NAVAL AIR PROPULSION TEST CENTERTRENTON, NEW JERSEY 08828

NAPTC-PE-71 APRIL 1976

SA STUDY OF THE FACTORS AFFECTING DEPOSITION CHARACTERISTIC',

OF SYNTHMMIC LUBRICANTS FOR GAS TURBINE ENGINES

By A. J. D'OrazioP. A. KarpovichC. J. Nowack

APPROVED FOR PUBLIC RELEASDISTRIBUTION UNLIITED C

'7NCLAS SIF lEDSECURITrY-C.ASStFICATION OF TmIS PAGE (Whon Dael Entered)

REPORT DOCUMENTATION PAGE BFREA INSTRUCTIONSOR

I REPORT NUM tm 12. GOVT ACCE SSION NO. S. RECIPIENT'S CATALOG NUMBER

NAPTC-n-EPE Oor REPOurT & PERIOD COVERC,)

AStudy.: of the Factors Affecting DepositionCharac";eristics of Synthetic Lubricants for/A0'

FGas Turbine Engines*e

AUTHOR(s)I OYA.RGAYUOAd

V A. J.iD1')OrazioiP. A."lKarpovich/r, T __ _ __ _ __ _ __-k__ _ _

NAME AND ADDRESS 10. PROGRAM CLEMENT. PROJECT, TASK

Naval Air Propulsion Test Center NVAIR AIRTASK A3303300/052B/Lubricants & Power Drive Division, PE72 4 P30

It. CONTROLINGOFIC NAME AND ADDRESS X Aprix-

14. MONITORING AGENCY NAME 6 AODRESS(I114011ognt hea Cmnitmlin01,1J ffc) IS. SCCURITY CLASS (of hid. rOpMe

Unclassified.2154 09C&ASIICATION/OOWNORAOING

Approved for Public Release; Distribution Unlimited

17. 01ISTRIBUT ION ST ATEMENT (of I%. abodtenteref i tmlbSooh 20.f AII oo how kmPp, D

ID~

It. KEV WORDS (Conitinu an t1oero aide jief 34my a" sava~i by Nock nfAmb)

* - temperatures without forming excessive harmful coke deposits Vasinvestigated. This study consisted of an engineering evaluation of thelubricant deposition-degradation characteristics which could have as .ignificant bearing on the maintainability and reliability of futurehigh thrust to weight ratio engines to be incorporated into the Navy

*inventory. Relationships were then established between these characteristics

DD , ANIT 1473 ED0ITION 'Or' I O 6 ' OUL UNCLAW37=I5/ft ~ ~ ~ ~ "UPT CLAM01 q66011 I -P^ guryc.ISIA-m !

i UNCLASSIFIED,. u.ITY CLASSIFICATION OF THIS PAGE(When DI|me•rials, altd)esser dge

and chemical composition of the " asestock materials, and to a lesser degree,the oxidation inhibiting packages. The intention of this investigationwas to provide cause and effect relationships for the phenomena whichwere either the strengths or weakneses of the current formulationphilosophies. Such an understanding ild open the way to meet the moredemanding future requirements.

It is equally desireable that engine designers utilize the informationpresented as a guideline for avoiding those environments which are mostconducive to the generation of lubricant degradation products that canjeopardize engine operation.,

40. .iT14• II

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UNCLASSIFIEDSECURITY CLASSIFICATION OP THIS PAGE(Wh' Data Enterend)

• NW-

[ NAPTC-PE-71

TABLE OF CONTENTS

REPORT DOCUMENTATION PAGE -- DD FORM 1473

I TITLE PAGE

TABLE OF CONTENTS ........... ........... . . i1LIST OF FIGURES. . . . . . . . ........ . .iii . .

INTRODUCTION .... .................. . . . . . . 1

CONCLUSIONS ... ....................... . 2

j RECOMMENDATION ... ........... . . . . . . . . . . . . . .2-3

DESCRIPTION . ......... . . . . . . . . . . . 3 - 7

DISCUSSION ................................... .7 -.

FIGURES 1 THROUGH 21 ........... ...... ........... ... 19 -39

REFERENCES ..... ...................... . . . .. ..

I APPENDIX A . . .. . . . . . . . . . . . . . . . . . . A I- 1 - A1 -5. .

APPENDIX B ........................... . B3-I - BI-6

.APPENDIX C ................. ....................... Cl-1 - C1-5

DISTRIBUTION LIST ........ ..................... .... Inside rear cover

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NAVAL AIR PROPULSION TEST CENTER

TRENTON, NEW JERSEY 08628

PROPULSION TECHNOLOGY AND PROJECT ENGINEERING DEPARTMENT

NAPTC-PE-71 APRIL 1976

A STUDY OF THE FACTORS AFFECTING DEPOSITION CHARACTERISTICSOF SYNTHETIC LUBRICANTS FOR GAS TURBINE ENGINES

Prepared by: ,ji,

Program Manage(

TA| ,P.A* APO HProject Engineer

C.-J. NOWACKConsulting Chemist

Approved by: / '.. 4.- . J(A. L. LOCKWOOD

AkPPROVED FOR PUBLIC RELEASE:DISTRIBUTION UNLIMITED

AUTHORIZATION: NAVAIR AIRTASK A3303300/O52B/4F5i45330CWork Unit Plan 637

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LIST OF FIGURES

No. Page No.

1 High Temperature Deposition Tester Test Section Schematic 19

2 Vapor Phase Coker Test Section Schematic 20 t

3 Basestock and Additive Effects on Deposition 21

Basestock and Additive Effects on Viscosityiy 21

4 Basestock and Additive Effects on Viscosity Stability 22

5 Basestock and Additive Effects on TAN 23

6 Lasestock Effect on Deposition Characteristics 24

7 Basestock Effect on Bulk Oil Degradation Characteristics 25

8 Coking Characteristics in Non-Oil Washed Areas 26

9 Deposit Formation in Vent Line of Vapor Phase Coker at 27Various Vent Control Temperatures

10 Deposit Formation in Vent Line of Vapor Phase Coker for 28Seven MIL-L-23699 Lubricants

11 Deposit Weights Versus Average Acid Chain Length 29

12 Deposits Formed by Various Ratios of Dipentaerythritol 30and Trimethylol Propane Ester Basestocks

13 Yield From Various Ratios of Pentaerythritol and 31Trimethylol Propane Ester Basestocks

14 Yield Versu.; Average Acid Chain Length for Mixtures of 32Pentaerythritol and Trimethylol Propane Ester Basestocks

15 Deposit Formation in Vent Line of Vapor Phase Coker for 33Various Ratios of Trimethylol Propane and DipentaerythritolEsters

16 Deposit Formation in Vent Line of Vapor Phase Coker for 34Various Ratios of Trimethylol Propane and PentaerythritolEsters

17 Yield Versus Average Acid Chain Length for Esters of 35Pentaerythritol and Trimethylol Propane

18 Deposit Formation in Vent Line of Vapor Phase Coker for 36Various Alcohol Acid Combinations of the Base Ester

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:111 NAPTC-PE-71

LIST OF FIGURES

"No. Page No.

?9 Temperature at Location of Maximum Deposit Density for 37Various Acid Chain Lengths of the Basestock Acid Component

20 Deposition RegiJon Related to Chain Length of the Acid 38Component of Ester Basestocks

21 Deposit Forming Patterns as Related to Acid Chain Lengths 39

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INTRODUCTION

The gas turbine lubricant, as it performs its function as bearinglubricant and ccolant, is surrounded by an environment that is conduciveto its oxidative and thermal decomposition. A breakdown of the lubricantcan seriously jeopardize engine performance since decomposition products,such as coke and sludge, interfere with the proper functioning of bearings,seals, oil scavenge pumps and breather systems. Severe failure of thelubricant in any one of these areas can produce conditions which willlead to catastrophic engine failure.

For more than a decade, Navy gas turbine aircraft engines have beenoperating on lubricants that are generally classified as neopentyl polyolesters. As a class, these lubri'cants possess satisfactory thermal andoxidative stability as well as the physical properties that permitoperation over a wide temperature range. Since their introduction asMIL-L-23699 lubricants into Navy aircraft systems in 1963, oil drainintervals have been extended, consumption reduced, sludging virtuallyeliminated and coke formation decreased below levels experienced withthe previously used diester type lubricants.

However, the ability of these oils will be tax. I as designers striveto derive more power out of lighter more compact powerplants. With theresultant increase in what may be termed "energy density" of the engine,it was anticipated that the temperature of surfaces to which the lubricantis exposed would increase to the point where the present ester lubricants"would decompose to form objectionable amounts of carbonaeceous deposits.Therefore, the Naval Air Propulsion Test Center (NAPTC) undertook aresearch program, authorized by reference 1, to determine what potentialwas inherent in the neopentyl polyol esters to survive higher surfacetemperatures without forming excessive harmful coke deposits. This studyconsisted of an engineering evaluation of the lubricant deposition-degradation characteristics which could have a significant bearing on themaintainability and reliability of future high thrust to weight ratioengines to be incorporated into the Navy inventory. Relationships werethen established between these characteristics and the chemical composi-tion of the basestock materials, and to a lesser degree, to the oxidationinhibiting packages. The intention of this investigation was to providecause and effect relationships for the phenomona which were either thestrengths or weaknesses of the current formulation philosophies. It wasbelieved that such an understanding could open the way to new approachesin the development of lubricants needed to meet the more demandingfuture requirements.

It is equally desirous that engine designers utilize the informationpreqented as a guideline for avoiding those environments which are mostconducive to the generation of lubricant degradation products that canjeopardize engine operation.

Publication of this report completes the requirements specified by

reference 1.

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NAPTC-PE-71

CONCLUSIO! --

1. Impri ;ement in ester lubricant technology is needed in order toprovide satisfactory deposit characteristics when 3uch lubricants areexposed to the anticipated environments of future Navy engines. Toward

this end, the following findings can be considered:

(a) Quantitatively, deposit formation is a strong function of thechain length of the acid component of the basestock. Deposits increasewith increasing chain length.

(b) The basestock alcohol component has a small effect on depositweight with deposits increasing in the order dipentaerythritol, penta-erythritol and trimentylol propane.

(c) An interaction effect between acids and alcohols exists wherebythe differences in deposit weights from two alcohols increase withincreasing acid chain length.

(d) Qualitatively, deposit formation patterns are strongly dependenton the acid component of the ester. As acid chain length increases, thedeposit area and temperature range over which deposits form increasewhile average temperature of the deposit forming zone decreases.

(e) The performance of single acid esters is identifiable in theperformance of mixed acid esters. Therefore, mixed acid ester performancecan be predicted by the performance of the component individual singleacids.

(f) Evaluations of neat basestocks agree with the trends establishedfor fully inhibited lubricants.

(g) Additive systems employed to control bulk oil oxidation contributesignificantly to deposit formation.

2. The differences in the deposit forming tendencies among the lubricantstypical of current (MIL-L-23699) technology can be considerably trouble-some to engine designers striving to provide lubricant environments thatare compatible with all lubricants within this class.

RECOMMENDATIONS

1. Findings of this investigation should be used as a basis fordeveloping a new class of ester lubricants which can better withstandthe (anticipated) more severe environments of future Navv eneines^

2. Attention should especially be given to developing basestocks which,as a class, are very consj tent in deposition characteristics and todeveloping additive packages which perform the necessary function of

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inhibiting bulk oil oxidation without adding to the formation ofobjectionable types of deposits.

3. Future lubricant specifications should define those environmentsin which the formation of objectionable types of deposits is unavoidable.These definitions should then be utilized as guidelines by the enginedesigner to avoid engine lubrication and breather system configurationsthat are conducive to gross deposit formation.

DESCRIPTION

Lubricants

1. The gas turbine engine lubricants of concern to this researchprogram are based on a group of "hindered" esters formed from organicacids and polyfunctional alcohols. These esters, known as neoperitylpolyol esters, depend primarily upon a unique five-carbon structurein the polyfunctional alcohol used for esterification with the selected

• monofunctional acids. Polyols derive an added measure of thermal(pyrolytic) stability by virtue of the hinderence provided by the

neopentyl structure which eliminates hydrogen from the "beta" carbon.

2. The esters most frequently employed in products that concern thisprogram are mixed acid esters of pentaerythritol, dipentaerythritoland trimethylol propane. The chemical structures of these esters areshown below:

Pentaerythritol (PE)'.

CH2 -O_-R

InS.... ~ ~R-C-O-CHf----CCH2-O-C-R.ii•i

CH2 -O.4C-R

Dipentaerythritol (DPE)

R-6O-0 CH2 CH 2-0-P-R

-uOC2 -C -CH 2-O-CH -C-CH 2-O-C2-

R-C -0 - CH2 , H2-c-•-K -o -U 0,--d

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NAPTC-PE-71

Trimethylol Propane (TMP)

IH2-0- -R5 i

CH2 -0-9-R

For each of the above structures R=C5 to Cl0 either straight or brenchedchain hydrocarbons, the chain length being derived from that of themonocarboxylic acid used for the esterification of the alcohol.

i•3. The nti•pentyl polyol esters are the basis for all lubricants qualifiedunder specification MIL-L-23699 which currently meets the needs of all

Navy gas turbine propulsion systems. In 1963, the Navy introduced theselubricants to replace the MIL-L-7808 lubricants whicn were classifiedas monohydric alcohol esters of dibasic acids. The hindered polyol estershave been shown to possess e-'cellent thermal and oxidative stabilityup to h000F. Additionally, they have most of the other properties of agood lubricant, including good viscosity-temperature characteristics,fairly low pour point, and good lubricity without the use of additives.

4. In order to meet the physical property requirements of thespecification, the esters are generf ly produced from mixtures of acidsof various chain lengths. The finished product then contains a statisticaldistribution of acid-alcohol combinations. It is the effects of the.various monobasic acids on deposition characteristics that constitute themajor effort under this program.

5. More detailed information on the chemical structure and specificproperties and characteristics of neopentyl polyol esters is containedin references 2, 3 and 4.

Test Methods

6. The evaluations conducted under this program employed two testmethods commonly used in the evaluation of deposition and degradationcharacteristics of gas turbine lubricating oils. The methods referredto are the High Temperature Deposition (HTD) test and the Vapor PhaseCoker (VPC) test.

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7. (NAPTC-PE-71High Temperature Deposition Tester

7. The HTD test simulates, with simplified equipment, the basicparameters that influence deposition and oil degradation throughouttypical gas turbine engine lubricat'on systems. Deposition is definedas the formation, on a surface, of a solid or semi-solid material(e.g. varnish, coke or sludge) whicL is the result of decompositionof the lubricant upon contact with the hot surface. Degradation refersto the change in chemical structure, usually evidenced by changesin viscosity and acid number, of the lubricant due to its thermaland/or oxidative decomposition in the bulk state.

8. The apparatus used for HTD testing is the Alcor Deposition Tester,Model HTDT1003, manufactured by Alcor, Inc. of San Antonio, Texas.Figure 1 presents a *chematic drawing of the test section. Oil iscirculated at 300 ml/min by a high temperature pump. Since this is aprecision constant displacement pump, constant flow is maintainedby a constant-speed drive. Air, saturated with distilled water, isinjected into the oil just prior to entering the deposition tube section.After leaving this section, the oil-air mixture discharges into a s'umpwhere oil temperature is maintained at the desired value by a controlledflow of an air-water mist that passes through a jacket surrounding the !

oil chamber. A filter screen is incorporated in the bottom of thecooler-sump. The oil level may be observed from the sight glass, andthe level is maintained by the automatic leveling device.

9. The deposition tube is seamless (Type 440) stainless steel and the

test area is 1/4 inch in diameter and 10 inches long. The tube isheated by passing AC current directly through the metal. Two thermo-couples are inserted into the inside of the tube. One thermocoupleenters from the bottom and is positioned at exactly three inches fromthe centerline of the oil inlet point for monitoring the tube temperature(5-3/4 inches from the bottom). Another thermocouple is located at themaximum temperature point, which is 3/4 inch from the oil outlet center-line (3-1/2 inches from the top end of the tube). The increase in theupper tube temperature point is an indication of the deposit build-up.S~All metal components contacting the test oil are stainless steel exceptfor the pump which is made of high temperature steel.

10. The entire test section is enclosed in an insulated cabinet in whichtemperature is controlled by forced convection of air through a 3000watt heater. The test rig is equipped with a total of six thermocoupleprobes, the outputs of which are recorded on a six-point strip chartrecorder as follows:

a. Maximum deposition tube temperature

b. "Lower" deposition tube tempert re (control point on lower endof tube).

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NAPTC-PE-71

c. Oil-out temperature.

d. Oil-in temperature.

e. Cabinet ambient temperature.

f. Bus bar connector temperature (used for test integrity purposes). f

Dual thermocouples are utilized for controlling and recording the oil-intemperature and the cabinet ambient temperature.

11. The test conditions applicable to this method are as follows:

Oil-in Temperature 300°F +2 0 F

Initial Oil Charge 250 ml +20 ml.

"Lower" Deposition Tube 500'F, 5250F, or 550°FTemperature

Air Injection 1000 ml/min +15 ml/min

Cabinet Temperature 2000 F+5°F

Bus Bar Connector Temperature 100OF to 130 0 F

Test Duration 48 hours +0.5 hr.

The above test conditions result in a temperature gradient along the tubewith initial maximum tube temperature, in the order of 5900, 6400 and 700°Fcorresponding to the lower tube temperatures of 5000, 5250 and 550 0 F.For a fluid which forms little or no deposits, the temperature rise ofthe maximum temperature point will only be slight whereas for a fluidwhich forms heavy deposits this temperature will gradually increase andis indicative of deposit baild-up at that point.

12. The complete method of :,st is described in detail in specificationXAS-2354A (reference 5). ....e only deviation from this method is thevariation in "lower" deposition tube temperature.

Vapor Phase Coker Tester

13. The VPC test simulates, in a simplific" rig, those portions of agas turbine engine where hot surfaces are con.- -,ed by oil mists and/orvapors only, i.e. no oil washing occurs at these surfaces. The test isused primarily to assess the tendency of lubricants to form depositseither as mists con t act the hot surfaces or as vapors condense in coolerregions and reflux over hotter surfaces.

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14. The test apparatus used for VPC testing is the Eppi Vapor Pha-eCoker, model 5400, manufactured by Eppi Precision Products, Inc. ofClaredon Hills, Illinois. The Eppi Coker (Figure 2) consists essentiallyof a three neck flask (oil rese-voir) surrounded by an electric heatingmantle, an intermediate "heating" tube surrounded by a coiled rod typeheater, and a stainless steel venit" tube inside which the depositsare formed. In operation, air is fed through a tube entexing one neckof the flask, bubbled through the oil and permitted to escape throughthe center neck. Upon leaving the flask, the air and vapors passthrough the heater tube and then directly into the cokiny "vent" tube,where the deposits are formed. A thermocouple is inserted through thethird neck of the flask and immersed in the oil for monitoring andcontrolling oil temperature. A second thermocouple is located in theheater. Temperature profiles of the vent tube are determined by meansof a specially instrumented reference tube. The capacity of the oil.reservior flask is 2000 ml. The stainless steel coker tube is 6 incheslong, 0.5-inch on the inside diameter with a 0.049-inch wall thickness.

15. The conditions applicable to the VPC test are as follows:

Bulk Oil Temp. (Sump) 400F !

Tube Heater Temperature Variable

Air Flow 0.027 scfm (dry air)

Time l1 hours

16. More complete details uf the test method and equipment are foundin Appendix A of this report.

DISCUSSION

1. At the initiation of this program, several gas turbine enginelubricants of the neopentyl polyol class were in use by thecommerical airlines and the U.S. Navy. These products representedseveral years of research effort on the part of ester manufacturers andlubricant formulators. Their advantages in bulk oil stability, clean-liness and consumption over the diester lubricants, having beendemonstrated in the laboratory, were confirmed by flight evaluationsShrough rapid and wide acceptance by the airlines and by early Navyoperating experience.

2. Among the products qualified against MIL-L-23699 were severalformulations resulting from various research and development effortsto produce fluids that would meet the performance criteria of thespecification. The basestocks of these products consisted of mono, di,and tri- pentaerythritol esters (hereafter designated PE esters) or mixturesof trimethylol propane and pentaerythritol esters (hereafter designatedTMP/PE esters). Within these basestock categories, oxidation inhibitionwas achieved by several different types of additive packages.

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3. Each of these lubricants was designed to meet the needs of existinggas turbine engines. In laboratory evaluations, as well as in actual service,no large differences in performance characteristics were observed amongthese lubricants. However, since it was anticipated that future militaryengines would impose more severe thermal requirements on these oils, itwas decided to evaluate representative formulations at temperaturesexceeding those applicable for current engines.

Evablation of Current Technology - Phase I Testing

HTD Test

4. The rationale behind this first testing phase was to explore whatpotential for higher temperature operation existed in fluids developedby the current technology and to reveal what new approaches for researchwere indicated. These investigations were concerned with the bulk oil

stability and particularly the deposition characteristics as affected bythe basestock composition of the lubricant. For this purpose, the HighTemperature Deposition (HTD) Tester was chosen as the research tool forevaluating the performance of seven formulations (5 PE and 2 TMP/PEesters). M

5. The lubricants tested are identified b' code ers which categorizethe products by basestock and additive corposition %e.g. PE-I or TMP/PE-5).In the case of the fully blended oils, PE means a basestock consistingof mono, di or tri-pentaerythritol esters. TMP/PE means that trimethylol

propant esters are the dominant basestock components with mono, di ortri pentaerythritol being minor components. The number in the codingidentifies an additive package concert. Note that the codes PE-5 andTMP/PE-5 would indicate the same typ. additive package in differentbasestocks.

6. The tests were conducted at three "control" temperatures i.e. 500,525, and 550OF (see Description Section for test details). The twolower temperature tests were considered representative of the range ofconditions existing in current operating engines. The 550OF testsimulated the estimated environment of future engines.

7. The results of the Phase I testing are plotted on Figures 3 to 7and the data are presented in Appendix B. The bar graphs of Figures 3,4, and 5 show the effects of basestock, additive package and temperatureon the three selected evaluation parameters: deposit formation, viscositychange (A V) and Total Acid Number change (A TAN). The data for the500OF and 5250 F test conditions demonstrate generally how, regardles, ofbasestock or additive package, the performance is not significantly different.

8. It is quite obvious from inspection of the plotted data that themost pronounced effects on all three evaluation parameters are caused byraising the temperature above the range indicative of current engineenvironment, i.e. to 550 0 F. In this environment, bulk oil degradation

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(Figures 4 and 5) becomes significant in most cases, whereas the effecton deposits is not very significant for four of the five PE esters.This observation is especially noteworthy since the main thrust of thisk study was the evaluation of the effects of basestock on depositioncharacteristics. Additive effects on performance are apparent from the fplotted data, but their significance was not tested since the emphasiswas on basestock effects. In comparing the FE and TMP/PE base esters,the assumption was made that equal levels of additive technologyexisted for both type basestocks and that the differences notedbetweenbasestocks (if real) were attributable to the response of the basestockto oxidation inhibition by additives as well as to inherent basestockstability. Therefore, to make comparisons between the basestocks, alldata generated for each basestock was averaged for each test temperature.These mean values also include all repeat data on each formulation(see Appendix B).

9. To further illustrate both the temperature effect and the basestock ARMeffect, the mean values of the basestock data for each evaluationparameter are plotted versus temperature on Figures 6 and 7. From theseplots, it is clear that the basestocks dc not exhibit differences inany performance category at the two lower test temperatures. However,at the 55G°F test condition, deposition formation appears to be widelyvariant between PE and TMP/PE with viscosity and TAN change displayingmuch smaller differences. If these differences are real, i.e. not theresult of chance or experimental error, it could be said that the PEesters are superior to the TMP/PE esters in both deposit formingcharacteristics and bulk oil stability.

10. In order to assess the significhnce of the differences in performancedisplayed at 550°F on Figures 6 and 7, the data were subjected tostatistical analysis which provided a means of distinguishing between realdifferences and differences caused by either experimental error or change.The analysis is included as a part of Appendix B. From the analysis, it wasshown that, in the 550°F HTD test, a) the PE and TMP/PE ester lubricantspossessed different deposit forming tendencies and b) no distinction couldbe made in their relative bulk oil stability characteristics (A viscosityandATAN). Therefore, at this time in the evaluation, it was concludedthat with the existing lubricant formulation technology, the PE esterlubricants were superior to the TMP/PE ester lubricants in depositioncharacteristics. With regard to bulk oil stability, the test methodemployed was not capable of discerning between the performance of the twoester basestocks.

11. During the course of the early investigations under this program,considerable experience had been gained from the use of polyol basedlubricants in Navy engines (references 6 and 7). Inspections of engineshad shown that carbonaceous deposit formation occurred predominantly inthe non-oil-washed areas, i.e. either on surfaces contacted by fine mistsof the lubricant or by condensed vapors in vent lines. Since thedeposition data generated in the HTD tester was on oil washed surfaces, itwas decided that the above findings would be more meaningful if someadditional testing was performed in an apparatus where oil washing was

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not present. The test rig chosen for this phase of the testingwas the Eppi Vapor Phase Coker.

Evaluation of Current Technology - Phase II TestingN Vapor Phase Coker Test

12. The Vapor Phase Coker (VPC) testing was conducted, per theprocedure of Appendix A, at a series of vent control temperatures rangingfrom 550°F to 7500 F. The lubricants tested were the same as those usedin the Phase I evaluations. The criteria for evaluation were theweights of the vent tube deposits which are shown in Figure 8 plottedagainst vent control temperature. Test results for this phase of theinvestigation are included in Table C-I of Appendix C. It should benoted here that the control vent temperatures are irdicative of a selectedtest condition and are not the temperature of the surface on which thedeposits were formed. Actual surface temperatures at the deposit locationscan be determined from the tube temperature profile given in Appendix A.The relationship between actual surface temperatures and deposit formatiorwill be discussed in later sections of this report.

13. Before proceeding with the analysis of the Phase II results, somegeneral observations associated with the interpretation of resultsgenerated during this phase should be made. It can be noted from Figure 8that, as control temperature is increased, deposit formation reachesa peak and then falls off as temperature is further increased. The maximumdeposits were formed at the TOOOF test condition for all lubricants testedexcept TMP/PE-6. However, for this lubricant as well as for each of theothers, the difference in deposit weight between the 650OF and 700OFcontrol temperature was quite small. The key point is the fact that allof the formulations, regardless of basestock composition or additivepackage, formed maximum deposits at approximately the same test condition.

l. Another observation, which was made by employing a glass vent tube,was that deposits are formed primarily from vapors that condense and/orfrom very fine droplets that agglomerate on the upper tube surface andreflux down to the hotter section. This refluxing material then formshard carbonaceous or varnish type deposits as previously observed in the

.metal vent tubes. From these observations it was believed that thereare several competing factors, e.g. surface temperature, volatility andstability of the refluxing material, that affect the formation of deposits.Volatility and surface temperature are the prime factors governing the po

residence time that the refluxed materials remain on the hot surface.The oxidative stability determines quantity and the type of the depositsformed.

15. It was also noted that, for the vent control temperatures used,deposits were always formed in the vent tube at locations of approximatelyequivalent temperatures. As an example, note from photographs of splittubes (Figure 9) that, as the vent control temperature is increased, ' -"

the location of the deposits moves up the tube to the region approximatelycorresponding in temperature to that of the previous test condition.Therefore, it appears that, although the products leaving the sump were

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subjected to different temperatures (in the individual tests) on theirway to the vent tube, the deposits were formed in the same temperatureregime on the tube surface. It is also clear from Figure 9 that therefluxing zone (area above deposits) diminishes as vent controltemperature is increased. With this condition occuring, one mightexpect that reducing the length of the refluxing area would reduce thequantity of material available for deposition. The expected result wouldthen be that as vent control temperature was increased the deposit levels•ould decrease. However, this was not the case as seen from Figure 8.This phenomenon may be explained by theorizing that, since the sunptemperature was the same for all tests, the propensity to form depositswas altered as the partially oxidized esters leaving the sump passedthrough the heater sectaon. Up to the 700°F vent control temperature,the increase in coking propensity outweighed the effect of reduced refluxarea resulting from the shift in the temperature profile of the vent tube.Above 7000 F, although coking propensity of the materials leaving theheater section may still be increasing, the combination of increasedvolatility of these materials and the decreasing reflux area result in alessening of the amount of the deposits formed. It is believed that thispb~Icnmenon represents a realistic approximation of what would occur in

ues since the products which did not condense at the tube exit•ature of 250°F would most likely be vented overboard from an actual

ae and therefore would not be available for forming deposits.

16. Considering all of the above observations, it was concluded thatevaluation of polyol type lubricants at either the 650 or 700OF testcondition provided a realistic means of assessing performance at conditionsmost conducive to deposit formation. Having established a rationalefor the interpretation of vapor phase coker results, the evaluation of the

relative performance of the seven lubricants tested can be discussed.

17. An examination of the plots of deposit weights versus vent controltemperature (Figure 8) supports the findings of the Phase I (HTD) testingthat TMP/PE ester lubricants are more prone to deposit formation than thoseof the PE ester basestocks. This condition is especially apparent as thecontrol temperature is increased as also was the case with HTD testing.At the 650°F and 700°F test conditions, those most conducive to depositformation, lubricants TMP/PE 5 and 6 generated more deposits than anyof the PE esters tested. Since this part of the evaluation was performedto confirm the trends seen in the Phase I testing and since generalagreement was acheived, no statistical analyses into the significance ofthe observed differences were performed.

18. Photographs of the deposit formations in the vent tubes from theVPC testing at 650°F are shown in Figure 10. After w6ighing to determinethe deposit weights, the tubes were split axially for examination of thedeposits. The photographs show a very striking uniformity of appearanceamong the tubes within each basestock category. The PE esters developeda rather heavy (thick) deposit over a very short length of tube whereas

- - the TMP/PE esters generated a thinner layer of deposits over a much longer

J

a

ziý$4

( iNAPTC-PE-71

section of the tube. Considering the temperature profile of the tube(sep Figure 10), the TMP/PE esters produced degradation products thatwere sensitive to deposit formation at lower surface temperatures andover a much broader temperature range than were the PE esters. Th.-observation prompted further consideration into explaining the differencesdetected between the PE and TMP/PE lubricants.

19. The lubricants tested up to this point of the evaluation were allfully blended products containing idditive packages tailored to meetspecific requirements of Specification MIL-L-23699. The effect ofadditives on deposits within a basestock category was shown in the HTDtesting of Phase I, and possibly these additives contribute to thedifferences in performance in the Phase II VPC testing, particularlywithin the PE ester category. Especially noteworthy is the fact that I

in both testing phases the PE-1 lubricant formed considerably moredeposits than any of the other PE products. Whether or not the additiveshad an over-riding effect on either tne quantity or the temperaturesensitive range of deposits between the basestock categories wasa subject iof test Phase III.

20. Another factor to be considered regarding its effect on depositionwas the average carbon atom chain length of the acid component of theester. Since, in order to achieve the desired physical properties ofthis class of lubricant it is necessary to esterify TMP with longerchain acids than is necessary with PE, it was decided to assess thepossibility that acid chain length was a dominant factor affectingdeposit formation. For this purpose, the average deposit weights forthe 650°F and 700F test conditions (from Figure 8) were plotted againstthe averagp acid chain length of each lubricant (Figure 11). As wasanticipated from the previous analysis, a trend of higher deposit levelassociated with the longer acid chain lengths of the TMP esters isquite apparent when considering all the deta points of Figure 11. Thisdata establishes that chain length can be very influential, but it isinconclusive because the two data points most responsible for the

correlation with chain length are also the only data associated withTMP/PE es'.ers. In order to exclude the alcohol basestock component, thePE ester results alone were considered. From inspection of the plotteddata (Figure 11), a slight trend was conceivable, bu ' unclear. Regressionanalysis confirmed (correlation coefficient =0.54) that these datacould not support a chain length effect. However, repeatability of thetest and the small differences among the PE ester besestock average acidchain lengths may have been responsible for the inability to discern thepotential effect of chain length. Additional testing with neat baseesters of specific and varying chain lengths would be required for thedetermination of the relative effects of alcohols, acids and additives.

1

SIM

NAPTC-PE-71

Summary of Assessment of Current Technology

21. The evaluation of several qualified MIL-L-23699B oils in the firsttwo testing phases of this program has produced the following conclusionsconcerning the current technology for this type of lubricant:

a. When subJec'.ed to test conditions considered representative ofcurrent engine environments, lubricant deposition and degradationcharacteristics were independent of either basestocks or additivesemployed in the various formulationp.

b. In higher temperature environments, the PE ester lubricants'deposition characteristics were superior to those of the TMP/PE type,but it could not be determined (from the data available at the time)whether the differences were attributable to either the alcohol or acidcomponents of the basestocks. Aside from the deposit formations beingo quantitatively different between the two basestocks involved, therewere very consisteut and distinct £eatures associated with the particularbasestock compositions. The PE aster lubricants formed deposits over afairly small temperature range and at an average temperature which

• ~exceeded that for the TMP/PE based products. The 1WP/PE esters formeC

deposits over a relatively broad temperature range.

22. It can, therefore, be seen that at "advanced" conditions theproducts developed by current technology exhibited varying performancecharacteristics. It was the cause of these characteristics that became Uthe subject of the remainder of this program. It was believed that theestablishment of the relationships between engineering evaluations of

performance characteristics and lubricant chemistry would benefit andguide formulators in the development of the lubricants required forfuture gas turbine engines.

Evaluation of the Effects of Alcoho) and Acid Composition on Deposition

Characteristics of Esters - Phase III Testing

Vapor Phase Coker Test

23. The assessment of the effects of alcohol and acid components ondeposition included Vapor Phase Coker testing of: a) baseqtocks ofmixed acid esters in which the trimethylol propane (TMP) - pentaerythritol(PE) (moio or di) alcohol ratio was varied and b) single acid estersof DPE, PE (mono) and TMP esters. The mixed acid esters represent thetype of basestock used in MIL-L-23699 oils and the results were used to -Ishow the response of deposit formation to both TM?-PE ratio and averageacid chain length without the presence of additive effects. The sin6leacid esters were used to separate the deposition response with respectto alcohol content and acid chain length.

2h. This testing phase made use of the Vapor Phase Coker at the 650°Ftest condition. Experimentation with test conditions had shown that forevaluating uninhibited basestocks, the 650OF test gave good separationamong the various esters with the deposits forming at tube locationswhere losses of vented products from the top of the tube would not

~ii - -- 13 -

NAPTC-PE-71

F significantly affect results. In addition, a second evaluation

parameter, "yield", was introduced. Yield is the weight of deposits

formed expressed as a percentage of "through-put" (by weight) or the

amount of lubricant that was lost from the sump and, therefore, passedthrough the vent tube. The yield is used in lieu of deposit weightsin order to normalize the data with respect to the quantity of materialavailable for the formation of deposits. This parameter is'particularlyuseful in the evaluation of uninhibited basestocks of single acid esterswhere "through-puts" can vary over a wide range as the acid chain

length is varied.

25. The effect of basestGck components on deposition was studied bypreparing test samples of various ratios of mixed acid esters of

DPE and TMP and subjecting them to Vapor Phase Coker tests at6500F. Test data are included in Appendix C (Table C-II). Depositweights and yield versus DPE-TMP content are shown in Figures 12 and

13 respectively. These plots indicate the same trends observed fromthe results obtained with the inhibited lubricants. The higher theTMP content, the greater was the deposit formation.

26. However, it was also observed that these deposit levels were

considerably lower than those generated by the inhibited lubricants.Note from Figure 8 that both TMP/PE inhibited esters formed depositsin excess of 450 mg at the 650°F test condition, but the maximum depositfor the neat basestock (also at the 650°F test condition ) was about300 mg (Figure 12). The yields likewise showed the same trend. The

average yield for the two TMP/PE inhibited esters of Figure 8 is 5.72percent whereas the maximum yield for the neat TMP/DPE esters is 1.04percent (Figure 13). Of significance is the fact that, although theinhibitors grossly affected "through-put" (see Appendix C) and deposits,the results obtained on the neat basestocks still conform to the general

trends established with the fully blended materials.

27. The assessment of the relative effects of TMP and PE esterscontinuedby conducting VPC tests on a series of blends prepared from(mono) pentaerythritol tetravaleric ester (a single acid ester) and a

trimethylol propane (mixed acid ester). Data from this testing is

given in Table C-III of Appendix C. The yields obtained from these

blends are plotted against PE-TMP content in Figure 13. Again

the trend of increasing deposit level with increasing TMP content is

quite apparent. It was also seen that the mixtures containing DPE

(Figure 13) were somewhat lower in deposit forming tendency than those

containing PE (mono-pentaerythritol).

28. Since the TMP used in the above blends has an average acid chain

length of 7.9 as opposed to 5.0 and 6.2 for the PE and DPE respectively,

one could also predict that the yield would increase with increasingchain length of the blend. This relationship is shown graphically in

Figure 14. Least square regressions line through the data points show

the definite relationship between yield and chain length. As anticipated,

the correlation coefficients of 0.94 and 0.99 for the TMP/PE and TMP/DPE

14 Z

NAPTC-PE-71

blends respectively indicate a high degree of significance associatedwith this data, as was the case with correlation with TMP content(see Figure 13).

29. Photographic records of the deposit formation patterns in thevent tube for both sets of blends are given in Figures 15 and 16.By comparing the deposit distribution of the blends of TMP (mixed acids)and PE (C acid only), Figure 16, to those of the fully inhibited oils,Figure l05 certain relationships between acid composition and depositpatterns are discernable. Valeric acid (CO) is the dominant acid ofthe PE inhibited oils of Figure 10 and the deposits are distributed ina manner comparable to those on the left side of Figure 16 where the PEtetravaleric ester and, therefore, C5 is dominant. As TMP (Cavg.= 7.9)begins to dominate the basestock blends (the right end photographs ofFigurr: 16), the deposit patterns become more like the heavy deposit

areas of the TMP/PE oils (Figure 10) where the C7 and C are the majoracids. The thin deposit layer at the bottom of the TMP 7PE -5 and -6tubes is a characteristic of the additive package and, therefore, shouldbe ignored when comparing to basestock (uninhibited) results. If thesame type of comparison is made between the TMP, LPE blends (Figure 15)and the inhibited oils (Figure 10), the same treads with acid contentare not apparent. However, in the case of the blends containing the DPE,the C5 content was not as high as in those using PE. It should alsobe noted that the pattern of the deposit formation to the left inFigure 15 (high DPE content) is, as will be seen later, dominated bythe DPE deposit chai cteristic.

30. In order to more clearly separate the effects of alcohols and acids,single (normal) acid esters of DPE, PE and TMP with acid chain lengths aof 5, 7 and 9 were obtained. Since iso valeric acid (iC 5 ) is commonlyused in the fully blended oils to achieve certain low tenperatureproperties, it also was obtained for inclusion in this portion of thetesting. Each.of the above esters was then subjected to Vapor PhaseCoker tests Eiz the same conditions used for evaluation of the basestockblends. The resuls of the testing of these esters are reported in . iAppendix C (Table C-IV), and yield is plotted against the acid chainlength in Figure 17. Note from the tables of Appendix C that the estersof iC 5 performed very much like those of the normal C5 (nC5 ) ane. thereforeonly the nC5 results are used in the following analysis. The graphicalpresentation of the results (Figure 17) shows that deposit formation(yield) is very markedly affected by both the alcohol and the acidchain length. An interaction effect between the alcohol and chainlength is also evidenced by the large differences at the C9 chain lengthbetween TMP ester and PE esters compared to small differences at C5.Therefore, at any particular acid chain length, TMP esters form thelargest quarz'ty of deposits with respect to throughput followed by FEand DPE esters. As the chain length is increased to C7 and C9 , which is

15

-______ -. 1.- - -- - --- W r

NAPTC-PE-71

typical of current practice to obtain the desired physical propertiesof TMP base lubricants, the propensity of the TM ester to form harddeposits increases at a greater rate than that of the PE esters.

31. This analysis is based on "yield" which is a measure of thedeposit fcrming tendencies per unit ester material available forconversion to deposits. Therefore, it represents a means of ratingthe relative quantities of deposits formed from a given quantity of theparticular ester made available for conversion to solid products ofdegradation. it may be argued that the yield based analysis is notrepresentative of what would occur in actual engine operation where,depending on b.lk oil stability,various quantities of the particularacid esters would be available for deposit formation. It should beno-Led, however, from the data in Appendix C,that deposit weight per se,wou-d display the same trends as the yields, although the differenceswould not be as great.

32. These findings should not be mistaken as a measure of molecularstability of the esters. An analysis of relative inherent stabilityof the individual esters would entail a comparison of deposits formedwith respect to the molecular weights of the esters involved. Thiswould be necessary since, due to the various molecular structures ofthe esters concerned, molecules of equal stability could generatedifferent quantities of deposition products. Detailed investigationinto the relative resistance to decomposition of particular chemicalspecies is beyond the scope of this program. It is not the stabilityof a particular molecule to resist pyrolysis or oxidation that is ofconcern here, but quantity and type of deposits that are formed fromvarious chemical components as they undergo changes in a particularenvironment.

33. Therefore, as a result of the analysis performed thus far, itcan be deduced that the pentaerythritol esters provide a more favorablebasis than trimiethylol propane esters for the development of lubricantsfor the more hostile environments of future engines. The analyses showa twofold reason for this conclusion - a) the PE and DPE alcohols produceesters which form less deposits than esters of TMP alcohols at equal acidchain lengths and b) PE esters, due to physical property requirements,employ shorter chain length acids which display lower deposit formin.tendencies than the longer chain acids which must be used with theTMP esters. Of course this advantage could be less pronounced ifphysical property requirements were revised.

34. The deposit formation patterns of the single acid esters used forthe above testing are shown in Figure 18. It is quite clear frominspection of the deposits in these tubes and the data of Table C-IVof Appendix C that the pattern of deposit formation is very stronglydependent on the acid component of the ester. Regardless of thealcohol component, the C5 acid ester formed deposits over a shortsection at the lower (higher temperature) end of the tube. As thechain length increased to C7 and then to C9 , the length of the

16 -

NAPTC-PF-71

* ! deposit areas increased and it occurred further .up the tube (at alower temperature) than the C5 deposit area. These observations aredisplayed graphically on Figiires 19 and 20. By careful visualexamination of the deposit formations and by comparison to the tubetemperature profile, the temperature at which deposit build-upwas maximum and the temperature range of deposit formation weredetermined. The temperature at maximum deposit location is plottedagainst acid chain length for TMP and PE in Figure 19. DPE is notinclrded in this plot since deposits were so light that a maximumbuild-up point could not be discerned. The graph sqhows how, as acidchain length increased from C5 to C9 , the temperature at which maximumdeposits formed decreased. It also shows that the TP esters' maximumdeposits occurred on surfaces with temperatures approximately 153 to251F higher than PE esters. Considering the subjectiveness involvedin selecting the area of maximum deposits and possible small shifts intemperature profiles from test to test, the significance of such smalltemperature differences may be questioned. However, a comparison ofthe range over which deposits were formed between PE and TMP for theC7 and C9 acids supports the idea that the TMP esters form deposit'oat higher temperatures than PE esters. AN35. In Figure 20, the least square regression lines for maximum andminimum temperatures at which deposit formation occurred are plotted

SI. against acid chain length for the combined data of the DPE, PE and TMP Iesters. This graph displays the fairly large temperature gradient(135 0 F) over which C9 acid esters form deposits and the small gradient(70°F) over which C5 acid esters are prone to deposition, Extendingthese ranges, via the tube temperature profile, to the tube length over

which deposits form (Figure 20) it can be seen that thc C9 acid ester

deposits are spread over an area 5 times greater than those of C5acid esters. This fact can be interpreted as an advantage for theesters produced with longer acid chains despite the fact that they Nform greater quantities of deposits. Inspection of the deposit

.see Figure 18), especially for the PE and TMP esters,indicates how, at a given time, the smaller quantities of depositsformed with C5 esters could result in a more severe physical ormechanical condition than would result from use of longer acid chainlengths. With both nC5 and iC 5 esters of PE and TMP, the thickdeposit formations are choking off a considerable cross-sectionalarea of the tube, but the C7 and C9 acid esters of the same alcoholshave not yet "grown" out significantly from the surface. Therefore,the shorter acids may be m.re prone to causing such engine problemsas vent line plugging or the formation of crusty deposits (sometimescalled stringers) which easily break loose from the metal surface andmigrate thrcugh the lubrication system.

36. A point in the above analysis that should be emphasized is thatboth the quantitative and qualitative trends indicated by the testingof single acid esters are also seen with mixed acid ester basestocks Jand with fully blended (inhibited) oils. As an example, Figure 21compares the deposit formation patterns of. a) a single C7 acid ester,a mixed (Cavg = 7.5) acid ester and a fully blended oil -ith a mixed

U17

NAPTC-PE-71

(Cavg = 7.5) acid ester basestock and b) a single 05 acid ester anda fully blended oil with a mixed (Cavg = 6.1) acid ester basestock.In the above examples, C7 end C were major components of therespective mixed ester basestocks. Note also that for both examplesthe presence of oxidation inhibitors caused a slight upward shiftin the deposit location. By examining the photographs of the tubedeposits in Figure 21, the similiarities in deposit forming characteristicsthat exist between the single acid esters and the mixed acid esterscontaining acids of the same chain length as the single acid ester canbe seen. The importance of this fact 's that, by evaluating a singleacid esterinferences can be made regarding the effect (on deposition)nT a siitgle acid which is a major component of a maixed acid esterformulation.

Summary of Phase III Findings

37. The results of this phase of the evaluation are summarized asfollows:

a. The quantity of deposits formed is a strong function of thechain length of the acid component cl2 the basestock. Deposits increaseas chain length increases from C5 to CQ. The alcohol component hasa lesser effect on deposit weights witý increases occurring in the orderDPE, PE and TMP. There is an interaction effect between acids andalcohols such that differences in deposits formed between PE and TNPeste-s are greater with C9 acids than with C5 acids.

b. The deposit forming patterns are also strongly dependenton the acid component of the ester. Although quantitatively formingless deprsits, C5 esters form thicker build-ups of deposits over asmall area and temperature range. This area (covering a 0.8-inch lengthof tube, see Figure 20) occurs at temperatures ranging from 400°F to4700F. As acid chain length increases, tChe deposit rrea and temperature

range increase while the average temperature in the deposit areadecreases. For example, C; acid esters form deposits at temperaturesranging from 250°F to 3850F which spans a 4-inch section of the tube.Although lesser in quantity, the deposits formed from the C5 acidssre more concentrated and, therefore, may result in build-ups thatcan lead to problems in shorter times than those formed from the longerchain acids.

c. The evaluation of single acid esters may be used to predictthe deposition characteristics of particular acids when used as majorportions of a mixed acid ester.

d. The results obtained on neat basestocks agree relatively withthe trends established with fully inhibited lubricants.

N1

DEPOSTION ESMRNAPTC-PE-71F'IGURE 1: HIGH TEMPERATURE DEOIINTSE

TEST SECTION SCHEM4ATIC

MAKE UPVENT HEATED CABINET

OIL .E

TC.MAX. TEMP

TRANSFORMER

S OOE NT COOLER L

ANDCOKING HEAD

SUMP

WI TH

RESISTANCE -

HEATED TUBE

AIR

OIL IN

AIR ~T.C.

COOLANT

T'EMPER!ATURE

CONTROL T. C.

PUMP

DRAIN

TO VALVE

PRESSURE

GAGE

19

r NAFTC-?E-71

FIGURE 2: VAPOR PHASE COKERES' SECTION SCHEMATIC

-~ A AIR/VAPOR MIXTURE

HEATER

'iAIR CONTROL T.C.

HEATINGMANTLE

5

tI

INAPTC-PE-71

FIGURE 3: BASESTOCK AND ADDITIVE EFFECTS ON DEPOSITION

Higa Temperature Deposition Test Results

7O

550°F 5250 F 500OF

6o

0 50

40

E-4Ea 30

0 20

4--

101

- -• ,n - ""• 3- - W;• $r- r'Y• . 'T '

NAPTC-PE-71

FIGURE 4: BASESTOCK AND ADDITIVE EFFECTS ON VISCOSITY STABILITY

High Temperature Deposition Test Results

1140

Hf

11N\\I1N\NJ

120 V V V

550OF 5250F 500 OF

~100

, •80 I,

S60

4o4

20ZR

4.

-- "'C- U22

a -

.• NAPTC-PE-71

FIGURE 5: BASESTOCK AND ADDITIVE EFFECTS ON TAN

High Temperature Deposition Test Results

550OF 525 0F 500OF

10

ri

8 8

E-4

22

i '4

o 6~

- --

I' I

NAPTC-PE-71

FIGURE 6: BASESTOCK EFFECT ON DEPOSITION CHARACTERISTICS

High Temperature Deposition Test ResultsDeposit Weight vs Temperature

75% Trimethylol Propane25% Pentaerythritol

Ester Basestock

50

~4o

J1-M

H S 30-

20 -

100% PentaerythritolEster Basestock

10

500 525 550

Test Temperature F

(Lower Tube)

Li2

NN

NAPTC-PE-71

FIGURE 7: BASESTOCK EFFECT ON BULK OIL DEGRADATION CHARACTERISTICS

High Temperature Deposition Test Results

TAN & Viscosity Change vs Temperature

5

4

3 -

22S75%

Trimethylol Propane,-25% Pentaerythritol1 •Ester Basestock

100 0 100% PentaerythritolEster Basestock

80

60S~/

0 0

4 oH

0

H

S20 --A

20

500 I;c ;~

0 I I I

Test Temperature OF

(Lower Tube)

NAPTC-PE-71

FIGURE 8: COKING CHARACTERISTICS IN NON-OIL WASHED AREAS

Vapor Phase Coker Test Results0

E-'

/ I

/N 0D U M

\• 0

$4

N Nn

NN I .:• . I • .~~-1 -, - r

(Y N 0_ r

N 04 1\.. C -4

•,.% _ .26--

N.N

NAPTC-PE-71

FIGURE 9: DEPOSIT FORMATION IN VENT LINE OF VAPORPHASE COKER AT VARIOUS VENT CONTROL

TEMPERATURES

4450F

440cF

500 OF

450• F 495 0 F I

490cF

600°F 650°F 700OF 750°r-

VENT CONTROL TEMPERATURELU . •.i

NAPTC-PE-71FIGURE 10: DEPOSIT FORMATION IN VENT LINE OF VAPOR

PHASE COKER FOR SEVEN MIL-L-23699 LUBRICANTS

0

0 LLIZnL

>La

0.

10

OJO1YA~niHd L283

( Awh

FIGURE 11: DEPOSIT WEIGHTS VERSUS AVERAGE ACID CHAIN LENGTH

Vapor Phase Coker Test Res-sults !

El 500

ll PointsCorrelation Coef. = 0.91 i

00m C 300 -o Q TMP/PE-6

Si •hTMP/PE-5 N.

200 - E,.PE-5"-" /•PE- 5

A PE-I41.S Data onlyCorrelation Coef.= 0.54<> PE-3

100 - PE-2

0 PE-I

SII 1, I

6.0 6.4 6.8 7.2 7.6

AVERAGE NUMBER OF CARBON ATOMS INBASESTOCK ACID COMPONENT

29 ,1

~ NAPTC-PE-71

FIGURE 12: DEPOSITS FORMED BY VARIOUS RATIOS OF DIPENTAERYTHRITOLAND TRIMETH YLOL PROPANE ESTER BASESTOCKS

Vapor Phase Coker Results650OF Vent Control Temperature

300

200-

0~

H

1 00

S0DPE 00 8o 6o o 20

Tmp 0 20 4o 6o 80

BASESTOCK COMPOSITION

30

W-' §4-

NAPTC-PE-71

FIGURE 13: YIELD FROM VARIOUS RATIOS OF PENTAERYTHRITOL ANDTRIMETHYLOL PROPANE ESTER BASES'CKS

Vapor Phase Coker Results

650°F Vent Control Temperature

1.2 0

1.0

0.80-

S~TMP + PECorrelation Coef. 0.92 X

Coer

S 0.60

H0

°a

- TMP + DPE

o.04 Correlation Coef. = 0.98

.150.2

PE or DPE 100 80 60 40 20TMP 0 20 4o 60 80

BASESTOCK COMPOSITION

L •31

* NAPTC-PE-71

FIGURE 1)4: YIELD VERSUS AVERAGE ACID CHAIN LEN~GTH FOR MIXTURES OFPENTAERYTHRfITOL AND TRI1WHYLOL PROPANE ESTER BASESTOCKSf

Vapor Phase Coker Test Results65OOF Vent Control Temperature

'rat

0\0

0- 0~

00

00H

0x rM4 C-0

E-4

W0\

P4

x

00

r4 0

Ssori rIIO x'if/li/s1IoaQ-cIXu

32

NAP TC P E .71

FIGURE 15: DEPOSIT FORMATION IN VENT LINE OF VAPOR PHASECOKER FOR VARIOUS RATIOS OF TMP AND DPE ESTERS

iimIIN,

TMP-MIXED ACID ESTER 0 10 30 50 70

DPE-MIXED (C6 .2 ) ACID 100 90 70 50 30

ESTER-%A

33I

--

T

N APTC- PE -71FIGURE 16: DEPOSIT FORMATION IN VENT LINE OF VAPOR PHASE

COKER FOR VARIOUS RATIOS OF TMP AND PE ESTERS

IA

NI

"I I

j ,

•,(MIXED kCID ESTER)

i PE--% 100 86 72 58 44 30S ! (SINGLE, C3, ACID ESTER)

173

•-

-~ ~ -t IMFNAPTC-PE-71

FIGURE 17: YIELD VERSUS AVERAGE ACID CHAIN LENGTH FOR ESTERSOF PENTAERYTHRITOL AND TRIMETHYLOL PROPANE

3 _

I III _

ITMP

1

o 2

51

pq H0

5 6 7 8 9

AVERAGE NUMBER OF CARBON ATOMSIN BASESTOCK ACID COMPONENTS

35

NAPTC-PE-71

FIGURE 18: DEPOSIT FORMATION IN VENT LINE OF VAPORPHASE COKER FOR VARIOUS ALCOHOL-ACIDCOMBINATIONS OF THE BASE ESTER

OPE PE TMP

007 C

ics fi(5 IC5 a($ ICS ACS

36

Ai - 73:

NAPTC-PE-71

FIGURE 19: TEMERATURE AT LOCATION OF MAXIMUM DEPOSIT DENSITYFOR VARIOUS ACID CHAIN LENGTHS OF THE BASESTOCK ACID COMPONENT

Vapor Phase Coker Test Results650OF Vent Control Temperature

01

TMI'

H oo

S 300 P

FAAS 200

100

5 67 8 9

AVERAGE NUMBER OF CARBON ATOMSIN BASESTOCK ACID COMPONENT

37

NAPTC-PE-7'1

FIGURE 20: DEPOSITION REGION RELATED TO CHAIN LEN~GTHOF THE ACID COMPONENT OF ESTER BASES'1X.CKS

VENT TUBE TEMPERATUR~E OF

0 000

HL

040

H71-o

C14

HH

OE-- 0

0

E-

38

-Ml~i

NAPTC-PE-71

FIGURE 21! DEPOSIT FORMING PATTErlNS AS RELATED

TO ACID CHAIN LENGTHS

g~

TMP

ESTER ESTER INHIBITED OIL|SINGLE ACID) (MIXED ACIDI (MIXED ACID|

C7 Covg -7 5 Coa -7 5

IA,

PE

ESTER INHIBITED OIL(SINGLE ACIDI (MIXED ACIDI

Cs Covg= 6 1

39

7 .. ......

. .

• •.

.

NAPTC-PE-71

REFERENCES

1. AUTHORIZATION - NAVAIR AIRTASK A3303300/052B/4F54543300,Lubricants, June 1973 (Work Unit Plan 637)

2. TEXT - Guenderson, R.C. and Hart, A.W., "Synthetic Lubricants".Reinhold Publishing Company, N.Y., 1962

3. TEXT - Braithewaite, E.R., "Lubrication and Lubricants", ElsevierPublishing Company, 1967

4. REPORT - Bowers, R.C., and Murphy, C.M., "Status of Research onLubricants, Friction, and Wear" Naval Fesearch Laboratory, NRLReport 6466, January 19, 1967

5. SPECIFICATION - "Lubricating Oil,Aircraft Turbine Engine, SyntheticBapse",Experimental Development Specification XAS-2354A, of 8November 1974

6. WORK UNIT ASSIGNMENT - NAPTC-315-5R6-1-72, Qualification of AircraftEngine Lubricating Oil, 28 June 1971

7. WORK UNIT ASSIGNMENT - NAPTC-3B6-5R6-I-13, Analysis of ServicePerformance of Lubricating Oils, 28 June 1971p

IT,

'4

#1

w •40

APPENDIX A NAPTC-PE-?1OPERATING PROCEDURE FOR VAPOR PHASE COKER

INTRODUCTION

The Vapor Phase Coker consists essentially of a three neck flask

(oil reservoir) surrounded by an electric heating mantle, an intermediate"heating" tube surrounded by a coiled rod type heater, and a steel"breather" tube on which the deposits are formed. See Figure A-i

In operation, air is fed through a tube entering one neck of the

flask, bubbled through the oil and permitted to escape through the

center neck of the flask. Upon leaving the flask the vapors pass

through the heater tube and then directly into the coking "breather"

tube, where the deposits are formed. A typical vent tube temperature

profile is shown in Figure A-II.

A thermocouple is inserted through the third neck of the flask andimmersed in the oil for monitoring and controlling oil temperature. A

second thermocouple is located in the heater section.

The capacity of the oil reservoir flask is 2000 ml. The cokertube is 6 inches long, 0.500 inch on the outside diameter with a 0.049

inch wall thickness.

PREPARATION

Coker Tube

The preparation procedure is as follows:

1. Rinse the tube in SD-l solvent and wipe dry wi a lint-freecloth. Washi down with acetone and allow to dry.

2. Place the coker tabe in a clean storage tube fitted with a vented

stopper and labeled with the serial number of the tube.

3. Place the test tube containing the coker tube in a 220°F oven

for ½ hour.

4. After coker tube is cooled to roam temperature weigh to thenearest 0.001 gram. Record the weight on a tube inventory list and

on the storage tube label.

5. Return coker uabe to storage tube fitted with a solid stopperand store until needed.

Tent e t

1. Connect vapor phase coking tube to vent heater assembly.Tighten nut to 250 in-lb.

2. Place insulation around vapor phase coking tube and secure with 'wire. Place draft shield over insulation and vent tube.,I ;

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NAPTC-PE-71

3. Pour 900 ml of test fluid into flask.

h. Weigh flask with test fluid to nearest hundredth of a gram.Record on data sheet.

5. Place flask with test charge in mantle and zip mantle shut.

6. Position mantle and flask on insulation pad and connect mantle

power sources.

7. Insert air-inlet stopper and thermocouple (T/C) in glass stopperinto outside flask necks. Connect air line to air-inlet tube and positiontip of T/C so that it is located approximately one inch from the bottom Alof the flask.

8. Insert end of vent heater assembly with teflon stopper throughheater support and then into center neck of flask.

9. Connect power source to heater terminals.

10. Connect T/C lead from heater section to T/C block.

TEST - STANDARD

SConditionsBulk Oil Temp. (Sump) 400CFTube Heater Temperature VariableAir Flow 0.027 scfm (dry air)Time 17 hours

Test Sequenu.-

1. Adjust air supply to rotometer to 20 psi, then adjust air flowto 50 percent of maximum air flow on cabinet rotometer. (This adjustmentallows an air flow of 0.027 scfm).

2. Turn on cabinet power switch.

3. Adjust vent heater variac to obtain 120 volts.

-4. Set flask (bulk oil) temperature controller to h00°F.

5. When vent temperature reaches approximately 100°F below testtemperature, reduce voltage to approximately 30 volts.

6. Make necessary adjustments to stabilize test conditions.

7. Take and record bulk oil temperature, tube heater temperatureand air flow at one hour intervals.

Al-2

NAPTC-PE-71

8. At the end of 1.7 hours running time, turn off power switch,shut off air flow and allow to cool down.

9. Weigh flask with test fluid to nearest hundredth of a gram.Record on data sheet.

PROCEDURE FOR DETERMINING DEPOSIT WEIGHTS

1. Remove vapor phase coking tube from heater assembly.

2. Remove nut and ferrule from tube.

3. Fill a test tube with stoddard solvent and insert vapor phasecoking tube. Soak for one hour. Remove from test tube and allowsolvent to drain off.

4. Place in clean dry storage tube and put in a 20C)F ov' "or one

hour.

5. When cool, weigh coker tube to nearest milligram.

6. Record gross weight - determine deposit weight by subtract4ngtare weight of tube. Record deposit weight.

7. After review of results have the tube split lengthwise toexamine deposits.

POST TEST CLEANING PROCEDURES

1. Flask

Drain flask into a clean one quart sample bottle. Label bottlewith lube oil code number, test number, and date of test.

Examine empty flask for deposits remaining in flask.

Wash with acid dichromate solution. Rinse with distilledwater. Final rinse with acetone and allow to dry.

2. Heater Section

Wire brush inside of tube with SD-I solvent. Follow with

acetone rinse.

3. Wash air inlet tube and thermocouple with SD-l solvent.Follow with acetone rinse.

Al-3

NAPTC-PE-71

FIGURE A-I: VAPOR PHASE COKERTEST SECTION SCHEMATIC

tAIR/'VAPOiR MIXTUJRE

TEST SECTION DRAFT SHIELD

INSULATION

CONTROL T/C

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APPENDIX B

HIGH TEMPERATURE DEPOSITION TEST RESULTS AND ANALYSIS

High Temperature Deposition (HTD) Tests were run on severallubricants at three temperature levels at the lower section of thedeposition tube. The results of these tests are summarized in fTables B-I, B-II, B-III.

The results of Tables B-I, B-II, B-III are plotted and discussedin the main text. The differences in mean values at 500 and 5250Fbetween PE and TP/PE esters for deposits, viscosity change, and TANchange were considered too small to warrant statistical analysis toexamine for significance. However,the results shcwn in Table B-Ill(for lower tube temperature of 550 0 F) were szatistically analyzedto determine the significance of the differences that are apparentbetween PE and TMP/PE esters with regard to deposits formed, viscositychange and TAN change. The analysis provides a means of distinguishingbetween differences that are real, i.e. attributable to the two base-stocks, or the result of experimental error.

In the computations all factors except basestock variation (PE vsTP/PE) were ignored. Additive effects (e.g. PEI vs PE2 etc.) aswell as the duplication of tests on formulations were treated as con-tributing to experimental error. The analysis simply compared the pmean values of PE vs TMP/PE and orovided the information necessary to ff

make judgements on the reality of observed differences based on theprobability of either experimental error or chance contributing to thedifferences.

The results of the statistical analysis are given in Table D-IV.The method of analysis consisted of applying the "t" test for thecomparison of two randomized groups with unequal numbers of samples. ,.This method is described in detail in reference I of this Appendix.

The first step was to calculate the means and standard deviations pfor each of the two groups (PE and TW/PE) of data for each property(deposits, A viscosity and A TAN). These statistics are given inTable B-IV for each group. Since the computation of "t" is dependentupon the equality or inequality of the standard deviation, the nextstep was to perform an "F" test on the standard deviations in order tomake Judgements concerning their equality between the groups. The"F" values are also shown in Table B-IV. Tne calculated "F" values,when compared to the 5 percent level distribution of "F", (reference 1)show that the standard deviation between both groups for all threeproperties can be considered equal.

The next step was to calculete the "t" values for randomizedgroups having equal standard deviations. These "t" values along witha comparison to values from the distribution of "t" at specificprobability levels are also shown in Table B !V. These results showthat there is only about a 1 in 100 probability that the differencein the means for deposits formed by PE and TMP/PE are caused byexperimental error or chance. They also show that, forA viscosityand A TAN, the differences in the mean values between groups could occurabout 1 out of every 5 times as a result of error or ch&nce.

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NAPTC-PE-71

Therefore, the conclusions drawn from this statistical analysisare that, in the 550OF HTD test, PE ard TMP/PE esters possess differentdeposit forming tendencies, but no distinction can be made in theirability to resist change in viscosity or TAN.

References

1. Text - Snedect , G.W. , "tStatistical Methlods"f; The Iowa StateUniversity Press, Ames, Iowa 1956.

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APPENDIX C

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