NLGI

Enhancing High Temperature
Life Performance
of Lithium-Complex Greases

7

2014

N. K. Pokhriyal*, S.C. Nagar, J. P. Antony, T. P. George,
E. Sayanna and B. Basu
Research & Development Centre, Indian Oil Corporation
Limited, Sector-13, Faridabad

Abstract
Present research work focuses on the study of
enhancement of the high temperature life performance
of Lithium complex greases. Effect of complexing
agents, base oil type and viscosity, conventional and
solid additives, etc. has been studied in detail. Greases
were evaluated through tribological rig FE-9 and
high temperature life rig as per ASTM D 3336. A
novel approach was followed by applying rheology
to study apparent viscosity at subzero temperatures
and comparing rheological data with tribological data
obtained as per ASTM D 1092. An effort was also made
to correlate rig test data with Pressure Differential
Scanning Calorimetry (PDSC) study findings.

Keywords Complex grease, Tn bology,
Rheology, PDSC

Introduction
The NLGI Lubricating Grease Guide [1] describes the
easiest way to understand a Lithium complex grease
is to compare it with Lithium 12-hydroxystearate base
grease. Lithium complex grease has a significantly higher
dropping point (-~50° -100° C) than Lithium grease and
therefore has wide-temperature operability. This feature
is very important as ‘usually’ for every 10° C increase in

working temperature, 50% reduction in grease working
life is reported [2]. Fundamentally the high temperature
life of a lubricating grease depends on the oxidation
stability of base oil at that temperature, type of thickener,
presence of additives at application point and operating
conditions like load, rpm etc. [3]. Out of these factors
‘oxidation of base oil’ is an inherent property of base oil.
Therefore selection of an appropriate type of base oil
can significantly improve the high temperature life of
a lubricating grease. Similarly selection of appropriate
types of thickeners and additives can increase high
temperature life of a lubricating grease to 30 40% when
compared with non-additized grease [4].

A grease researcher faces three-fold challenges
during the development of a lubricating grease, namely
development of grease to meet specifications, simulation
of actual working conditions and application suitability.
To deal with these aspects a fundamental understanding
of grease manufacturing and lubrication mechanism
is required. Author’s laboratory earlier published an
exhaustive research article on the ‘Elevated Temperature
Performance’ of a variety of lubricating greases in rolling
element bearings [5]. In this study it was observed that
high temperature life [FE9-L10 at 140° C] of a Lithium
complex grease was a meager 27 hours compared to a

16VOLUME 78, NUMBER 3

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whooping 225 hours recorded by a Polyurea grease. This
observation motivated the author to develop a Lithium
complex grease to record at least 200 hours of high
temperature life [FE9-Li0 at 1400 C]. The research work
presented in this paper is the study of the development
of a multi-purpose Lithium complex grease having widetemperature operability.
Experimental
Grease Manufacturing
Lithium complex greases were prepared by batch
process. Four different types of complexing agents
were used in preparing Greases A, B, C and D. Greases
were also doped with nearly the same percentage of the
anti-wear, EP and anti-oxidant additives. Standard test
methods such as BIS/ASTM!IP were followed in the
determination of physico-chemical properties of the
greases.
Rheolagical Analysis
Rheometry analysis of lubricating greases was done
using ‘MCR 301 Rheometer’ (M/s Anton Paar). Sample
was taken in a disposable aluminum cylinder. Sample
was cooled! heated to desired test temperature and
was maintained at this temperature for half an hour
before running the experiment. To determine apparent
viscosity, shear rate was varied from 0.1 to 25 sec-i.
Apparent viscosity was also determined by using a rig as
per ASTM D 1092 test method. The temperature profile
runs were carried out in the range of 120° 180° C at
1000 sec-i shear rate. Oscillating Rheometry was applied
to determine the structure stability of greases.

Pressure Differential Scanning Calorimetry (PDSC) Analysis
Oxidation induction time (OIT) of the lubricating
grease at desired test temperature was determined as
per ASTM D 5483 test method using Q2OP Pressure
Differential Scanning Calorimeter from M/s TA
Instruments.

Scanning Electron Microscope (SEM) Study
To investigate the grease soap fiber structure, size and
morphology, SEM studies were performed on the grease
samples using a HITACHI S-3400N scanning electron
microscope at room temperature. Film of a speck of
grease sample was made on a micro-slide by means of a
spatula and this film was rinsed two times with a 70:30
v/v, Hexane/Toluene mixture to remove the oil from
the soap fiber matrix. The slide was then dried with air
blower and kept at ambient temperature for 24 hours in
a dust free chamber. Film was then mounted on a carbon
tape and was sputter coated with gold film and SEMs
were recorded.
High Temperature Life Analysis
High temperature life of the greases was determined by
following ASTM D 3336 test rig and FAG FE9 test rig.
Detailed description of FAG FE9 test rig and method of
determination of grease life is given in reference [5].

Results & Discussion
Basic Studies
Both organic (Benzoic acid, Adipic acid etc.) and
inorganic (Chlorides, Carbonates etc.) compounds
are widely used as complexing agents for Lithium
greases. Studies on the correlation between the type of
complexing agent used and the properties of resulting
greases have been well documented [6, 7 & 8]. It is
the structure of the resulting co-crystailised salt that
determines the oil retention, additive transportation and
rheological properties of the resulting grease. Detailed
discussion of these factors with respect to a variety of
the complexing agents has been reported [91. Properties
of greases prepared with different complexing agents are
given in Table 1. SEM micrographs depicting the micro
structures of the complex salts for Grease A, B, C and D
are given as Figures 1, 2, 3 & 4 respectively.

– 17 NLGI SPOKESMAN, JULY/AUGUST 2014

NLGI
S. No.
I.
2.
3.
4.
5.
6.

Property
NLGI grade
Penetration (P60)
Dropping point, (°C)
Oil separation, %
Ll0 life, hours
OIT, 180° C, mm

Grease A
Grease B
2
2
270
279
>270
260
1.5
1.4
34.28
21.18
17.1 at2IO°C 97.6

Grease C
2
290
>260
1.8
27.63
106.7

Grease D
2
278
258
2.2
30.76
112.5

Grease D
It can be noted from Table-i that Grease A had overall
best properties. SEM micrograph of Grease A showed
a sheet-like cross-linked structure indicating more oil
retention capacity. Micro-structure of Grease D was
thin and fibrous in nature indicating thickener network.
Micro-structures for Grease B and C were in between
these two extremes. Highest dropping point, OTT and
L10 life of Grease A was related to its strong cross-linked
micro structure. Grease A was selected for further
research work.

Rheometry Analysis
Grease A was modified with a variety of lubricity
additives for improvement in the high temperature
life. Three more greases namely Grease Al (Ester 1),
A2 (Ester 2) and A3 (Diester) were prepared. To begin
with a temperature profile of greases AT, A2 and A3
was done at 1000 Sec-i shear rate. Shear rate was kept
high to simulate the actual application conditions and
eliminate the effects of sample history. Results of the
temperature profile run are given in Figure 5. High
Temperature life at 177° C (ASTM D 3336) for Grease

– 18 VOLUME 78, NUMBER 3

I

NLGI
samples Al, A2 & A3 were found to be 131, 85 and 80
hours respectively. Keeping in view the ASTM D 3336
data and temperature sweep it can be argued that Ester 1
had better compatibility with the thickener structure
of Grease A. A structural change was also seen during
the temperature range 135° — 160° C for Grease Al
indicating re-structuring of a sheet-like fiber network
to accommodate more oil. To understand this better,
an amplitude sweep run (Strain from 0.1 to 100 %, w
= 25 rad/sec) was done at 177° C on Grease A and Al.
The results are given in Figure 6. It can be seen that
storage modulus (G’) which is indicative of the solid-like
behavior of samples became nearly half for Grease Al
when compared with Grease A. A clear indication of the
structure transformation with Ester 1 which made the
grease sample more flexible and fluid like. Grease Al was
selected for study of extreme pressure additives.

Figure 6 Amplitude sweep for lubricating greases
A&Al at 177°C

6000

Pa

•-GreaseA
•-GreaseAl

5~000

4~5O0
4000

t

3~500
3 000
2500

100
Strain7

I.

quick assessing tool for it requires only half an hour’s
time for one experiment. Rheogram in Figure 7 shows
low temperature shear stability of Grease Al at (-)54°
C. Apparent viscosity at 25sec-1 by ASTM D 1092
was 1120 Pa.S whereas by Rheometry it was 1025
Pa.S. This data is encouraging as it falls well within the
reproducibility limits mentioned in ASTM D 1092.
However, more greases are to be tested at different
subzero and ambient temperatures at different shear
rates to increase the confidence interval in results

Figure 5 — Temperature sweep for lubricating greases
A,Al,A2 &A3

Figure 7 — Dependence of viscosity of~
Grease A I on shear rate at (-) 54° C
10

Apparent Viscosity by ASTM 0
1092at-54°C 1120 Pa.S

Pas

10

~

Ti

Keeping in view ‘wide temperature operability’ criteria
for the development, low temperature behavior of
Grease Al was assessed through ASTM D 1092 test
method and Rheometry. ASTM D 1092 test method
is used to estimate the pressure-drop or required pipe
diameters in a distribution system for grease to flow.
However, the method takes nearly 2-3 days time for
one sample testing say at (-)54° C. Rheometry can be a

IU

2

5

10
ShearRate~,

19

NLGI SPOKESMAN, JULY/AUGUST 2014

15
~

20

us

25

NLGI
interpretation obtained using rheometry.

Enhancing High Temperature Lire
First step taken to enhance the high temperature
life was prevention of the oxidation of grease at test
temperature. Three types of anti-oxidants were doped in
equal dosage in Grease Al and Grease A1AO (Aminic
antioxidant), Grease A1PO (Phenolic antioxidant)
and Grease A1APO (Amine Phosphate antioxidant).
Around 3 psi drop in oxygen gas pressure was observed
for all greases as per ASTM D 942. However, PDSC data
at 200°C clearly distinguished between the type of anti
oxidant and its efficacy. OTT was maximum for aminic
type of antioxidant.
Grease A1AO was selected for study of extreme
pressure and solid additives. Three greases were
prepared, namely Grease A1AOZ (Zinc chemistry
EP additive), Grease A1AOP (Phosphorus chemistry
EP additive) and Grease A1AOZS (Zinc with solid
additive). FAG FE 9 test rig were employed for study

Table 2

Generally, when solid additives are added in a liquid
dispersion medium, an increase in G’ or system
viscosity is observed with increasing shear rate.
Interestingly, when solid additive is doped in Grease
A1AOZ the resulting sample behaved more like a fluid
(lower G’ values) indicating a flexible nature of the
thickener fiber-network. Similar observations for solid
additives like Graphite, MoS2 and PTFE have been
reported earlier for Lithium greases [10].

QIT values of greases doped with different types of anti-oxidants

IS. No. Property (Grease AIAO
I I.
I OlT, minutes 153
Figure 8

of the high temperature life. Greases were also studied
using oscillating Rheometry (strain sweep from 0.01 to
100% equivalent to shear rate variation from 0.001 to
1.0 Sec-i ) for structural strength at 177° C. Rheogram
is shown as Figure 8 and data obtained for high
temperature life from FAG FE 9 test rig at 140° C are
given in inset.

Grease AIPO Grease Al APO

35

I 44

Amplitude sweep for Greases A IAOZ, A IAOP and A IAOZS at I 77° C
4000
Pa
3 500

Grease AIAOZ— 172 hours
GreaseAIAOP— 121 hours
Grease A IAOZS 215 hours

t

A Grease A IAOZ
D Grease A IAOP
0 Grease A IAOZS

G’

0
Strain7
– 20 VOLUME 78, NUMBER 3

NLGI
References
Conclusions
• A multi-purpose Lithium complex grease with
appropriately chosen complexing agent, lubricity
additive, anti-oxidant and solid additive developed
and resulted in more than 200 hours high temperature
life as determined using FAG FE 9 test rig.
• Complexing agent has a great effect on the thickener
fiber structure as seen through SEM and rheological
analysis.
• Addition of esters as lubricity enhancing additive
greatly alters the structure of the Lithium complex
grease.
• Rheometry is a very useful tool in determination
of the flow properties of the greases and structural
determination. It was successfully used to correlate
apparent viscosity data obtained as per ASTM D 1092
test method at subzero temperatures.
• Aminic anti-oxidant had better compatibility with
complexing agent and lubricity additive. PDSC
successfully distinguished efficacy of various types of
the anti-oxidants.
• Use of solid additive with Zinc type extreme pressure
additive significantly enhanced high temperature life
of the Lithium complex grease.

Acknowledgements
Authors are thankful to the management of JO CL,
R&D Centre, Faridabad, India for granting permission to
submit and present paper in NLGI —India Chapter’s 15th
Lubricating grease conference, Kovalam, India. Authors
also thank Tribology Department of IOCL, R&D Centre
for testing tribological properties of greases.

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Grease Institute, Fourth Edition, 1996.
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Possibilities and Limitations:’ Lubrication
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3. Hosoya, S. and Hayano, M., “Deterioration of
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– 21 NLGI SPOKESMAN, JULY/AUGUST 2014