Note: Descriptions are shown in the official language in which they were submitted.
CA 02266841 1999-03-24
LUBRICATING COMPOSITIONS
The present invention relates to the use of friction modifiers to reduce
micropitting of metal surfaces such as gear teeth, and to lubricant
compositions
comprising friction modifiers.
Micropitting is a type of surface damage which occurs predominantly in rolling-
sliding contacts of hard steel surfaces. Sometimes called "frosting",
"greystaining" or
"peeling" it typically occurs in rolling element bearings and most often on
gear teeth,
where it poses a significant practical problem. Micropitting may lead to
higher noise, to
significant rapid wear and to more serious surface damage, such as scuffing
and even to
tooth fracture in gears. Conventional lubricants are used to reduce friction
when metal
surfaces move in contact with each other but they do not prevent the
occurrence of
micropitting. Original equipment manufacturers require lubricants which can
lead to a
reduction in the amount of micropitting when compared with the conventional
lubricants.
It is an object of the present invention to meet this need.
The awareness of micropitting within the lubricant additives industry has
increased significantly. A micropitting test has been established by the FZG
Institute in
Germany and is called the FZG micropitting test. This test is run on gears
sets with the
same metallurgy and surface profile/roughness as gears used in the field. The
conditions
2 0 of the test (high load/low speed) are the optimum conditions for
micropitting to occur.
Equipment manufacturers believe that the FZG micropitting test correlates well
with field
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CA 02266841 1999-03-24
experience.
The FZG micropitting test is carried out using a standardized FZG test rig
according to CEC L-07-A-71, with C type case hardened gears of minimum 0.4 Ra
surface roughness. The test has a stepwise phase to investigate build up of
micropitting
and an endurance phase to investigate resistance to micropitting. The stepwise
phase runs
from load stage 5 to load stage 10, each stage lasting 16 hours. The profile
of the gears is
measured prior to testing and during the test. The variation from the original
gear profile
(the profile deviation) is calculated. Also evaluated are the percentage
micropitting (the
percentage of gear tooth which is micropitted) and the weight loss from the
gears. After
the stepwise phase the endurance phase is run for 80 hours at load stage 8 and
then at load
stage 10 until failure. Again, the deviation from the original profile
(maximum 20
microns), the level of micropitting and the weight loss are measured. A result
which
would be particularly acceptable to the industry would be a pass at load stage
10 in the
stepwise phase of the test. This corresponds to a profile deviation of less
than 7.5 m,
micropitting of less than 15% (approx) and weight loss of less than 15 mg
(approx) after
load stage 10. Extended performance in the endurance phase is also desirable.
The present invention is based on the surprising appreciation that certain
friction
modifiers may be included in lubricant compositions with the result that an
improvement
in micropitting performance is observed when the lubricant compositions are
used, i.e.
2 0 there is reduced micropitting. Accordingly, the present invention concerns
the use of at
least one friction modifier to reduce micropitting of a metal surface, which
comprises
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lubricating the metal surface with a lubricant composition comprising the at
least one
friction modifier, wherein the at least one friction modifier is selected such
that
micropitting is reduced when the metal surface is so lubricated.
The metal surface may be the surface of a gear tooth, in which case the at
least
one friction modifier may be added to a formulated gear lubricant composition.
In the present specification the term "friction modifier" is used to describe
additive compounds which are conventionally used in lubricant compositions to
reduce
friction. The friction modifiers which are useful in practising the present
invention are all
known in the art.
In accordance with the present invention it has been found that only certain
friction modifiers may be used to give the desired technical effect of reduced
micropitting. The efficacy of any given friction modifier in reducing
micropitting may be
assessed by comparing the amount of micropitting observed when a metal surface
is
lubricated with a lubricant composition comprising the friction modifier with
the amount
of micropitting observed when an identical metal surface is lubricated (under
the same
conditions) using the corresponding lubricant composition from which the
friction
modifier of interest has been omitted. The FZG micropitting test may be used
to assess
the relative performance of lubricant compositions.
Another way of identifying suitable friction modifiers is by reference to the
2 o friction coefficient of lubricants including them. It has been found that
the at least one
friction modifier may be selected such that, when measured at 130 C using a
high
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frequency reciprocating rig (HFRR) under the conditions described in SAE
Technical
Paper 961142, a lubricant which comprises the friction modifier and which has
a
viscosity grade of ISO 220 has a coefficient of friction of 0.100 or less. The
HFRR test
may thus be employed as a screen for useful friction modifiers. Lubricant
compositions
which have a viscosity grade of ISO 220 and which are useful in screening
friction
modifiers may be prepared by blending a conventional sulphur- and phosphorus-
containing gear additive package with a base oil having a viscosity of between
1.98 x 104
to 2.42 x 104 m2/s (198 to 242 cSt) at 40 C. Suitable additive packages
include those
comprising from 15-75 wt%, preferably from 45-65 wt%, of a sulfurized
isobutylene,
from 0-25 wt%, preferably from 3-15 wt%, of a phosphorus-containing antiwear
agent,
from 0-60 wt%, preferably from 5-25 wt% of a carboxylic-type or Mannich-type
ashless
dispersant, from 0-20 wt%, preferably from 1-10 wt% of corrosion and rust
inhibitors,
from 0-20 wt%, preferably from 1-10 wt%, of surface active agents and diluent
oil. Such
additive packages are commercially available. The additive package is used at
conventional treat rates. A suitable base oil to use in formulating the
compositions
includes a blend of 51 wt% ESSO 600SN and 49 wt% of 2500 Brightstock. Useful
additive package are described in EP-A-0744456 and EP-A-0812901.
A number of different classes of friction modifiers have found to be useful in
the
present invention. Mention may be made of phosphonate esters, phosphite
esters,
2 0 aliphatic succinimides, molybdenum compounds and acid amides.
Useful phosphonate esters include O,O-di-(primary alkyl) acyclic hydrocarbyl
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phosphonates in which the primary alkyl groups are the same or different each
independently containing 1 to 4 carbon atoms and in which the acyclic
hydrocarbyl group
bonded to the phosphorus atom contains 12 to 24 carbon atoms and is a linear
hydrocarbyl group free of acetylenic unsaturation. These compounds thus
comprise 0,0-
dimethyl hydrocarbyl phosphonates, 0,0-diethyl hydrocarbyl phosphonates, 0,0-
dipropyl hydrocarbyl phosphonates, 0,0-dibutyl hydrocarbyl phosphonates, 0,0-
diiso-
butyl hydrocarbyl phosphonates, and analogous compounds in which the two alkyl
groups differ, such as, for example, O-ethyl-O-methyl hydrocarbyl
phosphonates, 0-
butyl-0-propyl hydrocarbyl phosphonates, and O-butyl-O-isobutyl hydrocarbyl
phosphonates, wherein in each case the hydrocarbyl group is linear and is
saturated or
contains one or more olefinic double bonds, each double bond preferably being
an
internal double bond. Preferred are compounds in which both 0,0-alkyl groups
are
identical to each other. Also preferred are compounds in which the hydrocarbyl
group
bonded to the phosphorus atom contains 16 to 20 carbon atoms. A preferred
friction
modifier in this class is dimethyloctadecyl phosphonate. Phosphonate esters
useful in the
present invention are described in USP 4,158,633.
Useful phosphite esters are described in W088/04313. These include
dihydrocarbyl hydrogen phosphites in which the hydrocarbyl groups are the same
or
different linear aliphatic hydrocarbyl groups free of acetylenic unsaturation
each
2 0 independently containing 8 to 24 carbon atoms, and amine salts of these
phosphites. The
phosphites typically contain linear aliphatic hydrocarbyl groups, each of
which contains
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CA 02266841 1999-03-24
12 to 24, preferably 16 to 20 carbon atoms. It is also preferred that at least
50% of the
hydrocarbyl groups in the dihydrocarbyl hydrogen phosphite contain at least
one internal
double bond. It is preferred to use dioleylphosphite.
Preferred amine salts of the foregoing dihydrocarbyl hydrogen phosphites are
those in which the aliphatic group of the amine is a linear primary aliphatic
group having
8 to 24 carbon atoms, for example 16 to 20 carbon atoms, and in which at least
50% of
the aliphatic groups contain one or more internal double bonds.
Useful succinimides include those of formula:
O O
Z-CH-C~ Z-CH-C-NH2
I ~NH or I
CH2- ; CH2-U-NH2
O O
in which Z is a group R'RZCH- in which R' and R 2 are the same or different
each
independently representing straight- or branched-chain hydrocarbon groups
containing
from 1 to 34 carbon atoms and the total number of carbon atoms in the groups
R' and R2
is from 11 to 35. Such compounds are described in EP-A-0020037, EP-A-0389237
and
EP-A-0776964.
The radical Z may be, for example, 1-methylpentadecyl, 1-propyltridecenyl, 1-
pentyltridecenyl, 1-tridecylpentadecenyl or 1-tetradecyleicosenyl. Preferably
the number
of carbon atoms in the groups R' and R 2 is from 16 to 28 and more commonly 18
to 24.
2 0 It is especially preferred that the total number of carbon atoms in R' and
R2 is about 20 or
about 22. Preferably, the succinimide is a 3 - C1S-24 alkenyl-2,5-
pyrrolidindione. A
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sample of this succinimide contains a mixture of alkenyl groups having from 18
to 24
carbon atoms.
Useful molybdenum compounds are described in USP 5,650,381. These
compounds are typically substantially free of active sulphur. Examples of
suitable
compounds include glycol molybdate complexes as described in U.S.P. 3,285,942,
overbased alkali metal and alkaline earth metal sulfonates, phenates and
salicylate
compositions containing molybdenum such as those disclosed in U.S.P.
4,832,857,
molybdenum complexes prepared by reacting a fatty oil, a diethanolamine and a
molybdenum source as described in U.S.P. 4,889,647, molybdenum containing
compounds prepared from fatty acids and 2-(2-aminoethyl)aminoethanol as
described in
U.S.P. 5,137,647, overbased molybdenum complexes prepared from amines,
diamines,
alkoxylated amines, glycols and polyols as described in U.S.P. 5,143,633, and
2,4-
heteroatom substituted-molybdena-3,3-dioxacycloalkanes as described in U.S.P.
5,412,130.
Molybdenum salts such as the carboxylates are a preferred group of molybdenum
compounds. The molybdenum salts used in this invention may be completely
dehydrated
(complete removal of water during preparation), or partially dehydrated. They
may be
salts of the same anion or mixed salts, meaning that they are formed from more
than one
type of acid. Illustrative of suitable anions there can be mentioned chloride,
carboxylate,
2 0 nitrate, sulfonate, or any other anion.
The molybdenum carboxylate is preferably that of a monocarboxylic acid such as
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those having from about 4 to 30 carbon atoms. Such acids can be hydrocarbon
aliphatic,
alicyclic or aromatic carboxylic acids. Monocarboxylic aliphatic acids having
about 4 to
18 carbon atoms are preferred, particularly those having an alkyl group of
about 6 to 18
carbon atoms. The alicyclic acids may generally contain from 4 to 12 carbon
atoms. The
aromatic acids generally contain one or two fused rings and contain from 7 to
14 carbon
atoms wherein the carboxyl group may or may not be attached directly to the
ring. The
carboxylic acid can be a saturated or unsaturated fatty acid having from about
4 to 18
carbon atoms. Examples of carboxylic acids that may be used to prepare the
molybdenum carboxylates include butyric acid, valeric acid, caproic acid,
heptanoic acid,
cyclohexanecarboxylic acid, cyclodecanoic acid, naphthenic acid, phenyl acetic
acid,
2-methylhexanoic acid, 2-ethylhexanoic acid, suberic acid, octanoic acid,
nonanoic acid,
decanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid,
pentadecanoic
acid, palmitic acid, linolenic acid, heptadecanoic acid, stearic acid, oleic
acid,
nonadecanoic acid, eicosanoic acid, heneicosanoic acid, docosanoic acid and
erucic acid.
The preferred molybdenum carboxylate is molybdenum octanoate.
Useful carboxylic acid amides include aliphatic monocarboxylic acid amides.
These may be represented by the formula (R3CO)N(R4)(RS) in which R3 represents
an
alkyl or alkenyl group having 8 to 24 carbon atoms and R4 and RS which may be
the same
or different are each independently hydrogen or alkyl of up to 7 carbon atoms.
Typically,
R' represents a C14_,$ alkyl radical. Amides of this type are described in USP
4,280,916.
A preferred friction modifier falling within this class is oleyamide.
8
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Friction modifiers which are useful in the present invention are commercially
available or may be prepared by the adaptation or application of known
methods.
The amount of the at least one friction modifier which is used is at least
sufficient
for it to exert its intended function of reducing micropitting. The friction
modifier(s)
is/are generally used at conventional treat rates. Typically, the total
concentration of
friction modifier used is 0.125 to 1% by weight based on the total weight of
the lubricant
composition. Preferably, the total amount of friction modifier is 0.15 to
0.75% by
weight, more preferably about 0.5% by weight.
Mixtures of friction modifiers may be used. In this case friction modifiers of
the
same or different type may be used in combination. For example, satisfactory
results
have been obtained using combinations of dimethyloctadecyl phosphonate and a 3-
C18_24
alkenyl-2,5-pyrrolidindione. When mixtures of friction modifiers are employed
the total
amount of friction modifier is as described above.
It is important that the at least one friction modifier employed is
sufficiently
soluble in the lubricant composition at the treat rate at which it is used. It
is also
important that the at least one friction modifier is sufficiently compatible
with the
additional components commonly found in lubricating compositions. Such
components
include dispersants, detergents, antioxidants, extreme pressure agents,
antiwear agents,
foam inhibitors, viscosity index improvers and pour point depressants. These
additives
2 0 are themselves used in conventional amounts.
The base oil which is used to formulate lubricant compositions useful in the
9
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present invention may be natural or synthetic, or a blend thereof. Useful base
oils are
known in the art. The lubricant compositions are formulated in known manner by
blending the individual components. The at least one friction modifier
responsible for
improving the micropitting performance may be added at the time the lubricant
is
formulated. Alternatively, the at least one friction modifier may be added as
a top treat to
improve or boost the micropitting performance of an existing formulated
lubricant
composition.
The invention also provides lubricant compositions which exhibit excellent
micropitting performance relative to conventional lubricants. In one
embodiment the
invention provides a lubricant composition comprising an O,O-di-(primary
alkyl)acyclic
hydrocarbyl phosphonate as described above and a succinimide as described
above.
Preferably, the composition comprises dimethyloctadecyl phosphonate and a 3-
C18_24
alkenyl-2-pyrrolidindione. In another embodiment the composition comprises a
molybdenum carboxylate, such as molybdenum octanoate, and a sulfurized
isobutylene
extreme pressure agent. The compositions may also include one or more of the
other
additive components described above.
The invention is illustrated in the following examples.
CA 02266841 1999-03-24
EXAMPLES 1-7
Lubricant compositions were prepared by blending the components listed in
Table
1 below. The sulphur- and phosphorus-containing gear additive package had the
following composition:
50 wt% sulfurized isobutylene (extreme pressure agent)
8 wt% mixed phosphite and phosphate anti-wear agent
17.5 wt% phosphorylated, boronated succinimide dispersant
9 wt% rust inhibitor package
2.6 wt% corrosion inhibitor
0.5 wt% defoamer
0.15% demulsifier
1.5 wt% 3-C18-24 alkenyl-2,5-pyrrolidindione
balance base (diluent) oil
The base oil was ESSO ISO 220. The viscosity grade of each composition was ISO
220.
11
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0 0 \
~ p O
V~ ~ N
O p
0 0
0
Cl N O
O
i ~ a ~ i p
N
~ o o \
M N
0
.--~ N
~ o a
to
a o .~ - v bn
o cd ~4
o o U v,
Q a C~ r, c4 O r~i~
CA 02266841 1999-03-24
The coefficient of friction for each composition was measured at 130 C using
an HFRR
operated under the conditions described in SAE Technical Paper 961142 (ball
diameter
6mm,
load 4N, frequency 20 Hz, stroke length 1 mm; ball and flat ANSI 52100 steel).
The
HFRR coefficient of friction for each composition is given in Table 2. Each
composition
was also subjected to the FZG micropitting test in accordance with CEC L-07-A-
71. The
results obtained in this test are also shown in Table 2.
The solubility/compatibility of the friction modifier(s) within the lubricant
compositions tested the appearance of the compositions was assessed visually.
The
presence of precipitate indicates poor solubility/compatibility. Table 2
reports the extent
of the solubility/compatibility.
The percentage micropitting and weight loss were assessed after load stage 10.
The weight loss was determined by comparing the initial weight of the gears
under test
with the weight of the gears after load stage 10. The results are also shown
in Table 2.
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0
~ U
o~ a H o O
~ o Q ~ vi
.-
O
U
~ w
~ a [ M
O
U
N
N O o Q ' o
00
-~
E-+ 1:4
0
U
Q r-I
a H o
00
. o Q Cv
o .-= tn .- rn
N 0 Q
O
O Q CLl ~ ~
N ~ A
O w O O
O O, C7 ~ N
kQ O
'" o o O
~
0
~ o
* ~
~
o
~ ~ a a cc
pq Q ~ ~ 0
c~ x ~N O O
Ln
CA 02266841 1999-03-24
In this table the FZG result is given as a load stage result (in the stepwise
phase).
A profile deviation of 7.5 m is used to differentiate a pass or fail result
at any given load
stage. For example, Runs 1 and 2 give "10 fail" and "9 fail" results
respectively which
means that the profile deviation exceeded 7.5 m after load stage 10 (Run 1)
and load
stage 9 (Run 2). Runs 4-7 on the other hand give an FZG result of " 10 pass"
which
means that the profile deviation has not exceeded 7.5 m after load stage 10.
The results in Table 2 show that the friction coefficient obtained in the HFRR
test
may be used to predict which friction modifier(s) is/are useful in improving
micropitting
performance. HFRR results of less than 0.100 are predictive of friction
modifiers which
give improved micropitting performance.
The lubricant composition used in Runs 1 and 2 is a conventional gear
lubricant.
This gives reasonable micropitting protection, the 7.5 m threshold being
exceeded after
load stage 10 (Run 1) or 9 (Run 2). For Run 3 the micropitting test was
aborted after load
stage 6 because the composition tested was found to contain precipitate. This
emphasises
the need for the friction modifiers used to be fully soluble/compatible in
lubricant at the
treat rate at which they are used. The compositions used in Runs 4-7
illustrate the present
invention and give improved FZG results of "10 pass" when compared with the
conventional lubricant compositions of Runs 1 and 2. A consistent "10 pass"
result
2 0 would be very acceptable in the industry. Runs 4-7 also showed acceptably
low levels of
percentage micropitting and weight loss.
CA 02266841 1999-03-24
= ' ' = .
Example 8
To confirm the accuracy of the friction modifier screening procedure glycerol
monooleate, a friction modifier which is known to exhibit poor micropitting
protection,
was included in a lubricant composition having a viscosity grade ISO 220 and
the
resulting composition tested using the HFRR test (in accordance with SAE
Technical
Paper 961142). The composition gave an HFRR result of 0.114, i.e. well above
the
threshold value of 0.100.
Examples 9 and 10
The HFRR screening procedure was repeated using compositions using a different
sulphur- and phosphorus-containing gear additive package. The base oil was
ESSO ISO
220. The viscosity grade of the formulated compositions was ISO 220. The treat
rate of
the various components and the HFRR results obtained are shown in Table 3
below.
TABLE 3
COMPONENT RUN
9 10
Dimethyloctadecyl - 0.25%
phosphonate
3-C18-24 alkenyl - 0.25%
2,5-pyrollidindione
S/P Containing gear pack 2.0% 2.0%
HFRR 0.105 0.070
The HFRR result for Run 9 of in excess of 0.100 is consistent with the HFRR
16
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results for Runs 1 and 2 in Table 1. The composition used in Run 10 included a
combination of friction
modifiers which are known to give improved micropitting performance (see the
result for
Run 6 in Table 2). The HFRR result for Run 10 is less than 0.100. This is
consistent
with the HFRR result obtained for Run 6 in Table 2 where a different gear
additive
package was used in formulating the composition under test. This shows that
the HFRR
screening procedure remains predictive of useful friction modifiers even when
different
gear additive packages are used in formulating the lubricant compositions.
17
