[0001] The present invention relates to a grease composition having excellent lubricity.
[0002] Grease is typically used for sliding surfaces where moving contact surfaces make
it difficult to keep a lubricant film adhered to the lubricated surfaces, for example,
such as in sliding portions and rotors of mechanisms. In recent years, there is an
increasing trend of smaller, lighter, higher-output, and longer-life devices along
with the advancement of machine technology in products such as automobiles and electrical
devices, and these products are used under ever demanding operating conditions. This
has created a demand for higher performance in the lubricity or other properties of
greases used in devices.
[0003] In order to improve the lubricity of grease, it has been proposed to select a base
oil, a thickener, and additives. For example, it is known to add a specific phosphite
ester (PTL 1), or incorporate organic molybdenum, zinc dithiophosphate, a polysulfide,
and a triglyceride (PTL 2). Further,
US 4 029 587 A discloses lubricants and functional fluids containing substituted sulfolanes as seal
swelling agents.
WO 2006/023317 A1 discloses lubricant compositions containing a combination of conditioning agents
for preserving the integrity of elastomeric materials, which include a swell agent
such as a sulfolane.
[0004] However, for further improvement of lubricity, there is a need for development of
a novel additive for grease.
[0006] The present invention has been made to provide a solution to the foregoing problem,
and is intended to provide a grease composition of excellent lubricity containing
a novel additive for grease.
[0007] The present inventors conducted intensive studies to develop a grease composition
of excellent lubricity, and found, rather by chance, that a specific sulfolane derivative
is effective as an additive for grease. It was confirmed that the sulfolane derivative
has synergy with other additives under specific conditions.
[0008] The present invention is based on these findings, and includes the following.
- [1] A grease composition comprising a lubricant base oil, a thickener, at least one
of diphenyl hydrogen phosphite and tricresyl phosphate in an amount of 0.1 to 10 mass%
with respect to the total amount of the grease composition, and a sulfolane derivative
represented by the following general formula (1).
In the formula, R1 represents a hydrocarbon group of 1 to 20 carbon atoms, and R2 and R3 each represent hydrogen, or a hydrocarbon group of 1 to 20 carbon atoms.
- [2] The grease composition according to [1], wherein the sulfolane derivative of the
general formula (1) is one in which R1 is an alkyl group of 4 to 18 carbon atoms, and R2 and R3 are each hydrogen, or a hydrocarbon group of 1 to 8 carbon atoms.
- [3] The grease composition according to above-described [1] or [2], wherein the sulfolane
derivative is contained in a content of 0.2 to 10 mass%.
[0009] The grease composition of the present invention has distinguishing effects including
a low coefficient of friction, high load bearing characteristics, and excellent lubricity.
Lubricant Base Oil
[0010] A lubricant base oil of the present invention may be any of a mineral oil, a synthetic
oil, an animal and vegetable oil, or a mixture thereof, provided that it is a lubricant
base oil for common lubricants.
[0011] The lubricant base oil has a kinetic viscosity at 100°C of preferably 1 to 200 mm
2/s, more preferably 5 to 50 mm
2/s.
[0012] The content of the lubricant base oil is preferably 50 mass% or more, more preferably
60 mass% or more with respect to the total amount of the grease composition. Typically,
the upper limit of the content is preferably 95 mass% or less, more preferably 85
mass% or less, accounting for the balance.
[0013] Mineral oil-type base oils for lubricant are generally prepared by distilling crude
oil under atmospheric pressure, optionally further distilling the atmospheric residual
oil under reduced pressure, refining the resulting distillate oil by various refining
processes to obtain a lubricating oil fraction. The fraction is used as a base oil,
as it is, or a base oil is prepared by blending various additives to the fraction.
Examples of the above refining processes include hydrotreating, solvent extraction,
solvent dewaxing, hydrodewaxing, sulfuric acid washing, and a clay treatment. A mineral
oil-type base oil for lubricant suitable for the present invention can be obtained
by combining these processes in an appropriate order and submitting thereto. A mixture
of two or more refined oils having different properties obtained by processing different
crude oils or different distillate oils in combination with different processes, or
in different orders may also be used as a suitable base oil.
[0014] For safety of handling, the mineral oil-type base oil for lubricant is preferably
one with an aromatic content of 20% or less.
[0015] Examples of the synthetic oil-type base oil for lubricant include poly α-olefin (PAO),
a low-molecular copolymer of ethylene and α-olefin, high-molecular polymers such as
polybutene and polyisobutene, oxygen-containing synthetic oils such as ester oils
and ether oils, silicone oils, fluorinated oils, and alkylnaphthalene. These may be
used either alone or in combination.
[0016] Preferred examples of the animal and vegetable oil-type base oils for lubricant
include milkfat, beef tallow, lard (pig fat), mutton tallow, whale oil, salmon oil,
bonito oil, herring oil, cod oil, soy oil, canola oil, sunflower oil, safflower oil,
peanut oil, corn oil, cottonseed oil, rice bran oil, sesame oil, olive oil, linseed
oil, castor oil, cacao butter, palm oil, coconut oil, hempseed oil, rice oil, and
tea seed oil.
Thickener
[0017] The thickener used in the present invention may be, for example, a metal soap- or
urea-based thickener commonly used for grease compositions, or an organic synthetic
resin powder, an inorganic powder, a gelatinizer made from an amide compound, or a
wax.
[0018] The metal soap-based thickener is a metal carboxylate (metal soap), and may be, for
example, a sodium soap, a calcium soap, an aluminum soap, or a lithium soap, of which
the lithium soap is preferred.
[0019] Examples of the carboxylic acids that form the soap include fatty acids, and aliphatic
divalent carboxylic acids. The metal soap may be a so-called composite metal soap.
[0020] Examples of the urea-based thickener include urea compounds such as diurea compounds,
triurea compounds, tetraurea compounds, polyurea compounds (excluding diurea compounds,
triurea compounds, and tetraurea compounds), urethane compounds such as urea-urethane
compounds, and diurethane, and mixtures thereof. Preferred are diurea compounds, urea-urethane
compounds, diurethane compounds, and mixtures thereof.
[0021] Examples of the organic synthetic resin powder include fluororesin particles (particularly,
polytetrafluoroethylene resin particles).
[0022] Examples of the inorganic powder include metal oxide such as silica, metal carbonates
such as calcium carbonate, metal sulfates such as calcium sulfate, and nitrides such
as h-BN (atmospheric-phase boron nitride) . The inorganic powder also may be, for
example, a mineral-derived metal oxide such as bentonite.
[0023] Examples of the gelatinizer made from amide compounds include monoamides, bisamides,
and triamides. Preferred are aliphatic monoamides, and aliphatic bisamides.
[0024] Examples of the wax include petroleum waxes, synthetic waxes, and natural resin waxes.
[0025] The thickener content is preferably 3 to 45 mass%, more preferably 5 to 30 mass%
with respect to the total amount of the grease composition.
Sulfolane Derivative
[0026] The sulfolane derivative of the present invention is a compound represented by the
following general formula (1).
In the formula, R
1 represents a hydrocarbon group of 1 to 20 carbon atoms, and R
2 and R
3 each represent hydrogen, or a hydrocarbon group of 1 to 20 carbon atoms.
[0027] The sulfolane derivative is preferably one in which R
1 in the general formula (1) is an alkyl group of 4 to 18 carbon atoms, more preferably
one in which R
1 is an alkyl group of 6 to 16 carbon atoms, and R
2 and R
3 are each hydrogen or a hydrocarbon group of 1 to 8 carbon atoms. More preferably,
R
2 and R
3 are both hydrogens.
[0028] The sulfolane compound is commercially available as a seal-swell agent, and may be
produced by, for example, the method described in
JP-A-2006-206580.
[0029] The content of the sulfolane derivative is preferably 0.2 to 10 mass%, more preferably
0.5 to 5 mass% with respect to the total amount of the grease composition.
Second Additive
[0030] In the present invention, a second additive, which is at least one compound selected
from a diphenyl hydrogen phosphite and tricresyl phosphate, is added in addition to
the lubricant base oil, the thickener, and the sulfolane derivative.
[0031] The content of the second additive is 0.1 to 10 mass%, preferably 0.2 to 5 mass%
with respect to the total amount of the grease composition. The content of the phosphorus
element is preferably 0.01 to 1 mass%, more preferably 0.02 to 0.5 mass% with respect
to the total amount of the grease composition.
Other Additives
[0032] The grease composition of the present invention may also contain known additives,
other than the second additive. Examples of such additives include phenol- or amine-based
antioxidants; anti-rusting agents such as carboxylates, and sulfonates; antiwear agents
such as polyalkylene glycol, and glycerine; extreme-pressure agents such as chlorinated
paraffins; oiliness improvers such as higher fatty acids, and synthetic esters; and
solid lubricants such as graphite, and molybdenum disulfide. These may be added either
alone or in a combination of two or more.
Preparation and Properties of Grease
[0033] The grease composition of the present invention may be prepared by using a common
grease preparation method, specifically by blending the thickener with the lubricant
base oil to produce a base grease, and blending and mixing the additives to the base
grease. Alternatively, the grease composition of the present invention may be prepared
by blending and mixing the lubricant base oil, the thickener, and the additives at
once.
[0034] The grease composition of the present invention is semi-solid or solid at room temperature,
and has a consistency of 000 to 6 in terms of a JIS K2220 consistency number.
Examples
[0035] The thickener shown in Table 1 was added to the lubricant base oil, and the additives
were added to make the compositions shown in Table 1. The mixture was stirred, and
passed through a roller mill to obtain grease compositions of Examples, Reference
Examples and Comparative Examples.
[0036] Here, "PAO" is poly α-olefin (kinetic viscosity at 40°C: 48 mm
2/s).
[0037] "Mineral oil 1" is a highly-refined mineral oil (kinetic viscosity at 40°C: 38 mm
2/s) obtained through hydrocracking of a crude oil-derived component.
[0038] "Mineral oil 2" is a solvent-refining paraffinic mineral oil (kinetic viscosity at
40°C: 100 mm
2/s).
[0039] "Urea" in "Thickener" is a urea-based thickener obtained through reaction of diphenylmethane-4,4'-diisocyanate
(MDI) and monoamine which is a mixture of 2 mole parts of p-toluidine and 8 mole parts
of cyclohexylamine.
[0040] "Li soap" in "Thickener" is lithium stearate.
[0041] "phosphite ester" is diphenyl hydrogen phosphite.
[0042] "Phosphate ester" is tricresyl phosphate.
[0043] "Sulfur compound" is polysulfide (sulfur element content of 40%) .
[0044] "Sulfolane derivative" is a compound of the general formula (1) in which R
1 is an alkyl group of 8 carbon atoms, and R
2 and R
3 are hydrogens.
[0045] ZnDTP (zinc dithiophosphate) have alkyl group of 6 carbon atoms, and a Zn element
content of 8.7 mass%.
[0046] MoDTC (molybdenum dithiocarbamate) have alkyl group of 4-12 carbon atoms, and a Mo
element content of 10.0 mass%.
[0047] The friction characteristics were determined by conducting a ball/disc SRV test,
in which the grease composition was subjected to friction for 5 min at a frequency
of 50 Hz, an amplitude of 1.0 mm, and a temperature of 40°C under a load of 50 N using
a 10-mm ball. The friction test was continued for the next 5 min under an increased
load of 100 N, and for another 5 min under 200 N. Subsequently, the load was increased
in an increment of 100 N, and the coefficient of friction was measured for each load
until 600 N.
[0048] The load at which the coefficient of friction showed an abrupt increase to 0.3 or
higher, and a burn occurred was measured as the burn load (load bearing characteristics)
. The burn load was measured to be 600 N or higher when burns did not occur under
600 N.
[0049] The coefficient of friction under 50 N was determined as a measure of friction characteristics
because the friction characteristics under a light load are important for saving energy
in an actual use environment such as in bearings.
[0050] The SRV test used ASTM D5706 testing equipment, and used an SUJ-2 ball and disc.
[0051] The results are presented in Table 1.
[0052] As is clear from these results, the grease compositions with the addition of the
sulfolane derivative had greatly improved load bearing characteristics compared to
the traditional grease compositions that contained a phosphite ester or a phosphate
ester.
[Table 1]
|
Exa.1 |
Exa.2 |
Exa.3 |
Exa.4 |
Exa.5 |
Exa.6 |
Exa.7 |
Exa.8 |
Exa.9 |
Exa.10 |
C.Exa.1 |
C.Exa.2 |
C.Exa.3 |
Base Oil |
PAO |
83 |
|
|
83 |
80 |
82 |
82 |
82 |
82 |
82 |
85 |
83 |
83 |
Mineral oil 1 |
|
83 |
|
|
|
|
|
|
|
|
|
|
|
Mineral oil 2 |
|
|
83 |
|
|
|
|
|
|
|
|
|
|
Thickener |
Urea |
15 |
15 |
15 |
|
|
|
|
|
|
|
|
|
|
Li soap |
|
|
|
15 |
15 |
15 |
15 |
15 |
15 |
15 |
15 |
15 |
15 |
Additives |
Sulfolane derivative |
2 |
2 |
2 |
2 |
5 |
2 |
2 |
2 |
2 |
2 |
|
|
|
Phosphite ester |
|
|
|
|
|
1 |
|
|
|
|
|
2 |
|
Phosphate ester |
|
|
|
|
|
|
1 |
|
|
|
|
|
2 |
Polysulfide |
|
|
|
|
|
|
|
1 |
|
|
|
|
|
ZnDTP |
|
|
|
|
|
|
|
|
1 |
|
|
|
|
MoDTC |
|
|
|
|
|
|
|
|
|
1 |
|
|
|
Consistency |
300 |
300 |
300 |
300 |
310 |
305 |
305 |
305 |
305 |
305 |
295 |
300 |
300 |
SRV lubrication test |
Burn load (N) |
600 or higter |
601 or higter |
602 or higter |
603 or higter |
604 or higter |
605 or higter |
606 or higter |
607 or higter |
608 or higter |
609 or higter |
100 |
500 |
500 |
Coefficient of friction |
0.16 |
0.16 |
0.16 |
0.16 |
0.17 |
0.14 |
0.15 |
0.16 |
0.16 |
0.16 |
0.17 |
0.15 |
0.17 |
[0053] Examples 1 to 5 and 8 to 10 are Reference Examples
[0054] The grease composition of the present invention has a small coefficient of friction,
and high load bearing characteristics, and excels in lubricity. The grease composition
of the present invention has use as a lubricant for, for example, sliding portions
or rotors of automobiles and electrical devices.