[0001] The invention relates to a grease composition that utilizes a metal soap-based thickener.
[0002] Grease has been mainly used for slide bearings, rolling bearings, and sliding surfaces
where it is difficult to maintain adhesion of a lubricant film due to the movement
of the contact surface. Metal soap grease that utilizes a metal soap-based thickener
exhibits excellent water resistance, heat resistance, and mechanical stability. In
particular, lithium soap-based grease has been most widely used as universal grease,
and has been used for bearings, gears, ball joints, pinions, and the like which have
steel-resin sliding parts.
[0003] A grease composition that includes silicone oil, a lithium soap thickener, 3 to 25
mass% of a polytetrafluoroethylene resin powder, and 1 to 15 mass% of a saturated
fatty acid amide has been proposed as resin grease used for steel-resin sliding parts
(see PTL 1).
[0004] However, since the grease composition disclosed in PTL 1 utilizes silicone oil, namely
a high-density base oil, the dispersibility of the solid lubricant is poor. Moreover,
the friction coefficient cannot be sufficiently decreased, and a stick-slip phenomenon
easily occurs.
[0006] An object of the invention is to provide a grease composition that improves the dispersibility
of a solid lubricant, that provides a sufficiently low friction coefficient in steel-resin
sliding parts, and that prevents a stick-slip phenomenon.
[0007] The inventors of the invention conducted extensive studies in order to achieve the
above object. As a result, the inventors found that a grease composition that improves
the dispersibility of a solid lubricant, that provides a sufficiently low friction
coefficient in steel-resin sliding parts, and that prevents a stick-slip phenomenon
can be obtained, by utilizing a lubricant base oil having a specific density.
[0008] The invention was completed based on the above finding, and provides the following.
- (1) A grease composition comprising a lubricant base oil having a density of 0.75
to 0.95 g/cm3 at 15°C, an amide compound, a solid lubricant, and a metal soap-based thickener,
wherein the solid lubricant is boron nitride, and wherein the content of the lubricant
base oil is 60 to 85 mass% based on the total amount of the grease composition, the
content of the amide compound is 5 to 30 mass% based on the total amount of the grease
composition, the content of the solid lubricant is 0.2 to 5 mass% based on the total
amount of the grease composition, and the content of the metal soap-based thickener
is 5 to 15 mass% based on the total amount of the grease composition.
- (2) The grease composition according to (1), wherein the amide compound is a monoamide
or a bisamide.
- (3) A method for lubrication using a grease composition according to (1) or (2) between
various resin sliding members and various metal sliding members.
[0009] The grease composition according to the invention ensures that the solid lubricant
is sufficiently dispersed therein, provides a low friction coefficient in steel-resin
sliding parts, and prevents a stick-slip phenomenon.
[0010] The grease composition according to the invention comprises a lubricant base oil
having a density of 0.75 to 0.95 g/cm
3 at 15°C, an amide compound, a solid lubricant, and a metal soap-based thickener.
[Lubricant base oil]
[0011] A mineral oil-based lubricant base oil or a synthetic lubricant base oil may be used
as the lubricant base oil used in connection with the invention as long as the density
is 0.75 to 0.95 g/cm
3 at 15°C. If the density of the lubricant base oil falls outside the above range,
the dispersibility of the solid lubricant may decrease, and the friction coefficient
may not be sufficiently decreased. The density of the lubricant base oil is more preferably
0.8 to 0.9 g/cm
3.
[0012] It is preferable that the lubricant base oil has a kinematic viscosity of 1 to 500
mm
2/s at 40°C, and more preferably 5 to 100 mm
2/s. If the kinematic viscosity of the lubricant base oil falls outside the range of
1 to 500 mm
2/s at 40°C, it may be difficult to easily prepare a grease composition having the
desired consistency. In order to prepare grease having excellent lubricity, it is
preferable to use a lubricant base oil having a viscosity index of 90 or more in particular,
95 to 250, a pour point of -10°C or less in particular, -15 to -70°C, and a flash
point of 150°C or more.
[0013] Examples of the mineral oil-based lubricant base oil include lubricant fractions
obtained by distilling crude oil under atmospheric pressure optionally followed by
distillation under reduced pressure to obtain a distillate, and refining the distillate
using various types of refining process. Examples of the refining process include
hydrotreating, solvent extraction, solvent dewaxing, hydrodewaxing, washing with sulfuric
acid, clay treatment, and the like. The base oil used in connection with the invention
can be obtained by combining these processes in an appropriate order. A mixture of
a plurality of refined oils having different properties is also useful, wherein the
mixture is obtained by using different types of crude oils or distillates and by a
different combination and/or order of processes. The base oil obtained by each method
may preferably be used as long as the properties of the base oil are adjusted to satisfy
the above density range.
[0014] It is preferable to use a material that exhibits excellent hydrolytic stability as
the synthetic lubricant base oil. Examples of the synthetic lubricant base oil include
polyolefins such as a poly-α-olefin, a polybutene, and a copolymer of two or more
olefins, polyesters, polyalkylene glycols, alkylbenzenes, alkylnaphthalenes, and the
like. It is preferable to use a poly-α-olefin from the viewpoint of availability,
cost, viscosity, oxidation stability, and compatibility with a system member. A polymer
of 1-dodecene or 1-decene is more preferable as the poly-α-olefin from the viewpoint
of cost.
[0015] These synthetic lubricant base oils may be used either alone or in combination. The
synthetic lubricant base oil may be used in combination with the mineral oil-based
lubricant base oil.
[0016] When using a mixture of a plurality of types of lubricant base oil including a synthetic
lubricant base oil, the properties of each base oil are not necessary to fall within
the above range as long as the base oil mixture satisfies the above properties. Therefore,
each synthetic base oil need not necessarily satisfy the above properties, but it
is preferable that the properties of each synthetic base oil fall within the above
ranges.
[0017] The content of the lubricant base oil in the grease composition is 60 to 85 mass%
based on the total amount of the grease composition. If the content of the lubricant
base oil is outside the range of 60 to 85 mass%, it may be difficult to easily prepare
a grease composition having the desired consistency.
[Amide compound]
[0018] The amide compound used in connection with the invention is a compound that includes
at least one amide group (-NH-CO-). A compound that includes one amide group (monoamide),
a compound that includes two amide groups (bisamide), and a compound that includes
three amide groups (triamide) may be used as the amide compound. The bisamide and
the triamide have an advantage in that the frictional resistance in the sliding part
can be reduced even when the amide compound is used in a relatively small amount.
Therefore, the bisamide and the triamide are most suitable as the amide compound.
[0019] The bisamide may be an acid amide of a diamine or an acid amide of a diacid.
[0020] It is preferable to use an amide compound having a melting point of 40 to 180°C (more
preferably 80 to 180°C, and still more preferably 100 to 170°C) and a molecular weight
of 242 to 932 (more preferably 298 to 876).
[0021] The monoamide is represented by the following general formula (1), the bisamide is
represented by the following general formulas (2) and (3), and the triamide is represented
by the following general formula (4).
R
1-CO-NH-R
2 (1)
R
1-CO-NH-A
1-NH-CO-R
2 (2)
R
1-NH-CO-A
1-CO-NH-R
2 (3)
R
1-M-A
1-CH(A
2-M-R
3)-A
3-M-R
2 (4)
[0022] wherein R
1, R
2, and R
3 are independently a hydrocarbon group having 5 to 25 carbon atoms. The hydrocarbon
group may be an aliphatic hydrocarbon group, an alicyclic hydrocarbon group, or an
aromatic hydrocarbon group. R
2 in the general formula (1) may be a hydrogen atom. A
1, A
2, and A
3 are independently an aliphatic hydrocarbon group having 1 to 10 carbon atoms, an
alicyclic hydrocarbon group, an aromatic hydrocarbon group, or a divalent hydrocarbon
group having 1 to 10 carbon atoms formed by combining these groups, and M is an amide
group.
[0023] It is preferable that R
2 is a hydrogen atom or a saturated or unsaturated chain hydrocarbon group having 10
to 20 carbon atoms when the amide compound is the monoamide.
[0024] It is preferable that A
1 is a divalent saturated chain hydrocarbon group having 1 to 4 carbon atoms when the
amide compound is the acid amide of a diamine.
[0025] Some of the hydrogen atoms of the hydrocarbon group represented by R
1, R
2, or A
1 in the general formulas (2) and (3) may be substituted with a hydroxyl group (-OH).
[0026] An amide compound in which A
1, A
2, and A
3 are an aliphatic hydrocarbon group is referred herein to as "aliphatic amide", an
amide compound in which at least one of A
1, A
2, and A
3 is an aromatic hydrocarbon group is referred herein to as "aromatic amide", and an
amide compound in which at least one of A
1, A
2, and A
3 is an alicyclic hydrocarbon group or an aromatic hydrocarbon group is referred herein
to as "non-aliphatic amide".
[0027] It is preferable that R
1, R
2, and R
3 is a saturated or unsaturated chain hydrocarbon group having 10 to 20 carbon atoms
when the amide compound is the aliphatic amide.
[0028] It is preferable that R
1, R
2, and R
3 is a saturated or unsaturated chain hydrocarbon group having 10 to 20 carbon atoms
or an aromatic hydrocarbon group when the amide compound is the aromatic amide.
[0029] The non-aliphatic amide may also be used as the amide compound, but it is preferable
to use the aliphatic amide as the amide compound. It is preferable that A
1 is a divalent saturated chain hydrocarbon group having 1 to 4 carbon atoms when the
amide compound is the acid amide of a diamine (general formula (3)).
[0030] Specific examples of the monoamide include saturated fatty acid amides such as lauric
acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and hydroxystearic
acid amide, unsaturated fatty acid amides such as oleic acid amide and erucic acid
amide, substituted amides of a saturated or unsaturated long-chain fatty acid and
a long-chain amine such as stearylstearic acid amide, oleyloleic acid amide, oleylstearic
acid amide, and stearyloleic acid amide, and the like.
[0031] Specific examples of the acid amide of a diamine represented by the general formula
(2) include ethylene bis-stearic acid amide, ethylene bis-isostearic acid amide, ethylene
bis-oleic acid amide, methylene bis-lauric acid amide, hexamethylene bis-oleic acid
amide, hexamethylene bis-hydroxystearic acid amide, and the like. Specific examples
of the bisamide of a diacid represented by the general formula (3) include N,N'-bis-stearylsebacic
acid amide and the like.
[0032] Among these, the amide compounds represented by the general formula (2) or (3) in
which R
1 and R
2 are independently a saturated chain hydrocarbon group or an unsaturated chain hydrocarbon
group having 12 to 20 carbon atoms are preferable.
[0033] There are various triamide compounds that are represented by the general formula
(4). Specific examples of a compound among the compounds represented by the general
formula (4) that can be suitably used in connection with the invention include an
N-acylamino acid diamide compound. The N-acyl group included in the N-acylamino acid
diamide compound is preferably a linear or branched saturated or unsaturated aliphatic
acyl group having 1 to 30 carbon atoms, or an aromatic acyl group in particular, a
caproyl group, a capryloyl group, a lauroyl group, a myristoyl group, or a stearoyl
group. The amino acid included in the N-acylamino acid diamide compound is preferably
aspartic acid or glutamic acid. The amine of the amide group included in the N-acylamino
acid diamide compound is preferably a linear or branched saturated or unsaturated
aliphatic amine having 1 to 30 carbon atoms, and more preferably butylamine, octylamine,
laurylamine, isostearylamine, or stearylamine. In particular, N-lauroyl-L-glutamic
acid-a,y-di-n-butylamide is preferable.
[0034] These amide compounds may be used either alone or in combination. The content of
the amide compound in the grease composition is 5 to 30 mass% based on the total amount
of the grease composition.
[0035] When the amide compound is heated and melted in the presence of the lubricant base
oil, the lubricant base oil is held by the amide compound that forms a three-dimensional
network structure. Therefore, the friction coefficient in steel-resin sliding parts
further decreases, in comparison with the case of merely dispersing and mixing the
amide compound in grease. Moreover, a stick-slip phenomenon does not occur.
[Solid lubricant]
[0036] The solid lubricant is not particularly limited as long as the solid lubricant is
boron nitride.
[0037] A solid lubricant having an appropriate particle size may be selected depending on
the application. It is preferable to use a solid lubricant having a particle size
(diameter) of 0.2 to 50 µm, and more preferably 1 to 10 µm.
[0038] The content of the solid lubricant in the grease composition is 0.2 to 5 mass% based
on the total amount of the grease composition.
[Metal soap-based thickener]
[0039] The metal soap-based thickener is a thickener that includes a metal carboxylate.
The carboxylic acid may be a carboxylic acid derivative that includes a hydroxyl group
or the like.
[0040] The carboxylic acid may be either an aliphatic carboxylic acid such as stearic acid
and azelaic acid or an aromatic carboxylic acid such as terephthalic acid. An aliphatic
monocarboxylic acid or dicarboxylic acid in particular, an aliphatic carboxylic acid
having 6 to 20 carbon atoms may be used. It is preferable to use an aliphatic monocarboxylic
acid having 12 to 20 carbon atoms or an aliphatic dicarboxylic acid having 6 to 14
carbon atoms. An aliphatic monocarboxylic acid that includes one hydroxyl group is
preferable.
[0041] The metal may be an alkali metal such as lithium and sodium, an alkaline-earth metal
such as calcium, or an amphoteric metal such as aluminum. An alkali metal in particular,
lithium is preferably used.
[0042] The thickener may be added in the form of metal soap. The carboxylic acid and a metal
source (e.g., metal salt and metal salt hydroxide) may also be separately added and
reacted to produce a metal soap thickener, when preparing grease.
[0043] Such metal carboxylates may be used either alone or in combination. For example,
it is particularly preferable to use a mixture of lithium 12-hydroxystearate and lithium
azelate.
[0044] The content of the metal soap-based thickener in the grease composition is 5 to 15
mass% based on the total amount of the grease composition.
[Additive]
[0045] The grease composition according to the invention may optionally include a detergent,
a dispersant, an antiwear agent, a viscosity index improver, an antioxidant, an extreme
pressure agent, a rust-preventive agent, a corrosion inhibitor, and the like that
are normally used for a lubricant or grease in addition to the above components.
[Preparation method]
[0046] The grease composition according to the invention may be prepared using a normal
grease preparation method. It is preferable to heat a mixture comprising the amide
compound to a temperature equal to or higher than the melting point of the amide compound
at least once, after mixing the amide compound.
[0047] Specifically, the grease composition may be prepared by heating the amide compound
and the lubricant base oil to a temperature equal to or higher than the melting point
of the amide compound, cooling the mixture, and then physically mixing the cooled
mixture with normal grease that comprises the solid lubricant, the thickener, and
the lubricant base oil. Alternatively, all of the components including the thickener
may be mixed, heated to a temperature equal to or higher than the melting point of
the amide compound, and then cooled.
[Lubrication target]
[0048] The grease composition according to the invention may suitably be used for lubrication
between various resin sliding members and various metal sliding members. Examples
of the resin sliding member include sliding members formed of a-nylon resin, a polycarbonate
(PC) resin, a polybutylene terephthalate (PBT) resin, a polyacetal (POM) resin, and
the like having a long-term heat resistant temperature (UL standard) of 50 to 150°C.
In particular, the grease composition is suitable for a member where a nylon 6 (PA6)
resin is used. Examples of the metal sliding member include sliding members formed
of bearing steel, carbon steel, stainless steel (SUS), and the like. In particular,
the grease composition is suitable for a member where bearing steel SUJ-2 is used.
Examples
[0049]
- 1. Lubricant base oil
- (1) Mineral oil-based lubricant base oil
- Lubricant base oil obtained by distilling atmospheric distillation residue under reduced
pressure and subjecting the resulting distillate to solvent refining
Kinematic viscosity at 40°C: 68 mm2/s
Density at 15°C: 0.87 g/cm3
Viscosity index: 100
Pour point: -10°C
Flash point: 250°C
- (2) Synthetic lubricant base oil
- (a) Poly-α-olefin ("Durasyn 170" manufactured by INEOS)
Kinematic viscosity at 40°C: 68 mm2/s
Density at 15°C: 0.83 g/cm3
Viscosity index: 133
Pour point: -45°C
Flash point: 250°C
- (b) Polydimethylsiloxane (silicone oil; "KF-96" manufactured by Shin-Etsu Chemical
Co., Ltd., 50 cs, 100 cs)
Kinematic viscosity at 40°C: 68 mm2/s
Density at 15°C: 0.96 g/cm3
- (c) Linear perfluoro polyether (fluorinated oil; "FOMBLIN M15" manufactured by Solvay
Solexis)
Kinematic viscosity at 40°C: 85 mm2/s
Density at 15°C: 1.83 g/cm3
- 2. Amide compound
- (1) Aliphatic amide
- (a) Ethylene bis-stearic acid amide (special grade reagent)
- (b) Ethylene bis-oleic acid amide (special grade reagent)
- (c) Stearic acid monoamide (special grade reagent)
- (2) Aromatic amide
- (a) m-Xylylene bis-stearic acid amide (special grade reagent)
- 3. Solid lubricant
- (1) Melamine cyanurate (MCA, average particle size: 4 µm, "MELAPUR MC25" manufactured
by BASF)
- (2) Molybdenum disulfide (average particle size: 0.5 µm, "Nichimoly M-5 Powder" manufactured
by Daizo Corporation)
- (3) Polytetrafluoroethylene (PTFE, average particle size: 4 µm, "KTL-8N" manufactured
by Kitamura Limited)
- (4) Boron nitride (average particle size: 2 µm, "HP-P1" manufactured by Mizushima
Ferroalloy Co., Ltd.)
- (5) Silicon oxide (average particle size: 35 µm, special grade reagent)
The average particle size was measured by laser diffractometry.
- 4. Metal soap-based thickener
- (1) Lithium 12-hydroxystearate ("Lithium stearate" in Tables 1 and 2)
- (2) Mixture of lithium 12-hydroxystearate and lithium azelate (mixing ratio: 2:1)
("Lithium soap mixture" in Tables 1 and 2)
- 5. Additive
[0050] Diphenylamine was added to each composition as an antioxidant.
[Preparation method]
[0051] Each component was charged into a vessel in the amount (mass%) shown in Table 1 or
2, heated to 150°C, which is a temperature equal to or higher than the melting point
of the amide, stirred using a magnetic stirrer, and then cooled to room temperature.
The mixture was dispersed under pressure using a roller (triple roll) to prepare a
grease composition.
[Evaluation method]
[0052] An evaluation test was performed using a reciprocating friction tester utilizing
a ball and a disk. A sphere made of SUJ-2 and having a diameter of 1/4 inches was
used as a metal sliding member, and a plate made of nylon 6 ("N6 (NC)" manufactured
by Toray Plastics Precision Co., Ltd.) was used as a resin sliding member.
[0053] The test load was set to 2000 gf, the sliding speed was set to 10 mm/s, and the amplitude
was set to 20 mm. The grease was applied to the disk, and the presence or absence
of a stick-slip phenomenon was evaluated from the friction coefficient and the frictional
force waveform during sliding. Specifically, it was determined that a stick-slip phenomenon
occurred when the frictional force during sliding in one direction was not constant.
[Evaluation results]
[0054] When only the mineral oil and the lithium soap-based thickener were mixed, the friction
coefficient was high, and a stick-slip phenomenon occurred (Comparative Example 7).
[0055] When only the mineral oil, the lithium soap-based thickener, and the solid lubricant
were mixed, the friction coefficient slightly decreased, and a stick-slip phenomenon
was somewhat suppressed insufficiently (Comparative Examples 1 to 6).
[0056] When the mineral oil, the lithium soap-based thickener, the solid lubricant, and
the aliphatic amide were mixed, the friction coefficient further decreased, and a
stick-slip phenomenon was suppressed (Examples 1 to 13; Examples 4 to 6 are not in
accordance with the present invention).
[0058] Since the grease composition according to the invention ensures that the solid lubricant
is sufficiently dispersed therein, provides a low friction coefficient in steel-resin
sliding parts, and prevents a stick-slip phenomenon, the grease composition is useful
for lubrication between a resin sliding member and a metal sliding member (e.g., bearing,
gear, ball joint, and pinion).