Redesigning alkylated diphenylamine antioxidants for modern lubricants

Copyright © 2006 John Wiley & Sons, Ltd.

Redesigning alkylated diphenylamine antioxidants for modern lubricants

Vincent J. Gatto*,†, Hassan Y. Elnagar, William E. Moehle and Emily R. Schneller

Albemarle Corporation, Baton Rouge, LA, USA

SUMMARY

This paper describes a new alkylation technology that is very effective at synthesizing structurally different alkylated diphenylamine chemistries relative to those currently available to lubricant formulators. Examples areprovided showing how this technology can be used in a practical way to produce a variety of chemically modi-fied nonylated diphenylamine types. One example is also provided illustrating the preparation of a higher molec-ular weight dodecylated diphenylamine. Engine oil performance data utilizing pressurized differential scanningcalorimetry (PDSC), the thermo-oxidation engine oil simulation test, and a bulk oil oxidation test demonstratethat subtle changes in alkylated diphenylamine chemical composition can result in significant oxidation anddeposit control performance enhancements or losses. Examples of similar performance responses in industrialoils are demonstrated utilizing PDSC and the rotating pressure vessel oxidation test. These performance changesare directly related to specific structural changes in the modified products. Copyright © 2006 John Wiley & Sons,Ltd.

1. INTRODUCTION

Alkylated diphenylamine antioxidants have been used for many years to suppress the oxidation ofindustrial and engine oils. Their use in recent years has grown significantly due to the lubricant indus-tries’ need to reduce emissions and supply extended drain lubricants. In engine oils the mandatedreductions in ZDDP have resulted in the use of higher hindered phenolic and alkylated diphenylamineantioxidant treat levels. On the industrial oil side, the improved oxidation resistance of basestocks hasled to the development of longer life turbine oils, which have often required the use of higher levelsof antioxidants. These shifts in lubrication technology have changed the way industrial and engine oilsare formulated.

Despite the recent changes in basestock chemistry and lubricant specifications, very little changehas occurred in the types of antioxidants available to improve the oxidation stability of modern lubri-cants. The reasons for this relate to the cost associated with the development and commercializationof a new antioxidant component. Totally new antioxidant chemistries usually require a significant

LUBRICATION SCIENCELubrication Science 2007; 19: 25–40Published online 31 August 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/ls.28

*Correspondence to: Vincent J. Gatto, Albemarle Corporation, Baton Rouge, LA, USA.†E-mail: [email protected]

capital investment for manufacturing, regulatory approvals, engine testing and field testing. The sumof these costs rarely justifies the development of a new component. As a result, lubricant developershave generally relied on a limited slate of alkylated diphenylamines and hindered phenolics to solvetheir formulation challenges and commercialize new products.

A challenge exists in bridging between new antioxidant components and traditional establishedchemistries. One option is to devise an evolutionary approach where existing chemistries are modi-fied slightly to provide incremental improvements in performance and cost. The benefits of thisapproach are that capital investments and risk are managed while still solving formulation challenges.This type of component development approach, involving subtle changes to alkylated diphenylaminechemistries, will be presented in this paper. It involves making small changes in alkylated dipheny-lamine antioxidant chemical composition by utilizing a novel and highly selective alkylation catalyst.The structurally modified products are then tested for performance benefits in commonly used indus-trial and engine oil oxidation and deposit bench tests.

2. ANTIOXIDANT SELECTION

2.1. Alkylated Diphenylamine Chemistry and Selection

Structure activity studies comparing the antioxidant performance of highly purified alkylated dipheny-lamines have been reported in the literature.1 These studies, while useful towards understanding therelationship between chemistry and performance, are not practical for a real world situation wherechemistry is constrained by manufacturing process and cost. Our approach for selection of alkylateddiphenylamines involved utilizing a new manufacturing technology to produce alkylated dipheny-lamines of varied chemical composition. This concept is illustrated in Figure 1 for the reaction ofdiphenylamine with an olefin. This reaction can produce a variety of alkylated diphenylamine prod-ucts. Alkylation can occur ortho or para to the nitrogen atom. Furthermore, mono-alkylation, di-alkylation or tri-alkylation can occur. These two classes of chemical transformations lead to the selection of products shown in Figure 1.

Utilizing a novel and highly reactive Lewis acid alkylation catalyst, a number of alkylated dipheny-lamines derived from diisobutylene (C8), nonenes (C9), and propylene tetramer (C12) were synthe-sized. Reaction conditions were varied to produce subtle changes in chemical composition. Gaschromatography (GC) was used to characterize the individual chemical components in each productproduced. Examples of typical GC traces for nonylated diphenylamines are shown in Figure 2. TheGC method used has been reported previously for the characterization of nonylated diphenylamines.2

Note that GC provides reasonable resolution of the various mono-, di- and tri-nonylated diphenylaminecomponents as well as some information regarding isomeric ortho- and para-alkylation. This type ofchemical characterization has been carried out on all the synthesized samples with the results summarized in Table I.

The nonylated diphenylamine identified as reference C9 represents a commercially availableproduct. The remaining materials were all synthesized using the novel alkylation catalyst. The sampleidentified as high para C9 contains approximately 9% more di- and para-alkylated material comparedto the reference. The sample identified as high tri-alkylated C9 contains approximately 14.6% moretri-alkylated material than the reference. Finally, for the C9 series, the sample identified as high monoC9 contains approximately 35.8% more mono-alkylated material than the reference.

26 V. J. GATTO ET AL.

Copyright © 2006 John Wiley & Sons, Ltd. Lubrication Science 2007; 19: 25–40DOI: 10.1002/ls

For comparative purposes, samples of C8 (di C8 and high mono C8) and C12 (tetramer C12) alky-lated diphenylamines were also prepared. The composition of these materials is also presented in TableI. Note that ortho- versus para-alkylation could not be resolved in the C12 sample. Also note that theC8 products showed no evidence of ortho-alkylation.

2.2. Hindered Phenolics

It is well known that combinations of hindered phenolics with alkylated diphenylamines provide synergistic oxidation and deposit control in a variety of lubricants. As such, it was important tocompare the oxidative stability of lubricants formulated using combinations of alkylated dipheny-lamines and hindered phenolics. The hindered phenolics selected for this comparison are shown in Figure 3. A detailed base oil oxidation study evaluating this type of synergy has been reported previously.3

2.3. Sulfurized Antioxidant

Sulfur-containing antioxidants are commonly used to enhance the oxidative stability of engine andindustrial oils. It has also been shown that certain combinations of alkylated diphenylamines and sulfurcompounds can provide performance benefits in oxidation bench and engine tests.4 In this study methylenebis (di-n-butyl-dithiocarbamate) (MBDTC), shown in Figure 3, was chosen as a sulfur-containing antioxidant.

ALKYLATED DIPHENYLAMINE ANTIOXIDANTS 27


HNR

HNR

HNR

R''

R''

HN

HNR

R'

HN

R'

Olefin, Catalyst

mono-alkylated diphenylamines

di-alkylated diphenylamines

tri-alkylated diphenylamine

para-substituted ortho-substituted

all para-substituted ortho-para substituted

R'mixed ortho-para substituted

Figure 1. Alkylated diphenylamine process and chemistry.



Figure 2. GC analysis of nonylated diphenylamines (reference C9 and high para C9 examples).

Table I. Alkylated diphenylamine composition determined by gas chromatography.

ADPA Mono- and Mono- and Di- and Di and Tri-alkylatedortho-alkylated para-alkylated ortho-alkylated para-alkylated

Reference C9 2.0 16.1 7.5 67.5 6.2High para C9 0.1 15.7 1.4 76.5 5.4High tri-alkylated C9 1.0 17.6 8.9 50.1 20.8High mono C9 0.3 53.6 1.2 43.4 0.7Tetramer C12 Not resolved 21.3 Not resolved 66.7 11.9Di C8 Not present 1.5 Not present 97.5 1.0High mono C8 Not present 52.6 Not present 30.2 10.0

3. LUBRICANT PREPARATION

Model low phosphorus engine oils were blended having the following composition:

Group II basestock (150N) 88.3wt. %Succinimide dispersant 4.8wt. %Overbased calcium detergent 1.8wt. %Neutral calcium detergent 0.5wt. %Secondary ZDDP 0.6wt. %Test antioxidants and diluent 4.0wt. %

The finished oils contained 2400ppm calcium, 470ppm phosphorus, 520ppm zinc, and had a totalbase number of 7.5mg KOH/g oil. The test antioxidants were composed of various combinations ofthe alkylated diphenylamines, hindered phenolics and MBDTC. The diluent used was a Group II 150Nbasestock. Actual antioxidant formulations and treat levels are reported on the individual figures.

Model rust- and oxidation-inhibited industrial (R&O) oils were blended using Groups I, II, III andIV basestocks having the following composition:

Basestock 99.35wt. %Rust inhibitor 0.10wt. %Corrosion inhibitor 0.05wt. %Alkylated diphenylamine 0.50wt. %

4. OXIDATION STABILITY TESTING

4.1. Pressurized Differential Scanning Calorimetry

Pressurized differential scanning calorimetry (PDSC) was performed according to the ASTM D 6186test method. Details of the test’s utility and operation can be found in the literature.5 For engine oils



HOH2C OH

MBDTBP

HO CH2CH2COC8H17

O

HPE

OH

DTBP

HOH2C

HO

H2C OH

MBBP

[

]x

x = 0 - 5

CH2SC

S

N

2

MBDTC

[ ]

Figure 3. Hindered phenolic and sulfurized antioxidant structures.

the isothermal temperature was held at 180°C. For R&O oils the temperature was held at 170°C. Alltesting was performed in duplicate and average numbers are reported. Results for the engine oils arepresented in Figures 4–6. Results for the R&O oils are presented in Figure 7. Error bars in the figuresrepresent the 95% confidence intervals using pooled standard deviations. An increase in oxidationonset time corresponds to an increase in oxidation stability of the lubricant.



0

10

20

30

40

50

60

70

80

90

100

di C8 high monoC8

high paraC9

high tri-alkylate

C9

tetramerC12

referenceC9

high monoC9

ADPA (@ 1.5 wt. %)

On

set

Tim

e (m

in)

95 % Confidence Interval (pooled s) = +/- 4.0 minutes

Figure 4. PDSC evaluation of engine oils containing structurally modified alkylated diphenylamines.

0

10

20

30

40

50

60

70

80

90

100

HPE/reference C9

HPE/ highpara C9

HPE/ highmono C9

HPE/ high tri-alkylate C9

HPE/ tetramerC12

MBDTBP/reference C9

AOX (0.75 wt % phenolic /0.75 wt % ADPA)

On

set

Tim

e (m

in)


Figure 5. PDSC evaluation of engine oils containing combinations of hindered phenolics with the structurallymodified alkylated diphenylamines.



0

10

20

30

40

50

60

70

80

90

100

reference 2xreference

.75% HPE/reference

.75%MBDTBP/reference

.75%DTBP/

reference

.75%MBBP/

reference

.5%MBDTC/

reference

AOX (reference C9 @ 0.75 wt. %)

On

set

Tim

e (m

in)


Figure 6. PDSC evaluation of engine oils containing combinations of various antioxidants with the reference C9 ADPA.

0

10

20

30

40

50

60

70

80

90

100

high mono C9 high tri-alkylateC9

tetramer C12 reference C9 high para C9

ADPA Type (@ 0.5 wt. %)

On

set

Tim

e (m

in)

Group IIGroup IIIGroup IV

Figure 7. PDSC evaluation of R&O oils containing structurally modified alkylated diphenylamines.

4.2. Albemarle Bulk Oil Test

Albemarle has developed a bulk oil oxidation test (AlBOT) for monitoring the oxidation and thick-ening of fully formulated engine oils. The test involves passing pure and dry oxygen at 10L/h through300mL of a test lubricant held at 150°C. Oxidations were performed in the presence of 110ppm ofsoluble iron catalyst derived from iron naphthenate.

Samples were periodically removed from the test and analyzed for kinematic viscosity. Percent vis-cosity increase versus the fresh oil was calculated and plotted as a function of time. Group II engineoil results are presented in Figures 8, 10 and 12. An increase in viscosity over time corresponds to anincrease in the rate of lubricant polymerization and oxidation.

Fourier transform infrared (FTIR) spectroscopy measurements were also carried out on the abovesamples using the carbonyl peak area increase (PAI) method reported by Obiols.6 A potassium bromidecell path length of 0.037mm was used. Oxidation was measured by calculating the carbonyl area(C¨O) between 1650cm−1 and 1820cm−1 in the FTIR spectra. These results are presented in Figures9, 11 and 13. The carbonyl area is representative of the quantity of all products containing a carbonylfunction that are formed during the oxidation process (aldehydes, ketones, carboxylic acids, esters, oranhydrides). Therefore, an increase in area represents an increase in oxidation.

The data presented in Figures 12 and 13 represent single test results, while the data in Figures 8–11represent the average from duplicate test results.

4.3. Thermo-Oxidation Engine Oil Simulation Test

The thermo-oxidation engine oil simulation test (TEOST) was performed according to the ASTM D7907 test method. Details of the test’s utility and operation can be found in the literature.7 Total



0

100

200

300

400

500

600

20 30 40 50 60 70 80 90 100

Time (hours)

% V

isc.

Inc.

1.5 wt % reference C91.5 wt % high tri-alkylate C91.5 wt % tetramer C121.5 wt % high mono C91.5 wt % high para C9

Figure 8. AlBOT viscosity results for engine oils containing structurally modified alkylated diphenylamines.



0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80 90 100

Time (hours)

PA

I

1.5 wt % reference C91.5 wt % high para C91.5 wt % high trialkylate C91.5 wt % tetramer C121.5 wt % high mono C9

Figure 9. AlBOT PAI results for engine oils containing structurally modified alkylated diphenylamines.

0

100

200

300

400

500

600

20 30 40 50 60 70 80 90 100

Time (hours)

% V

isc.

Inc.

reference C9 and HPEhigh para C9 and HPEhigh tri-alkylate C9 and HPEtetramer C12 and HPEhigh mono C9 and HPEreference C9 and MBDTBP

0.75 wt % ADPA / 0.75 wt % phenolic

Figure 10. AlBOT viscosity results for engine oils containing combinations of hindered phenolics with thestructurally modified alkylated diphenylamines.



0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80 90 100

Time (hours)

PA

I

reference C9 and HPEhigh para C9 and HPEhigh tri-alkylate C9 and HPEtetramer C12 and HPEhigh mono C9 and HPEreference C9 and MBDTBP

0.75 wt % ADPA / 0.75 wt % phenolic

Figure 11. AlBOT PAI results for engine oils containing combinations of hindered phenolics with thestructurally modified alkylated diphenylamines.

0

100

200

300

400

500

600

20 30 40 50 60 70 80 90 100

Time (hours)

% V

isc.

Inc.

reference2x reference.75 % HPE/ reference.75 % MBDTBP/ reference.75 % DTBP/ reference.75 % MBBP/ reference.50 % MBDTC/ reference

reference @ 0.75 wt %

Figure 12. AlBOT viscosity results for engine oils containing combinations of various antioxidants with the reference C9 ADPA.

rod deposits were determined by the increase in depositor rod weight and reported in mil-ligrams. Recovered used oil and volatiles from the test were collected and analyzed by FTIR, but inthis case the method of Obiols was modified to report a parameter called total carbonyl oxidation,where:

Total carbonyl oxidation = [Volatiles carbonyl peak area × weight (g)] + [Used oil carbonyl peak area × weight (g)].

It was felt that PAI values of TEOST recovered oils might be somewhat misleading since signifi-cant volatiles are generated in the test and these volatiles are physically separated from the recoveredoil. This approach for analyzing TEOST recovered oils has been reported previously.8

Results for deposit formation in the Group II engine oils are reported in Figures 14 and 15, whileresults for total carbonyl oxidation are reported in Figures 16 and 17. Highlighted areas in the barcharts represent the 95% confidence intervals using pooled standard deviation. The values reported inthe bar charts represent the average deposit results, while the values above each bar indicate the numberof TEOST tests performed on each lubricant.

4.4. Rotary Pressure Vessel Oxidation Test

The rotary pressure vessel oxidation test (RPVOT) was performed according to the ASTM D 2272test method. This test is performed at 150°C using a copper coil oxidation catalyst and 50 g of testfluid. All testing was performed in duplicate. Results for the R&O oils in Group I, II, III, and IV base-stock are presented in Figure 18. An increase in oil life corresponds to improved oxidation stabilityof the lubricant.



0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80 90 100

Time (hours)

PA

I

reference2x reference.75 % HPE/ reference.75 % MBDTBP/ reference.75 % DTBP/ reference.75 % MBBP/ reference.50 % MBDTC/ reference

reference @ 0.75 wt %

Figure 13. AlBOT PAI results for engine oils containing combinations of various antioxidants with the reference C9 ADPA.



30

35

40

45

50

55

60

65

reference C9 high para C9 high tri-alkylate C9

tetramer C12 high monoC9

di C8

ADPA Type (@ 1.5 wt %)

To

tal R

od

Dep

osi

ts (

mg

)

41.2

55.6

50.6

54.8

38.2

48.9

4

3

2

2

2

2

Figure 14. TEOST deposits for engine oils containing structurally modified alkylated diphenylamines.

30

35

40

45

50

55

reference C9 reference C9 andHPE (1:1)

high para C9 high para C9 and HPE(1:1)

AOX System (@ 1.5 wt %)

To

tal R

od

Dp

osi

ts (

mg

)

4

3

3

2

48.9

41.2

39.7

45.4

Figure 15. TEOST deposit results for engine oils comparing reference C9 with high para C9.

5. RESULTS AND DISCUSSION

5.1. PDSC Oxidation Induction Times

Figure 4 provides the oxidation onset time results for the series of structurally modified alkylateddiphenylamines in a low phosphorus Group II engine oil. The results show that the high mono C8 and

high para C9 perform similar to the reference C9, the di C8 and high mono C9 perform better thanthe reference C9, and the tetramer C12 and high tri-alkylate C9 perform worse than the reference C9.

Figure 5 compares mixed antioxidant systems composed of the various alkylated diphenylaminesand hindered phenolics. Onset times are much shorter than those in Figure 4. In addition, much less



300

350

400

450

500

550

reference C9 high para C9 high tri-alkylate C9

tetramer C12 high monoC9

di C8

ADPA Type (@ 1.5 wt %)

To

tal C

arb

on

yl O

xid

atio

n

358

345

374

350

486

426

43

2

2

2

2

Figure 16. TEOST total carbonyl oxidation for engine oils containing structurally modified alkylated diphenylamines.

300

320

340

360

380

400

420

440

460

480

500

reference C9 reference C9 andHPE (1:1)

high para C9 high para C9 andHPE (1:1)

AOX System (@ 1.5 wt %)

To

tal C

arb

on

yl O

xid

atio

n

4

3

3

2

358

345

425438

Figure 17. TEOST total carbonyl oxidation for engine oils comparing reference C9 with high para C9.

differentiation is seen between the various alkylated diphenylamines. In these formulations high paraC9 and tetramer C12 are equivalent to the reference C9, and high mono C9 and tri-alkylate C9 performworse than the reference. Also noteworthy is that HPE and MBDTBP perform equivalently in theseformulations.

In Figure 6 the reference C9 is formulated with a variety of different antioxidant types. All the phe-nolic/reference C9 mixed antioxidant systems give similar onset time results, while the reference C9alone performs better than the mixed systems. This type of PDSC response has been reported previ-ously.9 An exception is seen with the mixed reference C9/MBDTC system where a much longer onset time is observed. Again, this antioxidant effect between alkylated diphenylamines and sulfurcompounds has been reported previously.4

PDSC results for the R&O oils are presented in Figure 7. In general, the Group III and IV fluidsperformed about the same, and significantly outperformed the Group II formulations. In this class ofoils the high mono C9 diphenylamine consistently outperformed all the other systems tested.

5.2. AlBOT Viscosity Increase and PAI Values

Figure 8 provides the viscosity increase results for the series of structurally modified alkylateddiphenylamines in a low phosphorus Group II engine oil. In this test the reference C9 and high paraC9 showed equivalent performance, the high mono C9 performed significantly better than the reference C9, and the tetramer C12 and high tri-alkylate C9 performed significantly worse than thereference C9. These trends are also confirmed in the PAI results presented in Figure 9.

Alkylated diphenylamine formulations containing hindered phenolics are shown in Figures 10 and11. In this case the reference C9 shows improved performance versus the high para C9, tri-alkylateC9 and tetramer C12, and is less effective than the high mono C9. It is also evident that the MBDTBP



0

200

400

600

800

1000

1200

1400

referenceC9

high paraC9

referenceC9

high paraC9

referenceC9

high paraC9

referenceC9

high paraC9

ADPA (@ 0.5 wt %) / Basestock System

RP

VO

T L

ife

(min

)

Group I Group II Group III Group IV

Figure 18. RPVOT evaluation of R&O oils comparing reference C9 and high para C9 alkylateddiphenylamines in various basestocks.

hindered phenolic performs significantly better than the HPE. When compared to Figure 8 it is seenthat the mixed phenolic/alkylated diphenylamine systems are less effective then the alkylated dipheny-lamines alone. The PAI results in Figure 11 show less differentiation but still highlight the benefits ofMBDTBP over HPE, and high mono C9 over the tri-alkylate C9.

The use of the reference C9 in combination with a variety of different antioxidant types is shownin Figures 12 and 13. These trends are similar to those seen in the PDSC results. The reference C9shows improved performance over the mixed phenolic/reference C9 systems. The data support equiv-alent performance between the reference C9, MBDTBP/reference C9 and MBBP/reference C9systems. The DTBP/reference C9 and HPE/reference C9 perform about the same as the reduced levelof reference C9. Finally, the performance of the MBDTC/reference C9 mixed system far exceeds thatof all the other antioxidant systems. These trends are confirmed by the PAI results in Figure 13 andfurther support the PDSC results and findings in the literature.4

5.3. TEOST Deposit and Oxidation Values

Figure 14 presents TEOST deposit results for the structurally modified alkylated diphenylamines. Thehigh para C9 and di C8 appear to outperform all the other alkylated diphenylamine types. Table I indicates that these two materials contain the highest levels of para- and di-alkylation. This suggeststhat products containing high para- and di-alkylation are more effective deposit control additives inthe TEOST. This is different from what was observed in the PDSC and AlBOT data where high monoC9 showed the best performance for viscosity control and oxidation. These results suggest that a mono-alkylated diphenylamine may be the best choice for viscosity control and oxidation, while thedi-alkylated diphenylamine may be the best choice for deposit control.

Figure 15 presents additional deposit results comparing reference C9 with high para C9 in the pres-ence of a mixed antioxidant system. Of interest is the performance reversal when shifting from thealkylated diphenylamine to the mixed systems. This suggests that performance rankings are impactednot only by alkylated diphenylamine type, but also by the overall antioxidant system type.

Figure 16 presents total carbonyl oxidation for the structurally modified alkylated diphenylamines.These results show very high oxidation for the system containing high mono C9, and are consistentwith the high deposit results presented in Figure 14. The higher than expected oxidation for di C8 isless consistent with the deposit results and illustrates that trends observed for TEOST deposits maynot always translate to TEOST oxidation.

Figure 17 presents additional total carbonyl oxidation results comparing reference C9 with highpara C9 in the presence of a mixed antioxidant system. Clearly in this comparison the two alkylateddiphenylamine types are equivalent, and the mixed antioxidant systems show higher oxidation.

5.4. RPVOT Oil Life Results

Much of the data presented thus far shows a possible performance advantage with high para C9 versusthe reference C9. In order to test this further, a series of RPVOT tests were performed comparing highpara C9 and reference C9 in the model R&O oils. These results are presented in Figure 18. A signif-icant basestock effect is seen in this test, with Group III and IV basestocks showing considerablyhigher oxidation stability compared to Group I and II basestocks. This type of response is now wellestablished in the technical literature.1,9 The only observed performance difference occurred in theGroup III comparison where high para C9 shows a significant improvement over the reference C9.



This difference represents a 12% improvement in response when high para C9 is utilized. These resultsconfirm the sensitivity of antioxidant performance and ranking to chemical structure and basestocktype.

6. CONCLUSIONS

This paper has demonstrated that subtle changes in alkylated diphenylamine chemical structure canproduce significant performance differences in oxidation bench tests. The work has demonstrated ageneral trend where mono-alkylated diphenylamines are the most effective in PDSC and bulk oxida-tion tests where oxidation onset times and viscosity control are the key parameters. For situationswhere deposit control is important, such as the TEOST, then products high in para- and di-alkylationhave the structure of choice. Data were also presented showing that higher molecular weight and tri-alkylated diphenylamines are less effective on an equal weight basis than the other structures examined. This is very reasonable as these structures contain less of the active diphenylamine moietythan the other lower molecular weight materials. Overall, the high mono C9 was most effective forincreasing oxidation onset time and controlling viscosity, while high para C9 and di C8 were mosteffective in controlling deposits.

While the structure–activity relationships discussed above are important, some of the data clearlyshow that the use of antioxidant combinations can be more important in producing high performancelubricants compared to what can be attained using only higher performing structurally modified prod-ucts. This is especially the case when alkylated diphenylamines are used with MBDTBP or MBDTC.Thus incremental improvements in antioxidant performance resulting from subtle structural modifi-cations of existing antioxidants, when coupled with a creative use of synergisms between differentantioxidant chemistries, may represent the most cost-effective way of meeting the challenges createdby the higher performance demanded of future lubricants.

REFERENCES

1. Gatto VJ, Grina MA, Tat TL, Ryan HT. The influence of chemical structure on the physical properties and antioxidantresponse of hydrocracked base stocks and poly-alpha-olefins. Journal of Synthetic Lubrication 2002; 19(1):3–18.

2. Aebli BM, Evans S, Gati S. Nonylated diphenylamines. US Patent 6,315,925, 2001.3. Gatto VJ, Grina MA. Effects of base oil type, oxidation test conditions and phenolic antioxidant structure on the detection

and magnitude of hindered phenolic/diphenylamine synergism. Lubr. Eng. 1999; 55(1):11–20.4. White WR, Reale J. Low ash, low phosphorus motor oil formulations. US Patent 4,330,420, 1982.5. Hsu SM, Cummings AL, Clark BD. Evaluation of automotive crankcase lubricants by differential scanning calorimetry. SAE

Tech. Paper No. 821252, SAE, Warrendale, PA, 1982.6. Obiols J. Lubricant oxidation monitoring using FTIR analysis — Application to the development of a laboratory bulk

oxidation test and to in-service oil evaluation. JSAE Tech. Paper No. 20030124, JSAE, Tokyo, Japan, 2003.7. Selby TW, Florkowski DW. The development of the TEOST MHT bench test of engine oil piston deposit tendency. In

Proceedings of the 12th International Colloquium on Tribology, TAE, Ostfildern, Germany, Supplement, 2000, pp. 55–62.8. Gatto VJ, Moehle WE, Schneller ER, Cobb TW. Utilizing the TEOST MHT to evaluate fundamental oxidation processes in

low phosphorus engine oils. 60th STLE Annual Meeting, May 15–19, 2005, Las Vegas, NV.9. Migdal CA. The influence of hindered phenolic and aromatic amine antioxidants on the stability of base oils. 213th ACS

Annual Meeting, April 13–17, 1997, San Francisco.



Documents

Redesigning alkylated diphenylamine antioxidants for modern lubricants