EP0475393A1

EP0475393A1 - Lubricant and grease composition for ceramic material

Info

Publication number: EP0475393A1
Application number: EP91115392A
Authority: EP
Inventors: Masafumi Masuko; Yoshitaka c/o Yodogawa Works Honda; Ikuo c/o Yodogawa Works Yamamoto
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 1990-09-11
Filing date: 1991-09-11
Publication date: 1992-03-18

Abstract

A lubricant for a ceramic material comprising 99-0 % by weight of a perfluoropolyether and 1 to 100 % by weight of a terminal-modified halogenated polyether and a grease composition containing this lubricant and a fluororesin, which prevent wear of a ceramic material and has good heat resistance and oxidative stability.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a lubricant and a grease composition for a ceramic material comprising a perfluoropolyether.

Description of the Related Art

With increase of the number of mechanical elements or parts (e.g. bearings) made of a ceramic material such as Si₃N₄, SiC and the like, a lubricant suitable for the ceramic material is increasingly required. Since the ceramics are very stable and inactive materials, they are often used under severer conditions, in particular, at a higher temperature than other conventional materials. Therefore, the lubricant for the ceramic material is required to be excellent in heat resistance and oxidative stability.
As a synthetic oil having good heat resistance and oxidative stability, hindered esters and perfluoropolyether (hereinafter referred to as "PFPE") are exemplified. As the lubricant which is stable at higher temperature, PFPE is preferred.
It is known that, when PFPE is used as the lubricant, the ceramic materials (e.g. Si₃N₄) is more worn than a steel (e.g. SUJ2 and SUS 440C). To reduce the wear of the ceramic material, it is known that the addition of a phosphorus additive is effective (see the Preprint for the 34th National Meeting (in Toyama) of the Japan Lubricant Society in 1989, page 33). However, since the phosphorus additive is not compatible with PFPE and has low heat resistance, it tends to decompose at high temperature and generate sludges.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a lubricant for a ceramic material which has good heat resistance and prevent wear of the ceramic material.
Another object of the present invention is to provide a grease composition for a ceramic material which has good heat resistance and prevents wear of the ceramic material.
According to a first aspect of the present invention, there is provided a lubricant for a ceramic material comprising 99-0 % by weight of a perfluoropolyether and 1 to 100 % by weight of a terminal-modified halogenated polyether.
According to a second aspect of the present invention, there is provided a grease composition comprising, as a base oil, a lubricant which comprises 99-0 % by weight of a perfluoropolyether and 1 to 100 % by weight of a terminal-modified halogen-containing polyether and 0.5 to 60 % by weight of a fluororesin based on the whole weight of the composition.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph showing the radii of the wear marks measured in Example 1 and Comparative Example at various temperatures,
Fig. 2 is a cross section of an apparatus for evaluating a radius of a wear mark on a ball,
Fig. 3 is a plan view of balls after the wearing test,
Fig. 4 is a plan view of a wear mark on a ball, and
Fig. 5 is a perspective view of a bench used in the measurement of a radius of a wear mark on a ball.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, preferably the lubricant for the ceramic material consists essentially of the terminal-modified halogenated polyether. In another embodiment, the lubricant for the ceramic material comprises 99 to 0.01 % by weight, preferably 99 to 1 % by weight of PFPE and 1 to 99.99 % by weight, preferably 1 to 99 % by weight of the terminal-modified halogenated polyether.
PFPE is preferably a polymer of the formula:

X¹-R_f¹-X² (I)

wherein R _f¹ is a group having a perfluoropolyether group, and X¹ and X² are the same or different and each a fluorine atom, a trifluoromethyl group or a pentafluoroethyl group.
Preferably, R _f¹ comprises at least one kind of a repeating unit selected from the group consisting of -CF₂CF₂CF₂O-, -CF(CF₃)CF₂O- and -CF₂O-.
A number (or weight) average molecular weight of the polyether (I) is usually from 1000 to 15,000, preferably from 2000 to 10,000.
Preferred examples of the polyether (I) are

(1) F(CF₂CF₂CF₂O)_nCF₂CF₃
(2) F[CF(CF₃)CF₂O]_nCF₂CF₃
(3) A polyether comprising the repeating units: -CF(CF₃)CF₂O-, -CF₂CF₂O- and -CF₂O-.

The terminal-modified halogenated polyether is preferably a halogenated polyether of the formula:

Y¹-R_f²-Y² (II)

wherein R_f² is a group having a halogen-containing alkyl polyether group, Y¹ is a fluorine atom or a polar group and Y² is a polar group.
A number (or weight) molecular weight of the terminal-modified halogenated polyether (I) is usually from 1000 to 15,000, preferably from 2000 to 5000.
Preferably, the terminal-modified halogenated polyether is compatible with the PFPE.
R_f² preferably comprises at least one kind of a repeating unit selected from the group consisting of -CH₂CF₂CF₂O-, -CHClCF₂CF₂O-, -CCl₂CF₂CF₂O-, -CHFCF₂CF₂O-, -CFClCF₂CF₂O-, -CF₂CF₂CF₂O-, -CF(CF₃)CF₂O-, -CF₂CF₂O- and -CF₂O-. R_f² may have, at one of its end to be bonded with Y¹ and Y², a group of the formula: -CH₂CF₂-, -CHFCF₂-, -CF₂CF₂-, -CCl₂CF₂-, -CFClCF₂-, -CHClCOO-, -COO-, -CF₂COO-or C₂F₄COO-.
Examples of the polar group are a group of the formula: -CH₂OH, -COOR¹, -CONR¹₂, -CH₂OR¹, -COOR²Ph or -R²Ph wherein R¹ is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms or an aromatic group a phenyl group, R² is an organic divalent group, and Ph is an aromatic group, a fluorine-containing alkyl group having 1 to 12 carbon atoms, a fluorine-containing ether group having 2 to 1000 carbon atoms or a polyoxyalkylene group. This polyoxyalkylene group comprises at least one kind of a repeating unit selected from the group consisting of a repeating unit: -C₂H₄O-, a repeating unit: -C₃H₆O- and a repeating unit: -C₄H₈O-, and has the tonal number of the repeating units of 1 to 200. In this connection, -C₃H₆- is -CH₂CH₂CH₂O-, -CH(CH₃)CH₂O- or -CH₂CH(CH₃)O-.
The polyoxyalkylene group is preferably a group of the formula:

wherein R³ is a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, R⁴s are the same or different and each a hydrogen atom, a methyl group or an ethyl group, and p and q are numbers which satisfy the equations: 0 ≦ p ≦ 100 and 0 ≦ q ≦ provided that the sum of p and q is at least 1 (one).
In the above formulas, R² is preferably an alkylene group having 1 to 5 carbon atoms and optionally 1 to 2 ether linkages. The aromatic group has usually 6 to 18 carbon atoms and may be substituted with a halogen atom, an amino group and the like. Examples of the aromatic group are a phenyl group and a naphthyl group.
The terminal-modified halogenated polyether may have the polar groups at both ends or at one end.
Y¹ is preferably a fluorine atom. Preferred examples of Y² are -CH₂OH,

-COOCH₂CF₂CF₃, -COOH, -COO(CH₂CH₂O)_nH in which n is 1 to 50 on the average,
-COOCH₂CH₂(OCH₂CH₂)_nOCH₃ in which n is 1 to 50 on the average,
-COOCH₂CH₂(OCH₂CH₂)_nOH in which n is 1 to 50 on the average,
-CH₂CH₂[OCH(CH₃)CH₂]_nOH in which n is 1 to 50 on the average,
-COOCH(CH₃)CH₂[OCH(CH₃)CH₂]_nOCH₂CH₂CH₂CH₂CH₃ in which n is 1 to 50 on the average,
-COOCH(CH₃)CH₂O[CH(CH₃)CH₂O]_mCH₂CH(CH₃)[OCH₂CH(CH₃)]_nOH in which the sum of m and n is 1 to 50 on the average, and
-COO(OCH₂CH₂)_nOCH₃ in which n is 1 to 50.
Preferred examples of the terminal-modified halogenated polyether are

F(CF₂CF₂CF₂O)_nCF₂CF₂CH₂OH

in which n is 20 on the average, and

F(CF₂CF₂CF₂O)_nCF₂CF₂COOH

in which n is 20 on the average.
The examples and the preparation method of the terminal-modified halogenated polyether are disclosed in Japanese Patent Kokai Publication Nos. 268664/1989, 131132/1989 and 131691/1991 (corresponding U.S. or European Patent Applications:
The lubricant for the ceramic material of the present invention can be formulated together with a fluororesin and used as a grease composition for the ceramic material.
The fluororesin to be contained in the grease composition is preferably a solid at room temperature. Examples of the fluororesin are polymers of tetrafluoroethylene such as polytetrafluoroethylene and copolymers of tetrafluoroethylene with a copolymerizable monomer (e.g. hexafluoropropylene, ethylene, fluoroalkyl-containing vinyl ether, etc.); polymers of chlorotrifluoroethylene such as high molecular weight polychlorotrifluoroethylene and copolymers of chlorotrifluoroethylene with a copolymerizable monomer; and the like. An average molecular weight of the fluororesin is usually from 1000 to 1,000,000.
The solid fluororesin having a smaller particle size is more preferred. When the fluororesin having a particle size of 0.01 to 10 µm, an amount of a thixotropic agent can be decreased for formulating a grease composition having the same consistency.
An amount of the fluororesin is generally from 0.5 to 60 % by weight based on the whole weight of the grease composition. When the amount of the fluororesin is less than 0.5 % by weight, the composition has a low dropping point and cannot be used at high temperature, and the consistency greatly changes according to temperature change. When the amount of the fluororesin is larger than 60 % by weight, the grease composition becomes too hard so that its production becomes difficult, and spreading during the application of the grease composition becomes poor. In addition, the grease composition easily solidifies at high temperature.
The grease composition may be prepared by adding the fluororesin to the lubricant for the ceramic material having a desired viscosity and mixed or kneaded in a three-roll mixer or a mill, or by mixing the solid fluororesin dispersed in a solvent such as trichlorotrifluoroethane with the lubricant for the ceramic material and evaporating off the solvent under reduced pressure.
The lubricant and the grease composition of the present composition can be used for any ceramic material, in particular, Si₃N₄ and SiC.
When the lubricant for the ceramic material of the present invention is used, the ceramic material is less worn. The lubricant and the grease composition of the present invention have good heat resistance and durability and can be used up to about 300°C.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be illustrated by the following Examples.

Example 1

A lubricant comprising 75 % by weight of a perfluoropolyether of the formula:

F(CF₂CF₂CF₂O)_nCF₂CF₃

having an average molecular weight of 8400 and 25 % by weight of a terminal-modified halogenated polyether of the formula:

F(CF₂CF₂CF₂O)_nCF₂CF₂CH₂OH

having an average molecular weight of 3500 was prepared.
Using this lubricant, wear resistance (a radius of a worn mark) of a bearing ball having a diameter of 11/32 inch was evaluated. The material of the bearing ball was Si₃N₄ or stainless steel SUJ2.
The radius of the worn mark was evaluated as follows:
Fig. 2 shows a cross section of an apparatus used for evaluating the radius of the worn mark. The apparatus comprises a rotating member 10 which rotates during friction and a stationary member 20 which is fixed during friction. The rotating member 10 consists of members 11 and 12 and connected with a motor (not shown). A test ball 13 is fixed by the members 11 and 12. The stationary member 20 consists of a container 21 and a support 22, and the container 21 contains three testing balls 23, 24 and 25. The wearing is carried out by rotating the rotating member 10 with contacting the ball 13 to the balls 23, 24 and 25. In the container, 4 cc of a lubricant is poured, and then the rotating member is rotated at 100 rpm under a vertical load of 30 kg for 30 minutes to wear the balls. The container is designed so that it is kept at a specific temperature. A lubricant temperature during friction is, for example, 50°C, 110°C, 160°C or 200°C.
Fig. 3 shows a plan view of the balls 23, 24 and 25 after the wearing test. On the balls 23, 24 and 25, worn marks 31, 32 and 33 were formed, respectively. Fig. 4 is a plan view of the worn mark. Sizes of the worn mark are measured in two directions which are perpendicular to each other to obtain a radius "a" and a radius "b". (a + b)/2 is a radius of a worn mark. An average of the radii of the worn marks 31, 32 and 33 is used as the radius of the worn mark for each lubricant.
Fig. 5 is a perspective view of a bench for measuring the sizes of the worn marks. The bench 40 has a hole 41, which engages with the support 22 of the stationary member 20. The stationary member 20 carrying the worn testing balls is placed on the bench 40 with inserting the support 22 in the hole 41, and then the bench is placed on a sample holder of an optical microscope for measuring the sizes of the worn mark. The bench 40 has a slant surface at an angle of 54.7° from a vertical plane as shown in Fig. 5, and the worn mark can be observed with the optical microscope from the vertical direction.
The results of the radius of the worn mark are shown in Fig. 1. From these results of Fig. 1, it is understood that the wear of the ceramic ball can be decreased to a level of the wear of the steel ball and the lubricant of the present invention can effectively prevent the wear of the ceramics.

Example 2

A lubricant consisting of 100 % by weight of the same terminal-modified halogenated polyether as used in Example 1 was prepared. Using this lubricant, a radius of a worn mark of a ball made of Si₃N₄ was measured in the same manner as in Example 1. The radius of the worn mark was 0.49 mm.

Comparative Example

In the same manner as in Example 1 but using the same perfluoropolyether as used in Example 1 alone as a lubricant, a radius of a worn mark was measured. The results are shown in Fig. 1.

Claims

A lubricant for a ceramic material comprising 99-0 % by weight of a perfluoropolyether and 1 to 100 % by weight of a terminal-modified halogenated polyether.
The lubricant for a ceramic material according to claim 1, which essentially consists of said terminal-modified halogenated polyether.
The lubricant for a ceramic material according to claim 1, which comprises 99 to 0.01 % by weight of PFPE and 1 to 99.99 % by weight of the terminal-modified halogenated polyether.
The lubricant for a ceramic material according to claim 1, wherein said perfluoropolyether is a polyether of the formula:

X¹-R_f¹-X² (I)

wherein R_f¹ is a group having a perfluoropolyether group, and X¹ and X² are the same or different and each a fluorine atom, a trifluoromethyl group or a pentafluoroethyl group.
The lubricant for a ceramic material according to claim 4, wherein said polyether is F(CF₂CF₂CF₂O)_nCF₂CF₃ or F[CF(CF₃)CF₂O]_nCF₂CF₃ having an average molecular weight of 1000 to 15,000.
The lubricant for a ceramic material according to claim 1, wherein said terminal-modified halogenated polyether is a halogenated polyether of the formula:

Y¹-R_f²-Y² (II)

wherein R_f² is a group having a halogen-containing alkyl polyether group, Y¹ is a fluorine atom or a polar group and Y² is a polar group.
The lubricant for a ceramic material according to claim 6, wherein said terminal-modified halogenated polyether is F(CF₂CF₂CF₂O)_nCF₂CF₂CH₂OH in which n is 20 on the average, or F(CF₂CF₂CF₂O)_nCF₂CF₂COOH in which n is 20 on the average.
A grease composition comprising, as a base oil, a lubricant which comprises 99-0 % by weight of a perfluoropolyether and 1 to 100 % by weight of a terminal-modified halogen-containing polyether and 0.5 to 60 % by weight of a fluororesin based on the whole weight of the composition.
The grease composition for a ceramic material according to claim 8, wherein said lubricant essentially consists of said terminal-modified halogenated polyether.
The grease composition for a ceramic material according to claim 8, wherein said lubricant comprises 99 to 0.01 % by weight of PFPE and 1 to 99.99 % by weight of the terminal-modified halogenated polyether.
The grease composition for a ceramic material according to claim 8, wherein said perfluoropolyether is a polyether of the formula:

X¹-R_f¹-X² (I)

wherein R_f¹ is a group having a perfluoropolyether group, and X¹ and X² are the same or different and each a fluorine atom, a trifluoromethyl group or a pentafluoroethyl group.
The grease composition for a ceramic material according to claim 11, wherein said polyether is F(CF₂CF₂CF₂O)_nCF₂CF₃ or F[CF(CF₃)CF₂O]_nCF₂CF₃ having an average molecular weight of 1000 to 15,000.
The grease composition for a ceramic material according to claim 8, wherein said terminal-modified halogenated polyether is a halogenated polyether of the formula:

Y¹-R_f²-Y² (II)

wherein R_f² is a group having a halogen-containing alkyl polyether group, Y¹ is a fluorine atom or a polar group and Y² is a polar group.
The grease composition for a ceramic material according to claim 13, wherein said terminal-modified halogenated polyether is F(CF₂CF₂CF₂O)_nCF₂CF₂CH₂OH in which n is 20 on the average, or F(CF₂CF₂CF₂O)_nCF₂CF₂COOH in which n is 20 on the average.