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When the going gets tough…
A heavy specialty oil for grease formulations
(Part II)
Mehdi Fathi-Najafi *
Rene Abrahams and Linda Maim
Nynas AB
Sweden
Presented at the NLGI 80th Annual Meeting
Tucson, Arizona, USA
June 15 -18, 2013
Abstract
The global ongoing rationalization of Group I
production and its potential impact on the future
availability of paraffinic bright stock has led several
lubricant formulators to start evaluating alternative
products. A highly viscous base fluid containing residue
from the vacuum distillation process with a kinematic
viscosity of approximately 700 (mm2/s) at 40°C has been
developed.
This base fluid can be used for various applications
where the color is not an issue, since it is black. However,
it is well known that bright stocks are usually used in
combination with solvent neutral 500 or/and naphthenic
oil with the same above-mentioned viscosity for
applications where higher film thickness is required such
as in the mining application.
to substitute this fluid for bright stock in grease
formulations with equivalent or better performance.
Consequently, a more cost effective grease formulation
could be achieved, assuming that color is not an issue.
Introduction
The global ongoing rationalization of Group I
production and its potential impact on the future
availability of paraffinic bright stock has led several
lubricant formulators to start evaluating alternative
products.
The alternative available in the market right now is to
reformulate by using a mix of heavy grade neutrals plus
a synthetic oil component such as polyalphaolefin, heavy
naphthenic and other sources.
This paper suggests a cost-effective alternative which
In order to explore the potential of this heavy specialty
oil as a substitute to bright stock in grease formulations,
several greases based on the different viscosities [235,
370 and 505 (mm2/s) respectively] have been prepared
and characterized. The residue oil based greases showed
interesting characteristics such as having a positive
impact on the thickener content, shear stability, water
resistance and rheological behavior.
However, the overall conclusion is that it is possible
VOLUME
may replace bright stocks in various grease lubricated
applications where the color is not an issue. The first part
of this work was presented by Dr. Valentine Serra-Hoim
at the 79th NLGI annual meeting where some of the
presented results measured by a mini traction machine
showed excellent tribological behavior at various
temperatures. This part of the work has been more
dedicated to a comparative study between a bright stock
and this residue-based naphthenic oil in lithium grease
formulation.
18 78, NUMBER 5
Preparation of the Oils and the Greases
It is well known that for industrial lubricated applications, higher base oil viscosities are required which usually
consist of heavy neutral and bright stock blends. Hence, the starting point for the experimental work was to prepare
some blends based on these two oils.
In order to make the comparison as realistic as possible, two paraffinic oils (BS200 & SN500) and a highly viscous
naphthenic oil (Ri) have been chosen, the characteristics of the base oils can be seen in Table 1.
Characteristics
Viscosity at 40°C (mm2ls)
Viscosity at 100°C (mm2ls)
Viscosityindex
Flash Point, PM (°C)
Pour point, (°C)
Aniline point, (°C)
Density at 15°C (g/dm3)
Method (ASTM)
D 445
D 445
D2270
D 93
D 92
D 611
D 4052
BS200
838.3
43.2
92
275
-6
113
918.9
SN500
95.4
9.6
71
227
-24
98.75
892.4
Ri
690.8
29.2
50
220
-15
90
940
Table 1 shows the characteristics of the base oils.
Preparation of the base oils: Three different viscosities (235, 370 and 505 mm2s-1) have been prepared by diluting
BS200 and Ri with SN500 respectively. Table 2 shows the characteristics of these blends; A, B and C are blends of the
two paraffinic oils (SN500+BS200), while A’, B’ and C’ are blends of paraffinic and naphthenic (SNOO+R1).
Characteristics
Viscosity at 40°C
(mm2/s)
Viscosity at 100°C
(mm2/s)
Viscosity index
Flash Point, PM, (°C)
Pour point, (°C)
Aniline point, (°C)
Density at 15°C, (g/dm3)
Cu corrosion, rating
Method
(AS TM)
D445
Blend A
Blend B
Blend C Blend A’
Blend B’
Blend C’
234.6
372.2
503.6
235.2
369.0
506.6
D445
17.9
24.7
30.4
16.2
20.8
24.8
D2270
D93
D92
D611
D4052
D130
81
229
-12
105.3
907.9
la
86
239
-12
109.9
912.2
lb
88
247
-12
110.4
914.5
lb
60
223
-24
94.3
921.3
la
56
223
-24
92.8
930.5
la
54
223
-15
91.7
936.1
la
Table 2 shows the characteristics of the base oil blends, usedfor production of the greases.
Results ad Discussion
The grease samples have been prepared in an open kettle in which the pre-blends (see Table 2) were used. The
target consistency for these samples was NLGI grade 2. Notable is that the greases used in this study do not contain
any additives.
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19
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NLGI SPOKESMAN, NOVEMBER/DECEMBER 2014
NLGI
Table 3 shows the characteristics of the greases based on the blends in Table2.
Characteristics
Grease A
Thickener content (%)
Penetration (60)
Dropping point (°C)
Cu~corrosion*
6.2
276
208
2c
Grease B
Grease C
6.6
268
206
lb
Grease A’
Grease B’
Grease C’
5.6
272
208
la
5.1
277
202
la
4.8
268
203
la
5.0
274
199
la
Penetration (100,000)
296
291
294
Water wash out (%) **
1.37
1.61
2.00
Water spray off (%)
55.6
53.8
31.2
*) 100 °C/24h; **) 79 °C/lh; ***) 38 °C/5min/4opsi (276 kPa)
275
278
277
2.73
43.8
1.38
N/A
2.12
18.8
~
Based on the measured properties of the greases, shown in Table 3, some interesting behaviors for Ri based greases
can be recognized, such as: lower thickener content, good mechanical stability and superior water spray off.
Tribological measurements
The load carrying capacity of the grease samples was measured by using a four ball machine. As can be seen in Table
4, the wear scar of all 6 grease samples indicates that there is no significant difference between the samples. This is
interesting since Grease A’, B’ and C’ contain lower thickener content as well as lower viscosity index than grease A, B
and C respectively.
Table 4 shows the measured wear scars of the greases.
Remarks
Wear scar (mm)
Grease A
2.73
Grease B
2.71
Grease C
2.77
Grease A’
2.73
Grease B’
2.70
Grease C’
2.67
The condition ofthefour ball test was: 140 kg, 60 sec, 1440 rpm
Rheological measurements
It is well known that lubricating grease is a viscoelastic material, in other words a material with a viscous part (the
base oil) and an elastic part (the thickener). Parameters such as temperature and shear stress affect the oil and the
thickener differently. Hence, flowability of the grease sample under controlled conditions can generate valuable
information such as storage or elastic modulus (G’) of the grease which could be interpreted as the real consistency of
the grease, at the applied temperature.
Strain Sweep: In an attempt to study the rheological behaviour of the greases described above, a strain sweep
program was applied in which the frequency and the temperature were kept constant (10 rad/s and 25 °C respectively),
and the strain was increased logarithmically from 0.01 to 1000%. Figure 2 describes the elastic (storage) modulus
as a function of the strain for B and B’. As it can be seen G’ for B is 13605 (Pa) while it is 878 (Pa) for B’ which is
consistent with the thickener contents which are 6.6% and 4.8% respectively. Notably, they have the same penetration
number after 60 strokes. However, LVR (the linear viscoelastic region) for B’ appears to be bigger than for sample
B which is indeed interesting since it indicates that B’ is a more mechanically stable product than sample B, in
accordance to the worked penetration after 105 strokes (see Table 3).
20
VOLUME 78, NUMBER 5
NLGI
Figure 2 shows the Storage modulus as afunction of Strain for B and B’ at 25 °C
Temperature sweep: The low temperature behaviour of the greases was investigated by using temperature sweep
program, from +50 to -30 °C. Then the change of G’ as a function of temperature was measured. Figure 3 shows the
obtained results for A, B and C and Figure 4 shows those for A’, B’ and C’.
S.,.
‘P’….
N,,
—
I6
•0 —
-00
25
-20
-55
20
-S
0
5
20
20,00.,*5
55
20
25
00
35
40
45
00
Figure 4 shows the behaviour ofstorage
modulus of 5N500+R1 based greases vs.
temperature.
Figure 3 shows the behaviour ofstorage
modulus of SN500+BS200 based
greases vs. temperature.
Figures 3 and 4 reveal some interesting information which could be summarized as: a) the change of the slope for
greases, A, B and C is around -5 °C which can be attributed to the presence of bright stock (BS200) and subsequent
assumed wax formation, despite a lower pour point for all paraffinic blends which is -12 °C. This phenomenon was
discussed in a separate publication [2]. And b) for all paraffinic based greases G’ is significantly higher at -30 °C than
paraffinic!naphthenic based, as a consequence of the impact of the further wax crystallization of the bright stock.
However, it is reasonable to believe that SN500, at least, is not the main source of this increase since both greases
contain SN500.
Differential Scanning Calorimeter measurements
In an attempt to verify the wax crystallization, described above, a differential scanning calorimeter (DSC) was used.
The condition of the test series was; cooling process (from 70 °C to -40 °C) followed by heating process (from -40 °C to
240 °C) with a rate of 5 °C per minute.
Figure 5 describes the data generated for Grease B and Grease B’. As it can be seen Grease B starts to crystalize
at a temperature of about -3.5 °C which is not far from the pour point of the bright stock (BS200) despite the
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NLGI SPOKESMAN, NOVEMBER/DECEMBER 2014
—
measured pour point of the Blend B which is -12 °C (see Table 2). Furthermore,
it was discovered that the starting temperature for the crystallization (transition
temperature) was more or less the same for Grease C (- 3.1 °C) while for Grease A
(where less bright stock was involved) was significantly lower, about -9.5 °C. This
behavior is in fact in-line with the previous published finding [2] where it was e.g.
shown that the wax crystallization (in a paraffinic oil) starts far above its pour point.
—
1500
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IL.
500
0
-so
ióo
ô
i~o
2óo
Temparature (‘C)
Exo L~
Figure~5 shows the thermal behavior of Grease B & B’ respectively.
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Reference
Summary
1. When the going gets tough… A heavy specialty oil
for grease formulations; Serra-Hoim V; 79th NLGI
Annual Meeting, June 2012 (US)
2. Low temperature tribology; Fathi-Najafi M. et.al,
24th ELGI Annual meeting, May 2012 (Germany)
• A highly viscous naphthenic oil, as a substitute
to bright stock, has been evaluated for grease
formulations. Based on the obtained results in this
study, it can be concluded that by substituting bright
stock with Ri:
• Significant reduction in thickener content.
• Better mechanical stability; in spite of lower thickener
content.
• Excellent copper corrosion inhibiting, without any
copper passivator additive.
• Very good water spray off, probably due to higher
adhesivity of Ri
• Similar wear scar as the bright stock based greases
despite lower thickener content and lower viscosity
index.
• Pour low temperature behaviors of Greases that
contain bright stock in comparison to Ri could be
explained by carrying out a multidisciplinary study,
using Rheometer and DSC.
However, the overall conclusion should be that it is
possible to provide a substitute for bright stock in grease
formulations with equivalent or better performance.
Consequently, a more cost effective grease formulation
could be achieved, provided the color is not an issue.
2
23 N1,GI$POKESMAN, NOYEMBERIDECEMBER 2014
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