Abstract
Well-formulated lubricating oils and greases are crucial to the function of modern
vehicles and industrial equipment. Unlike oils, greases are semi-solids. Therefore, at a
given temperature, greases have a greater ability to stay in place and can provide a much
greater film thickness than a lubricating oil. One of the advantages of a grease over
lubricating oil is its ability to adhere to a metal part or bearing. This property is called
adhesivity. A method has been proposed to determine adhesivity based on the probe
tack test (ASTM D2979). This paper focuses on a technique where a metal plate is pulled
away from a greased metal plate. The force required to separate the plates is calculated
from the known mass as measured by an inexpensive spring scale. The results correlated
with other well-known tests such as Cone Penetration (ASTM D2 17), Water Spray-off
(ASTM D4049) and Water Washout (ASTM D1264).
Introduction
Grease is widely used in lubrication applications including roller bearings and low
speed gear systems. A lubricating grease comprises three components: base fluid,
thickener, and performance additives. A grease is composed of liquid and solid
phases. The liquid phase is the base fluid and the solid phase is formed by a network
structure of soap molecules or a dispersion of solid particles such as inorganic clays
or other thickeners. The solid phase thickener can consist of soap molecules with or
without added polymer. The base oil in the grease is immobilized by the soap molecule
network structure, resulting in a semi-solid to solid structure. The base oil solubilizes
performance additives, including polymers.
The function of the thickener is to provide the gel-like network structure. Generally,
the soap thickener is a metallic salt of a long-chain, mono basic, fatty acid, e.g.
lithium 12-hydroxystearate. The soap thickener will form interlocked fibers in grease.
Incorporating polymers into the grease can further enhance the properties of the grease
~
~
~
.~
~ ~
~
such as consistency, shear stability, water resistance, adhesion, tackiness, and soap
yield. Polymers such as polyethylene, polypropylene, polyisobutylene, halogenated
polyethylene, and polymethacrylate are reported to improve the properties of greases. 1,,
Olefin copolymers (OCPs), Styrene-ethylene-butylene (SEBS) and OCP-anhydride
(OCP-A) were studied. Pull-off force was compared to cone penetration, water spray-off,
and water wash-out performance in greases containing polymeric additives. In a lithium
complex grease an additional component or components are added such as azelaic acid,
a dibasic acid. This forms a stiffer, more rigid 3-dimensional structure over that formed
using a monobasic acid.
The structure of the polymer has a significant impact on grease properties including
thickening efficiency and shear stability. The structure of the polymer determines the
overall shape of the molecule. The development of advanced polymerization methods
and catalysts allows polymers with a variety of structures to be synthesized. Figure 1
shows various polymer structures that may be obtained. Linear polymers are those with
repeat units connected in a single long chain. Branched polymers or comb polymers
comprise of structures with a long backbone and multiple side chains. In star polymers
or dendrimers, repeat units are arranged radially. The polymers studied in this paper
have linear or branched structures.
– 30 VOLUME 79, NUMBER 5
Unear
lTITr
*
Branched or Comb
Star
Dendri mer
Figure 1: Ilustration of various polymer structures.
Tackifiers are typically polymeric additives that impart
tack or stringiness to a lubricant. Tack is considered a
composite property; the ability of a material to function
as a tackifier is determined by its cohesive and adhesive
forces, viscosity and other factors such as the molecular
weight and concentration of the polymeric additives used
in the formulation of such additives. Tackifiers have high
cohesive and adhesive forces. High cohesive forces allow
the tackifier to remain together as a single mass while
high adhesive forces cause the tackifier to remain on the
surfaces to be lubricated.
Tack is a composite property and therefore must be
measured indirectly. Multiple tests are necessary to
fully understand how well a polymeric additive such as
a tackifier will perform as a grease additive. The water
spray-off and water washout tests quantify only a portion
of grease performance, the cohesiveness. The addition
of the pull-off test allows an understanding of another
important property of a grease tackifier, the adhesiveness.
A similar method to determine pull-off force in a
lubricant using inexpensive equipment was previously
developed.7
Current Test Methods to Measure Tack
Test methods for measuring tack are generally applied
to the adhesives market which includes pressure
sensitive adhesive tapes and adhesive coatings. Several
organizations provide test standards to the adhesives
market including the American Society of Testing and
Materials (ASTM), the Pressure Sensitive Tape Council
(PSTC), the European Association of the Self-Adhesive
Labelling Industry (FINAT), the British Standards
Institution (BSI) and the Tag and Label Manufactures
Institute (TLMI). Test methods currently used for the
pressure sensitive tape market include probe tack (ASTM
D2979), loop tack (BS EN 1719, TLMI LIB 1/2) and
rolling ball tack (ASTM D3121, BS EN 1721), as well as
tests for double-sided tapes (BS 71 16).8,9
These methods are useful when the adhesive can be
coated onto a solid support or a tape and can then
be placed in contact with a second surface. The force
required to separate the surfaces is then measured and
used as an indication of adhesiveness or tackiness. These
tests are useful for comparing adhesives to one another
but are not suitable for use in the grease industry;
these tests measure pressure sensitive tack rather than
adhesiveness as it relates to adherence of a grease to a
metal part. Cohesion is also an important property that
is generally not assessed using test methods designed
for use in the adhesives industry. Cohesion provides
the string-forming ability of a tackifier solution or a
grease due to the interaction of the individual polymer
molecules.
A grease should be able to adhere to a surface. This
property is determined by factors including internal
cohesiveness, surface adhesion, and tackiness or
formation of thin string -like grease filaments. By varying
the composition of the constituents, the properties
of greases can be adjusted. For example, tackifiers
containing high molecular weight polymers are added to
grease to increase the tackiness or string formation. Most
experimental methods to characterize lubricating greases
with respect to their cohesive and adhesive properties are
empirical in nature.
The cohesiveness of greases has been qualitatively
measured by cone penetrationlO and oil separation
measurements 11 while more recently rheological
measurements provide further insight. 12 Cone
penetration measures the consistency or cohesiveness of
grease by dropping a cone of known mass into a grease
filled trough and recording the penetration depth of the
cone. A low depth of penetration indicates stiff grease.
The adherence of grease to the substrate is characterized
by water spray-off or water washout measurements. 12
31 NLGI SPOKESMAN, NOVEMBER/DECEMBER 2015
In the water spray-off test, a greased metallic surface is
subjected to direct water spray at elevated temperature
and the adherence is determined based on the mass of
grease lost over a certain time period.13, 14 Another
method used for quantifying adherence is by subjecting
greased cylinders to centrifugal forces. Depending on the
mass of grease lost, the adhesion strength is ranked.15
In most cases, failure occurs within the grease, which is
technically cohesive failure.
Approach- retraction experiments are also used to
characterize adherence, cohesion, and tackiness of
greases. A greased substrate is moved towards a ball
attached to a flexible cantilever. On establishing contact,
the greased substrate is moved further until a certain
contact load is achieved. On reaching the target contact
load, the greased surface is retracted from the ball
until complete physical separation occurs. 16 The basic
technique is analogous to pull-off force experiments
done using an atomic force microscope.17 After the
approach-retraction cycle, the deflection force is plotted
as a function of distance.
This paper presents an empirical method using
inexpensive lab equipment to gauge the adhesivity of
greases. It can be used as in in-house method to compare
one grease to another. The results are correlated with
cone penetration, water spray-off and water washout
performance.
Adhesion and Cohesion
Cohesion is determined by the attractive forces
between the molecules of a substance that tends to hold
the substance together. Adhesion is determined by the
_-RCH2_C[i2-)_(CH2_CH)
CH3~’
]
attractive forces between dissimilar molecules and causes
one material to stay in place on another.
Adding a tackifler, such as an ultra-high molecular
weight PIB, at approximately 0.05% wt to a lubricant
package will tend to increase the cohesiveness and
adhesiveness of the lubricant. The cohesive forces within
a tackifler result in the string forming ability that is a
key component of tackiness. Cohesion also drives the
elastic nature of these materials. Greater cohesiveness is
required to keep the lubricant from being squeezed out
from between surfaces.18 Adhesion is also increased
when using a tackifler. Higher adhesiveness is required to
make a lubricant stick to surfaces.
Materials
The structures of the polymers used in this study are
shown below in Figure 2. The base grease was a Lithium
Complex Grade #2 grease with 10% soap content.
No additional additives were present. Polymers were
incorporated into the base grease by mixing in a Hobart
mixer at 90°C. The polymer additives used in this
study were in powder, liquid or pellet form. Powdered
polymers were added and mixed for 3 hours to ensure
complete solubilization of the polymer. Liquid additives,
OCP-A and OCP-P were mixed for 1 hour at 90°C.
Polymer OCP-B is in pellet form and required longer
mixing time and a higher temperature to complete
solubilization. For reference, the base grease was heated
and stirred using the same process as the samples
containing the powdered polymers.
Data for the greases prepared incorporating the
polymer additives is summarized in Table 1.
x
NH
R
Ethylene/propylene
copolymer (OCP)
Ethylene/propylene
copolymer grafted with an
acid anhydride (OCP-A,
OCP-B and OCP-P)
Figure 2: Structures of the polymers used in this study.
– 32 VOLUME 79, NUMBER 5
Styre ne/ethylene/b utylene
copolymer (SEBS)
Experimental Methods
A 36.0 x 76.0 mm piece of sheet steel was attached
to a wooden block. A 3mm deep grease containment
area having the same dimensions was constructed on
another piece of sheet steel. The volume of grease in
the containment area for each test run was constant at
8.2 cm3. A screw-eye was attached to the center of the
wooden block. Figure 3 on the following page shows the
set-up of the test apparatus.
The test grease was added and leveled to the top of the
containment area and the wooden block with a metal
plate equal in area to the grease containment area was
hung from the hand-held spring scale and pulled down
and the block was pressed into full contact with the
grease. The scale was calibrated such that a pull of 450
grams would be placed on the wooden block when the
block and grease were in full contact. When the wooden
bondingsprin
block was released a steady upward pull of 450 grams was
applied to the block, normal to the surface of the grease
until the block began to release. The time for the block to
be completely released from the grease was recorded. This
operation was repeated 5 times for each grease sample
and averaged.
Results and Discussion
The data in Figure 4 shows that pull-off force is inversely
related to cone penetration. As the grease becomes softer
with certain additives it requires less force to remove the
mass from the grease. Without any additives the base
grease exhibits poor water spray-off performance. For
certain additives, SEBS, OCP-A, OCP-B, and OCP-P
improved water spray-off values correlate with a greater
force required to remove the mass from the grease. This
demonstrates that the grease has more adhesivity to the
metal plate. The water washout test measures the
ability of a grease to be removed from a bearing
and is known to not typically correlate well with
the water spray-off test. However, for additives
OCP-A, OCP-B and OCP-P water washout
performance correlates well with water sprayoff performance. It can be understood from the
chemical structures shown in Figure 2 that polar
acid anhydride allows for greater attraction or
adhesivity to the metal surface.
Mechanism
In order to improve the water resistance
of grease, the polymer must form a network
structure. This network can be formed via
physical crosslinking through a crystalline
phase (e.g. semi-crystalline OCP), less soluble
hard phases (e.g. SEBS), by hydrogen bonding
(e.g. anhydride grafted OCP) or by long chain
entanglement.
Wooden Block
Grease Containment Area
Figure 3~ Test apparatus used for the pull-off experiment.
In a simple lithium soap the fiber network
structure formed by the soap thickener is
reversible. In the mixing process, the network
formed by soap thickener is partially dissociated
under heating. After cooling, the soap thickener
resumes its fiber structure in the presence of
the added polymer network. These networks
– 33 NLGI SPOKESMAN, NOVEMBER/DECEMBER 2015
325
300
275
250
225
200
• Pult.off(g)
175
~ Cone Penetration (1/10 mm)
150
A
>( WWO
125
(%)
• Release Time (s)
100
e
75
so
WSO(%)
e
A
25
A
–
0
I
A
-~
A
I
B
I
C
I
0
E
F
Figure 4: Grease test resultsfor samples listed in Table 1. Relative to the base grease, A, all of the polymer
additives improve the water spray-offperformance. However, pull-offforce is generally compromised.
Table 1: Grease performance data
Water Spray-off (%)
Water Wash-out
(%)
Cone Penetration
(‘ho
ASTM D4049
ASIM 01264
ASTM 0217
A
B
Base Grease
+1%OCP
52
20
23.50
10.75
314
317
C
D
+1%SEBS
+4% OCP-A
9
23
11.50
14.25
275
306
E
F
+0.25% OCP-B
-i-4%OCP-P
26
7
25.00
1.75
294
278
interpenetrate as shown in Figure 5. In a complex lithium
grease Lorimor has shown that little grease softening
(dissociation) occurs therefore an interpenetrating
network must form partly by another mechanism.’9
of the interpenetrating network of soap and polymer
fibers and be attached to the metal surface through
hydrogen bonding. The OCP-P polymer is a proprietary
composition that exhibits significantly improved
water washout and water spray-off performance. The
mechanism attributed to its performance is a combination
of multiple interpenetrating networks formed by semicrystalline phase and long chain entanglement linking
sites, as well as hydrogen bonding to the metal surface.
Adhesion to a metal surface is driven by the electron
rich oxygen of the acid anhydride group hydrogen
bonding to the hydroxyl groups generally present
on an iron surface, as shown below in Figure 6. As a
result, the OCP-A and OCP-B polymers will be part
–
VOLUME
mm)
34
79,
–
NUMBER
5
Add polymer
~eatinJ
Mix
(
)
Soap lldckener
Network
network
dissociation
Cool down
______
networks
~4oepthick~ner
1~ olymer
Figure 5: Schematic of the interpenetrating networks formed by the grease soap
thickener and the polymeric additive.
b
Hydrogen
6: Hydrogen bonding leading to adhesion of the anhydride
functionalized polymer network to a metal surface.
Figure
Conclusion
minimal equipment. Potential polymer additives can
be qualitatively evaluated and judgments can be made
about their performance. Based on the results of this
study, a potential polymer additive should have a high
pull-off force and excellent water spray-off performance.
Combined with other tests, such as mechanical stability
and oil separation, a grease additive can be developed
and evaluated more readily.
Relative to the base grease, all of the polymer additives
improve the water spray-off performance. However,
pull-off force is generally compromised. Both the pulloff force and water washout performance of a grease
are dependent on the cohesiveness of the material.
Cohesiveness is partly responsible for the property of a
grease known as tack.
The pull-off and water spray-off tests used in this study
are relatively quick and simple to perform and require
–
35
–
NLGI SPOKESMAN, NOVEMBER/DECEMBER 2015
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Quality Engineering
Services Bac!~ed
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E.
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S
G rea~se Maijufactu ring
and LubeOil Blendin’g
Co nceptu aI~Process,
and Detail Design
140 Corporate Place
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36
VOLUME 79, NUMBER 5
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