Low torque grease composition

Details Low torque grease composition Low torque grease composition

2003-01-03

2004-04-20

Howard, Jacqueline V. (Department: 1764)

Solid anti-friction devices, materials therefor, lubricant or se

Lubricants or separants for moving solid surfaces and...

Ptfe

C508S582000

Reexamination Certificate

active

06723684

ABSTRACT:

This application claims all rights of priority to Japanese Patent Application No. 2002-006792 filed on Jan. 15, 2002, (pending).
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a low torque grease composition, and more particularly relates to a low torque grease composition comprising perfluoropolyether oil as a base oil and polytetrafluoroethylene as a thickener, and a motor bearing for cars or aircraft to which such a composition is applied. Further, this invention relates to a bearing grease composition for electric controlling throttle motors, and more particularly relates to a fluorine grease composition of improved properties exhibiting low torque at low temperatures, long durability and effective applicability at low and high temperatures when the present grease is filled in a bearing.
2. Prior Art
Grease to be injected in a motor bearing for cars and aircraft is required to have constant properties in a wide range of temperatures postulated in the severely cold polar regions and the broiling deserts ranging from −40 to 180° C.
Although a variety of investigations have been done to satisfy such properties, in fact, there is yet room for further improvement thereof.
Requirements for a bearing grease composition used to cars and, in particular, electric controlling throttle motors include low torque, i.e., low rotational torque at room temperature and low temperatures, high acoustical properties, less leakage or volatility, long durability, and effective applicability in a wide range of low to high temperatures ranging from −40 to 180° C. Especially for bearings of severe requirements, there is used fluorine grease of high properties such as improved high-temperature durability, oxidative stability and chemical resistance or outstanding low-temperature performance.
Conventional fluorine grease comprises perfluoropolyether as a base oil, tetrafluoroethylene copolymer as a thickener and a small amount of other components such as a rust preventing agent or corrosion inhibitor. When further low-temperature performance is required, a straight-chain perfluoropolyether oil having low-temperature performance is used.
Because of recent strong requirements, it is impossible for such fluorine grease to satisfy a required level of resistance to low-temperature torque under load and durability. In order to meet such severer requirements, there is proposed a grease composition by Japanese Patent No. 7-102,274, which comprises a liquid fluorinated polymer and a hexagonal boron nitride powder of double-humped particle size distribution. The proposed grease composition has excellent heat resistance and bearing durability, while there is no disclosure with regard to low-temperature torque properties.
DETAILED DESCRIPTION
The inventors have eagerly investigated to develop a grease composition useful for bearings of cars and aircraft which is stable in a wide range of temperature of 40 to 180° C. and exhibits low torque properties under high loading at low temperatures. As a result, there are provided a grease composition of markedly lower torque compared with those conventional grease compositions comprising perfluoropolyether oil as a base oil and a low torque grease composition useful for motor bearings of cars and, in particular, of electric controlling throttle motor, or for those bearings of aircraft.
Further, the inventors have investigated to develop a fluorine grease composition of low torque and long durability under a condition of high load and low temperatures useful for rolling bearings, which meets requirements of bearings for electric controlling throttle motor.
SUMMARY OF THE INVENTION
It has been found that objects of the invention can be achieved by using a perfluoropolyether oil as a base oil limited only to a straight-chain polymer excluding branched one as a polymer compound of both high and low molecular weight, and a polytetrafluoroethylene fine powder as a thickener. This invention has been developed on the basis the above mentioned fact.
More concretely, a low torque grease composition having dynamic viscosity (40° C.) of 40 to 150 mm
2
/sec and stability in a wide range of temperature ranging from −40 to 180° C. can be obtained by using a base oil consisting of (a) straight-chain perfluoropolyether oil having dynamic viscosity (40° C.) of 25 mm
2
/sec or less and represented by the following general formula:
RfO(CF
2
O)
p
(C
2
F
4
O)
q
(C
3
F
6
O)
r
Rf
wherein Rf is a perfluoro lower alkyl group and p, q and r are 0 or an integer less than 10, and (b) straight-chain perfluoropolyether oil having dynamic viscosity (40° C.) of 250 mm
2
/sec or more and represented by the following general formula:
RfO(CF
2
O)
p
(C
2
F
4
O)
q
(C
3
F
6
O)
r
Rf
wherein Rf is a perfluoro lower alkyl group and p, q and r are 0 or an integer of 10 to 200; and (c) polytetrafluoroethylene as a thickener.
THE PREFERRED EMBODIMENTS
A straight-chain perfluoropolyether oil used in the invention is represented by the following general formula:
RfO(CF
2
O)
p
(C
2
F
4
O)
q
(C
3
F
6
O)
r
Rf
wherein Rf is a perfluoro lower alkyl group and p, q and r are an integer of 200 or less; and preferably a straight-chain perfluoropolyether oil of the above mentioned molecular structure wherein r=0.
The reason why a straight-chain perfluoropolyether oil comprising a group of chemical formula (C
3
F
6
O)
r
causes to increase the viscosity and thus the torque at low temperatures is because of lower viscosity index thereof compared with that of a straight-chain perfluoropolyether oil without the group (C
3
F
6
O)
r
. Accordingly, the torque at lower temperatures can be further decreased by using the above mentioned straight-chain perflouoropolyether of the molecular formula wherein r=0. More concretely, the following compound is used:
RfO(CF
2
CF
2
O)
m
(CF
2
O)
n
Rf
wherein m+n=3 to 200 and m
=10/90 to 90/10. This compound is obtained by photo-oxidative polymerization of tetrafluoroethylene followed by complete fluorination of thus polymerized precursor.
A straight-chain perfluoropolyether oil as a component (a) used herein has lower molecular weight and dynamic viscosity (40° C.) of 25 mm
2
/sec or less, and preferably 5 to 25 mm
2
/sec (according to JIS K-2283; −40° C.). When the dynamic viscosity (40° C.) is lower than the above mentioned level, it is impossible to use such a component (a) to heat resistant grease because of an increase in evaporation loss.
A straight-chain perfluoropolyether oil as a component (b) to be mixed with the component (a) has higher molecular weight and dynamic viscosity (40° C.) of 250 mm
2
/sec or more, and preferably 250 to 500 mm
2
/sec (according to JIS K-2283; −40° C.). The component (b) having dynamic viscosity (40° C.) higher than the above mentioned level causes an increase in torque at low temperatures if the grease is used in a general manner because of higher pour point thereof (JIS K-2283).
These perfluoropolyether oils of different viscosity have different properties each other. The perfluoropolyether oil (a) exhibits higher viscosity index, lower torque properties and lower abrasion resistance, although the heat resistance is not so good. On the other hand, the perfluoropolyether oil (b) exhibits lower volatility and improved oil film keeping properties at high temperatures, although the torque properties at low temperatures are not so good.
These straight-chain perfluoropolyether oils having different molecular weight and viscosity are mixed so that prominent properties of both components (a) and (b) can be retained in the thus formed base oil, thereby durability being exhibited together with torque properties at low temperatures. The base oil can be endowed with abrasion resistance and durability while retaining low temperature torque properties of the perfluoropolyether oil (a) by properly mixing it with the perfluoropolyether oil (b). A mixing rate of perfluoropolyether oils (a) and (b) may be variable depending on a condition to be applied, but is preferably 90/10

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