for current engines. All of these factors served to put greater stress on the lubricant, and it became apparent that oxygen absorption data alone did not present the whole picture. There

DIESTERS AS HIGH-TEMPERATURE

101

LUBRICANTS

was no indication of oil sludge build up which was, in the late Igiio’s, becoming a field problem. &IURPHY’S studies had suggested that many of the oxidation products of phenothiazine were, in fact, quite effective antioxidants. Unfortunately, they were also oil insoluble. This led to additive depletion which could be tolerated by virtue of low, but constant, oil make-up necessary to balance oil consumption from the 2-4 gallon oil systems of the aviation gas turbine engine. What could not be tolerated, however, was the steady build up of sludge, due in large part to the insoluble oxidation products of phenothiazine and perpetuated by oil make-up. In certain equipment, oilways to main shaft bearings became compacted with sludge, leading to oil starvation and subsequent bearing failure. ELLITOTT AND EDWARQS~ approached the problem of what became known as “phenothiazine dirtiness” by attempting to solubilise the oxidation products of phenothiazine, This was achieved by alkylation of the basic phenothiazine structure to give the 3,~ dioctyl phenothiazine. Their results showed that they had eliminated the problem of oil-insoluble matter derived from the oxidation inhibitor though not without some fall off in used-oil viscosity increase and copper corrosion performance (Table V). There had also been some drop in antioxidant activity, more than would be predicted by the increase in molecular weight of the alkylated product. Use of a secondary amine oxidation inhibitor in conjunction with phenothiazine improved used oil acidity and viscosity control and the addition of a corrosion inhibitor provided a balanced additive package greatly reducing engine sludge formation yet being non-corrosive and providing good control of used oil properties. Following these studies, other synergistic additive combinations have been evolved, serving to prolong the useful life of diesters as aviation lubricants. In re~tifyingthesludging~hara~teristi~ of diester-based formulations, however, it was inevitable that attention would be focused on some other aspect of lubricant quality. Thermal stability and volatility were the properties to receive attention and new requirements in these areas necessitated the development of hindered ester base stocks. TABLE THE

V

EFFECT

OF ADDITIVES

ON ESTER

PERFORMANCE

Oxidationcorrosiontests, 72 h at 347OF _.

._I Additives (wt. %)

iikylated ~ke~~th~az~~~

Secondary amine

_-.-~ Metal deactivator (cowosion inhibitor)

Weight change of copper specimen (wlcm*i -2.1

2.0

-1.2

- 1.6 -13.0 -7.4

1.0 2.0

I.5

3.0 o-5

-0.2

I.0

0.5

--I,?

0.j

14

0.75

0.75

0.1

-3.1 nil

1.0

1.0

0.1

-0.02

0.5 wt. % phenothiazine

Acidity increase (mg Kofffg)

Insolubles (%i

--

1.0

3.0

Change in K V at 100°F (%)

-8.2

-0.4

fI4.3 +6.7 1-5.2 + 16.5 t- 14.0 f23.5 i-4.9 14.7 +6.7 +3.0

16.0

0.50

II.1 2.9 3.8

Cl.20 nil 0.73

2.0

0.17

5.6 X.6 1.6

0.60 nil nil nil

0.26

0.1

2.6

0.92

I.93 Wear, 15 (1970) 97-104

nil 0.50

I:.

Carbon deposit5 and oil consumption were, bv the earl\. IqOo’s, beginning

engines

c~l1arac.tc~ristic.s 0I particular

I.

I~o\vI.I:li

turl)o-jc[

to suggest that siniplc alil)hatic, cstrrh were being thermally stressed to the limit of their performance. Sew htructurcs w(‘r(’ required as therca was e\rery indication that this trt,nd of increasing stLvt,ritv in operating

temperatures

would

continur

into

ttlt> 1070’s (Tablrl

VI).

As previously noted, Lvork by ZOKN et ul. and HUN) ASI) RLI:xK had suggested that the no-{j-hydrogen polyol would have a thermal stability somewhat better than the mn-hindered product. Elimination of the hydrogen atom in the fl position prevented the formation of a cyclic intermediate structure and suggested that thtl hindered

product

would

decompose

by a free radical

Ii, (‘X>H + K:!

mechanism.

Kn (‘=(

of the thermal decomposition temperature for Isoteniscope measurements esters gave values of hooM5o”F, indicating a IOO”F advantage in thermal over the non-hindered product. Use has been made of this enhanced stability in current synthetic lubricants. With the availability of a full range of C3P(‘1~) straight-chain monobasic acids together with tri-, tetra- and hexa-no-b-hydrogen polyols, products ranging from -3 to 25 cSt. viscosity at 210°F have been prepared and evaluated. In general, 5 cSt. (210~ 1; viscosity)-products offer the best compromise of low-temperature fluidity, volatility and load-carrying capacity. Fully hindered bases prepared from the polyols and ,X,X-substituted neo-acids were shown to provide a further improvement in thermal stability, but the additional branching introduced markedly affects volatility and \‘.l. (Table VII). These disadvantages outweighed the relatively minor improvement (cn. 25CI;) in thermal

hindered stability

DIESTERS

T_kBIX

AS Olga-TEMPERATURE

_.

_-

WINDGRED

--

------

ESTERS _..__._._~

Ester zi;. ______. Triester Tricster Triester Triester

103

Vl 1

vOJ.,ATII.ITYAND V.I. OF FULLY _.-

LUBRICANTS

_

..I._ - .---

(alcohol (alcohol (alcohol (alcohol

.-_-.-

-

--

.__ .-.- ..- -.

only hindered) and acid hindered) and acid hindered) and acid hindered)

~- ---

--.---

V.I.

Volatility loss after 6.5 A af 450°F I”/01 ---_

‘45 37 52

25.0 50.6

at z~o”F) -...

4.5 4.7 5.5

7.7 .-- -

6

60.0

40.0

stability, as other products, such as the polyphenyl ethers, could provide an even greater stability with structures possessing superior physical characteristics. Whilst oxidation inhibitor-base oil technology had now reached a stage where it was capable of meeting immediate and near-future oxidation and thermal stability requirements imposed by the engine temperature environment for sustained high speed flight, at least one important problem area remained requiring solution. It was the need to provide adequate load-carrying capacity for the lubrication of gearing tapping main engine power for aircraft accessories. Whilst a gear tooth loading of 140,000 p.s.c. Hertzian stress presented no major problem for uncompounded esters operating in a -40°C to +x10X temperature range, this level of loading for a bulk oil temperature of x80-ZOO’C presented a difficult target. All previous development of ester fluids for turbo-prop and turbo-jet applications had seen the desired level of load-carrying capacity built into the base ester structure. Physical properties, i.e. viscosity, had largely been tailored to accommodate load-carrying requirements and to avoid the use of extreme-pressure additives. Such additives, which function by reacting with the metal surface to form a low shear strength film thereby preventing or delaying the onset of scuffing, are labile molecules. They possess, therefore, considerable chemical reactivity which renders them less stable than an ester-base fluid and more prone to sludge formation and corrosion problems, Xany e.p. additives contain sulphur as the active element and this, in itself, may produce compatibility problems with rubber sealants currently employed. Studies have shown that, in the case of the SST aircraft, uncomI~ounded esters of the desired load-carrying capacity cannot be designed without seriously compromising viscometric or stability characteristics. The high oil system operating temperature permits a working viscosity of only 1.5~2.0 cSt. for the 5 &.-lubricant selected for SST operation. Recourse to e.p. additives was inevitab!e since a load-carrying capacity at least twice that of the 5 cSt.-base was required. Proprietary non-corrosive, stable e.p. additives have been developed in recent years. They show promise of meeting the future aviation demand for high temperature, high load-carrying, low viscosity synthetic lubricants. ACKNOWLEDGEMENT

The author wishes to thank permission to publish this paper.

the Directors

of the Esso Petroleum

Co, Ltd. for

Wear, rg (1970) 97-x04

Wmr,

‘5

(1970)

97-104