Charact ii csof SP-based
Lithi
lex Grease
•
.
.
-..
• NLGIAuthor Award
Development
—I
–
:
Govind Khemchandani, Ph.D.
The Dow Chemical Company
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Introduction
Most lubricating greases are made from petroleum
oils. However, for some applications, better performance
in one or more areas is required, justifying the use of
more costly lubricating fluid. Thus, a variety of fluids
have become available, each unique but all categorized
as synthetic oils. Greases have been made from all of
these oils, the thickeners being selected, in most cases,
from the ones commonly available. Among the common
synthetic oils worth mentioning are polyalkylene glycols
(PAGs), organic esters, polyalpha olefins (PAOs) and
alkylated aromatics [1].
Most greases consist of three basic parts: a base stock,
a thickener and chemical additives. There has been little
evolution with two of these parts. A 2011 survey by the
U.S. based National Lubricating Grease Institute indicates
that more than 90 percent of the grease consumed in
the world is formulated with conventional mineral oil
base stocks rather than synthetics. The survey also found
that more than 75 percent of greases are formulated
with lithium soap-based thickeners or lithium complex
thickeners, both of which were first patented in the
1940s. Much of the grease in use is based on a 60-yearold concept. Without change in base stocks or thickeners,
avenues for performance advances become limited [2].
However, a new class of oil soluble PAGs has recently
been introduced in the lube industry [3] [4]. The oil
soluble PAGs are based on downstream derivatives of
butylene oxide as one of the precursors. The use of higher
alkylene oxides, such as butylene oxide, increases the
ratio of carbon to oxygen in the PAG, which boosts oil
solubility~ This innovation is considered a step to change
a “base stock” in a grease formulation. New oil soluble
polyalkylene glycols (OSPs) have high viscosity index,
better friction properties and excellent solvency power as
indicated by very low aniline points. These characteristics
make OSPs a good candidate for grease formulations.
An OSP-based lithium complex grease (OSP grease) has
been prepared and some of its properties are discussed in
this paper.
Objective
Conventional PAGs are used for making greases with
inorganic and organic thickeners for specific applications
[5]. These conventional PAG based greases fall under
the category of “specialty greases” due to conventional
PAGs being water soluble and water insoluble types. This
restricts PAGs’ use as a base fluid of choice for 3general
purpose grease industries applications in the automotive,
steel and construction industry. A case in point is PAG
fluids for brake caliper applications. Conventional PAG
is the fluid of choice for this application since EPDM
(ethylene propylene diene monomer) is used in the
assembly.
Oil solubility characteristics of OSP fluids give grease
makers the option to blend them with mineral oils or
synthetic fluids to achieve additional characteristics
of lubricity and high temperature deposit elimination
which is inherent to PAG chemistry. Oil soluble PAGs
thus open the door for making widely used automotive
(GC-LB) grease similar to current greases based on
mineral oils and PAO. The main focus of the project was
to determine if a suitable lithium complex grease can be
made meeting GC-LB specifications as outlined in the
ASTM D4950 standard [6]. In the components of grease,
– 30 VOLUME 78, NUMBER 4
base oil accounts for about 75-95 percent, meaning that
the performance of a grease is largely determined by
the characteristics of base oil. Therefore, grease cannot
be any better than its base oil [7]. Consequently, before
selecting any base oil, its characteristics should be
examined in detail before making a grease for its desired
application. Table 1 describes the physical properties of
the range of OSP base oils.
Table 1: New Oil Soluble PAGs-Typical Properties
,
~STM
D445
ASTM
b445
‘ASTM
D2270,
OSP-18
OSP-32
OSP-46
•SP-68
osP-150
OSP-220
ASTM
D52’93
ASTM
D97
‘.ASTM
&92.
‘
ASTM,
D61 1-01
1750
2900
5400
17100
29100
OSP-32 0
OSP-460
OSP-680
Important Features of OSPs
Deposit and Varnish Control
OSPs are excellent in controlling deposit formation and sludge generation [8]. Every third atom in the polymer is
oxygen, imparting polarity and solvating capability. Figure 1 clearly demonstrates the non varnishing characteristics of
OSPs and how they contribute high temperature performance compared to mineral oils and synthetic hydrocarbons.
Figure 1: Modified ASTM D 2893B Extended Test, Inclusion ofan OSP improves deposit control
Mineral Oil, 50 days
Mineral Oil ÷10% OSP, 70 days
– 31 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
NLGI
A recent study by Quantitative Spectrophotometric
Analysis (QSA) test method [9] demonstrated reduction
in varnish by increasing the solvency power of API Group
II base oils. QSA measurements were performed on the
used fluid neat, with OSP added at treat levels of 10 and
20 percent. The samples were allowed to sit for 24 hours
after the OSP was added. Varnish potential ratings (VPR)
for a typical group II using turbine oil containing 20
percent OSP dropped from 90 percent to 10 percent. Even
the addition of 10 percent OSP produced a measurable
reduction in the VPR of the samples.
Tribological properties
The tribo- profile of OSPs was determined by a Mini
Traction Machine (MTM) test and exhibited low
traction coefficients across a wide slide to roll ratio range
compared to mineral oils [101. Recently, pressure viscosity
coefficients (PVC) data of oil-soluble polyalkylene glycols,
high oleic sunflower oil and their 50/50 (wt. %) blends
were investigated [11]. The PVC values calculated from
the literature data following this procedure showed
reasonable agreement with that from film thickness data
obtained from this work. This confirms the oil solubility
of OSPs even in vegetable oils and film forming ability
of OSPs. Optimum film thickness is required for better
protection of bearing surfaces.
Solvency Provider
As discussed in many published papers, synthetic fluids
like PAOs and other mineral oils have lower solvating
power for additives with the exception being naphthenic
base oils [12]. Switching from a Group Ito Group II
base oil has resulted in lower grease yields and additive
solubility is now an issue. This has led to the use of
naphthenic oils in combination with Group II base oils as
solvency providers. Alkylated naphthalene base stocks can
also be used to improve additive solubility in PAO [13]. It
is obvious from Table 1 that OSPs have very low aniline
points and should provide the best solubility for additives,
potentially higher yields and a uniform grease structure.
All of the base fluids mentioned above can improve their
solvency by using OSPs as a solvency booster component
in grease formulation. Such an effect of OSPs in reducing
aniline points of naphthenic oil and PAO is shown in
Figure 2.
Figure 2: Aniline Point Improvement- Naphthenic Oil (NO) &PAO by OSP 46
NLGI
OSP Lithium Complex Grease Formulation and Performance Tests
The first step in making an OSP-based lithium complex grease was the selection of OSP- 220 as the base oil.
Its important characteristics are listed in Table 1. 12-Hydroxystearic acid, azelaic acid, and lithium hydroxide
monohydrate were reacted in OSP fluid at the required temperatures well known in the art. The base grease was
additized with the commercial add-pack containing a combination of typical zinc/sulfur/phosphorous additives. It also
contained additional aminic/phenolic antioxidants. This finished NLGI 2 grade with 9 percent soap content was then
tested against all the GC-LB requirements. The results are shown in Table 2. The grease was also tested for prolonged
working W1OK, PDSC, and copper corrosion. The test results are shown in Table 3.
Table 2: Results of GC-LB Testing on Automotive OSP Lithium Complex Grease
Property
Worked Penetration, mm/I 0
Method
ASTM
D 217
NLGI Grade
Dropping Point °CASTM
Oil Separation, % wt. ASTM
Rust Protection, rating, max.
ASTM
Water Washout, 80°C, %
Fretting Wear Protection,
weight loss,
mg
Four Ball Wear, scar diameter,
mm
Elastomer Compatibility, AMS
32l7/3B
CR Type, 70 Hrs© 100°C
Volume Change, %
Hardness Change (Durometer
A Points)
Leakage Tendencies, g
Low Temp Torque, -40 °C
Four Ball EP, Weld Load, Kg
Load wearlndex
High Temperature Lire, Hours
Result
275
GC-LB Specs
220-340
2
303
0.9 I
Pass, No Corrosion
Mm. 220
Max. 6
Pass
D 1264
ASTM
D 4 I 70
3.2
2.3
Max. IS
Max. 10
ASTM
D2266
0.46
Max.0.60
ASTM
D4289
+5.71
-5
0 to +40
-IStoO
ASTM
D 4290
ASTM
D 4693
ASTM
D2596
0.7
Max. I Og
3.21 (See graph)
Max. 15.5 NM
315
60
Mm. 200 Kg
Min.3OKg
80
80
D 2265
D 6184
D I 743
ASTM
D 3527
Soap Content: 9.0 %
OSP-220: VI 196
– 33 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
5
NLGI
F
‘a
20
Tin…
~mpIo : ~-V12111.1
L~bV$ VIlJ~uf1U M~xImum Iorqu~
—
20
3540
n…:..y,.In
45
50
55
T..i~( ~
40 ‘C Dat,,: 06/14/I I
60 S~aurid Ri~adiri~j – 2.40 N in
~ 21 N-fl,
Figure 3: Low Temperature Torque per ASTM D 4693
Shear Stability, A, WI Ok
ASTM D2 I 7
4.7
No significant change
PDSC, 180 C, Minutes
ASTM D 5483
Graph attached
Copper Corrosion
ASTM D 4048
IA
Pass
Table 3: Results ofAdditional Testing on Automotive OSP Lithium Complex Grease
~s~npi.
.51 ~.
)tct~iod~.
C
DOX~ CI~MI~AL
X—0127i1–1.
ID Sc:D
A~Th D—~4O~3 ~00d~qC
LABffi1O6O7L0
Op~o~
Ri.m Ds.~e:
Jo1~ W.
29—lnl—J..1
11:04
Is
10-
0•
~4i
—
0w,~o~ I.,d~s~tio~ T~.
10
20
0~0
Ti~
40
(mi~1~
=
~
~1O
G=a~=.~.j. V4..LC Di..Pen~ 2200
Figure 4: Oxidation Induction Time per ASTM D 5483
NLGI
Results and Discussions
Conclusions
Currently, the high temperature life of a grease is
estimated using the ASTM D 3527 test method. The
disadvantage of this method is its poor test precision, long
endurance testing time and questionable correlation with
field vehicle performance under operating conditions
[14]. Subsequently, ASTM D 5483 was developed [15] and
became well known for measuring oxidation induction
time (OTT) for high temperature stability of greases [16]
[17]. In the present investigation, the ASTM D 3527 test
was still carried out to comply with GC-LB specifications.
Although PDSC ASTM D 5483 is not a requirement for
GC-LB specifications, the author wanted to have this
baseline data to indicate oxidation stability of the grease
at 180°C. The test conditions of the method and the
OTT curve Figure 3 is displayed per ASTM D 5483. OTT
provides an indication of the grease’s thermo oxidative
stability and is influenced by several parameters including
temperature, grease composition and anti-oxidant
additives [18]. Shear stability of the developed grease was
measured by worked penetration 10K strokes per ASTM
D 217. Test results show that lithium complex grease
can be made with OSPs as a base fluid using common
manufacturing methods for lithium complex greases.
An appropriate commercial adpack can be used to meet
NLGI automotive wheel bearing GC-LB requirements.
Higher drop points greater than 300°C can be achieved
easily with the use of OSP. It has good shear stability,
oxidation stability and high drop point with a favorable
low soap content of nine percent [19].
It has been shown that it is relatively straightforward
to manufacture and formulate an OSP Lithium complex
grease for automotive wheel bearings that meets GC-LB
performance levels. Formulators can now use oil soluble
PAGs as a primary base oil or a co-base oil in grease
formulations. OSPs offer options to upgrade hydrocarbon
oils to boost solvating power for additives and soaps
during grease manufacturing. The introduction of OSPs
to the grease market expands the restricted application
of conventional PAG based specialty greases to a wide
variety of requirements which include general purpose
greases and automotive greases.
I
– 35 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
NLGI
6. References
1. Lubricating Grease Guide National Lubricating
Grease Institute, Mo, pp 14-15
2. Tim Sullivan, Averse to Change, Lubes ~ Grease,
Europe-Middle East-Africa August 2012, pp 20-24
3. Neil Canter, New Type of Polyalkylene Glycol,
Tribology and Lubrication Technology, October
2010, ppiliO-11
4. Govii~d Khemchandani, Characteristics of New Oil
Soluble Poly~lkylene Glycols, STLE Houston Chapter,
February 9, 2011
5. US Patent WS 2011/0160110 Al, Lubricating Grease
Composition, June 30, 2011
6. ASTM D 4950-08, Standard Classification and
Specification of Automotive Service Greases, ASTM
International, West Conshohockn, PA
7. Luo Robin et al, A Study of Composition and
Technology of Complex Lithium Grease NLGI,
October 26-29, 1997
8. Martin Greaves et al, New Oil Soluble Polyalkylene
Glycols, STLE, Las Vegas, May 19, 2010
9. Gene Wagenseller, Reducing the Varnish Tendency of
a Group II Base Oil by Increasing the Solvency, STLE,
May 6-10,2012, St. Louis, Missouri
10. Govind Khemchandani, ~New Oil Soluble
Polyalkylene Glycol for Making High Performance
Grease, NLGI Volume 76, Number 2, PP 36-4 1
11. Grigor B. Bantchev et al, Film-Forming Properties of
Blends of High-Oleic Sunflower Oil with Polyallcyl
Glycol, J Am Oil Chem Soc (2012) 89:2227~2235
12. Valentine 5, and Luis Bastardo-Zambrano, The
changes in the global base oil market and their
potential impact on the grease industry, NLGI
Spokesman, Vol.74, 2010
,
,
,
13. Sandra Mazzo-Skalski, Synesstic Alkylated
Naphthalene Base Stocks for Incidental Food
Contact, Tribology and Lubrication Technology,
pp.44-46, November 2010
14. In-Sik Rhee, “Developing an Accleerated Endurance
Test for Grease A Status Report’~ BRDEC Technical
Report, 1987.
15. In-Sik Rhee, 37KH Development of a New Oxidation
Stability Test Method for Greases Using a Pressure
Differential Scanning Calorimetry (PDSC),’ NLGI
Spokesman, Vol. 55, pp 123-132, 1991.
16. M. J. Pohlen et al, 3DSC-a Valuable Tool for the
Grease Laboratory,’ N L G I, October, 1997
17. P. C. Hamblin, S. Laemlin, P. Rohrbac, J. Reyes
Gavilan and D. Zschech, 3Evaluation of the Thermo
Oxidative Characteristics of Greases by Pressurized
Differential Scanning Calorimetry,’ Euro Grease,
September/October, 2004
18. Jisheng E. et al, 3Comparison Between PDSC and
Oxygen Bomb Test Methods for Evaluation of
Grease Oxidation Stability,’ Euro G rease, October
2006
19. Richard E. Rush, 3A Review of the More Common
Stand~d Grease Tests in Use Today,’ Journal of the
Soci~ty of Tribologists and Lubrication Engineers,
March 1997, pp18-26
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– 36 VOLUME 78, NUMBER 4
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