EP0372559B1

EP0372559B1 - Coating composition for lubrication

Info

Publication number: EP0372559B1
Application number: EP89122596A
Authority: EP
Inventors: Yoshiyuki Hirokawa; Isao Tonomura; Hirohumi Michioka; Yoshio Fuwa
Original assignee: Takata Corp; Toyota Motor Corp
Current assignee: Takata Corp; Toyota Motor Corp
Priority date: 1988-12-09
Filing date: 1989-12-07
Publication date: 1992-07-29
Anticipated expiration: 2009-12-07
Also published as: EP0372559A1; JPH02155958A; JPH0674389B2; DE68902305T2; DE68902305D1

Description

The present invention relates to a coating composition for lubrication which provides an optimum friction coefficient to places to which it is applied and, at the same time, lowers abrasion loss.
Lubricants are supplied to surfaces of machines, devices, etc., to protect their surfaces from seizure and abrasion loss by replacing the dry friction between two surfaces by fluid friction or boundary friction.
Hitherto employed lubrication compositions comprise synthetic resins and solid lubricants, such as those described, e.g., in Japanese Patent Application (Laid Open Nos. 86,764/88; 157,664/80 and 200,463/82; and Japanese Patent Publication No. 1,314/80. The major objects of producing and using such lubrication compositions are, among others, as follows:

1) improvement of lubrication, namely, lowering of friction coefficient of sliding surfaces, and reduction of abrasion loss;
2) prevention of scuffing; and
3) improvement of corrosion resistance.

There are parts which must have a relatively high, but a moderate friction coefficient and, at the same time, a low friction loss For example, if the friction coefficient between a belt and a pulley is too small, the belt will slip on the pulley. In the case where the friction coefficient between a bolt, screw or the like and a surface with which it is in contact is too small, the bolt tends to become loose. In addition, clutch plates, friction discs of differential-limiting differential gear, etc., which are used under wet conditions, also require a high abrasion resistance and a moderately high friction coefficient.
Prior coating compositions applied to parts which require a moderately high friction coefficient and a low abrasion loss, lead to an undesirably low friction coefficient, although a low level of abrasion can be attained because of the characteristic mentioned in 1) above. In particular, when prior lubrication coating compositions are used for clutch plates, friction discs of differential-limiting differential gears, etc., which are used under wet conditions, the friction coefficient between their sliding surfaces becomes too low [friction coefficient (µ) will be around 0.05], and hence the frictional force necessary for the transmission of torque will not be attained. Therefore an increased number of friction plates is required, or an increased pressing load must be applied to attain sufficient frictional force.
US-A-3717576 and US-A-4438004 disclose lubricant compositions comprising a synthetic resin, a solid lubricant and a friction coefficient adjuster.
It is therefore an object of the present invention to provide a coating composition for lubrication which provides an appropriate level of friction coefficient (µ = 0.06 or above) and, at the same time, reduces abrasion loss.
Said object is achieved by a coating composition for lubrication comprising 20 to 80% by weight of a synthetic resin selected from the group consisting of epoxy resins, phenyl resins, amino resins, polyester-alkyd resins, polyurethane resins, vinyl resins and polyamide-imide resins, 20 to 80% by weight of a solid lubricant selected from the group consisting of sulfides, fluorides, graphite, melamine-cyanuric acid addition products, and boron nitride and 5 to 30% by weight of a friction coefficient adjuster characterized in that the friction coefficient adjuster is carbon fibers.
The coating composition according to the present invention is imparted with excellent lubricating properties by a solid lubricant and, at the same time, its lubrication property is adjusted by the friction coefficient adjuster to a desired moderate level without any impairment of its abrasion resistant properties.
The present invention will be explained hereinbelow in detail.
Any synthetic resin can be used, including thermosetting resins, cold setting resins and two-component curing resins. The resins can be either water miscible or oil miscible.
Synthetic resins include epoxy resins, phenyl resins, amino resins, polyester-alkyd resins, polyurethane resins, vinyl resins and polyamide-imide resins.
These resins can be used either individually or in combination of two or more. To use a combination of an epoxy resin and an amino resin can be particularly preferable since excellent adhesion, corrosion resistance and oil resistance, can be attained with regard to coated films formed therefrom. In this case, amino resins are used preferably in an, amount of 20 to 100 parts by weight, per 100 parts by weight of epoxy resins.
Synthetic resins are used in an amount of from 20 to 80% by weight, in particular, from 30 to 60% by weight, based on the total weight of the coating composition. When the amount of synthetic resins used is less than 20% by weight, insufficient adhesion, corrosion resistance on oil resistance, will result, whereas when it exceeds 80% by weight, it becomes difficult to control the friction coefficient.
Solid lubricants include sulfides, such as molybdenum disulfide and tungsten disulfide; fluorides, such as polytetrafluoroethylene and graphite fluoride; graphite; melamine-cyanuric acid addition products; and boron nitride. These solid lubricants can be used either individually or in combination of two or more. It is particularly preferable to use a combination of a sulfide and a fluoride because of their excellent withstanding load, fitting and feeling. In this case, fluorides are used preferably in an amount of 50 to 200 parts by weight, per 100 parts by weight of sulfides.
Such solid lubricants are used in an amount of 20 to 80% by weight, in particular, 30 to 60% by weight, based on the total weight of the coating composition. When the amount of solid lubricants incorporated is less than 20% by weight, there will result insufficient withstand load, fitting and feeling. In the case where it exceeds 80% by weight, there will result insufficient adhesion, corrosion resistance and oil resistance.
The friction coefficient adjuster is carbon fibers.
The use of carbon fibers is highly effective not only in the adjustement of friction coefficient, but also in the enhancement of abrasion resistance.
The friction coefficient adjuster preferably has a length of about 10 µm to 1 mm.
The friction coefficient adjuster is used in an amount of from 5 to 30% by weight, based on the weight of the coating composition prepared. When the amount of the friction coefficient adjuster incorporated exceed 30% by weight, there will result low adhesiveness and corrosion resistance of the coated lubricant film, and its abrasion resistance will not be sufficiently improved.
The coating composition according to the present invention can be easily prepared, e.g., by mixing a synthetic resin with a solid lubricant at a prescribed ratio, and then incorporating thereinto a prescribed amount of the friction coefficient adjuster. In addition, its friction coefficient can be varied over a wide range by adjusting the ratio of the components, while maintaining the abrasion loss at a low level.
If desired, an appropriate amount of solvent can be used upon mixing of the synthetic resin and the solid lubricant or after mixing of the three components to make their mixing easier and the adjust the viscosity of the composition to a level suitable for use.
Any solvent can be used if it is capable of dissolving synthetic resins. Although the amount of solvent to be used varies depending on the viscosity and quantity of synthetic resins used, it is preferable to use about 100 to 300 parts by weight of solvents, per 100 parts by weight of the composition. It is possible to use either one single solvent or a mixture of solvents. Specific examples of usable solvents include methyl ethyl ketone (M.E.K.) and xylene.
If desired, in addition to the above three components, the coating composition according to the present invention can be additionally incorporated with various additives, including quality modifiers. In this case, the amount of the additional additives should be 20% by weight or less, based on the weight of the coating composition.
The coating composition according to the present invention makes it possible to form a lubrication film having a friction coefficient controlled to a relatively high level and at the same time having a markedly reduced abrasion loss.
Accordingly, sliding friction and lubrication properties of such parts as differential- limiting differential gears, clutch discs and pulleys --- which require a low abrasion loss and, at the same time, a relatively high level of friction necessary to secure the transmission of torque --- and of parts which require a moderately high friction coefficient that prevents loosening of bolts can be markedly improved by using the coating composition according to the invention.
The present invention will further be explained by way of examples.

Examples 1 to 21 and Comparative Example 1

Coating compositions for lubrication were prepared by using the materials shown in Table 1 in ratios given in Table 2.
To be more specific, 100 parts by weight of a synthetic resin was dissolved in 100 to 200 parts by weight of a solvent (M.E.K., xylene, or the like), and then a solid lubricant was added thereto. The resulting mixture was subjected to a pulverization treatment by use of a ball mill, and a prescribed amount of friction coefficient adjuster was added thereto. The thus obtained mixture was stirred to give a sample of coating composition.
Samples of coating compositions obtained were applied to test pieces set forth below, and friction coefficient and abrasion loss were determined in the manner described below. The results obtained are shown in Table 2.

(1)Determination of Friction Coefficient

Friction coefficient was determined in the following manner, employing a tester, test pieces and test conditions described below.
To be more specific, a ring and a block were subjected to a surface treatment to form thereon a film of zinc phosphate, and then a sample of a coating composition prepared as above was air-sprayed on its surface at a thickness of about 20 µm, and then the coated film was cured under conditions given in Table 2. Friction coefficient was determined by pressing, with a constant load, the coated surface of the block onto the coated surface of the ring, while rotating the ring at a constant speed.
Upon determination of friction coefficient, the surface of the ring not contacted with the block was either dipped or not dipped in an oil (mineral oil) used for differential-limiting differential gears.

Tester

FALEX #1, Ring and Block Test Machine [manufactured by Faville Le Vally Co. (ASTM D 2714)]

Test Pieces

Ring: SAE 4620 Steel, Hrc 58-63, 6-12 r.p.min
Block: SAE 0-1 Steel, Hrc 58-63, 4-8 r.p.min

Test Conditions

Load: 100 lbs (45,4 kg)
Rotation Speed: 30 r.p.min

(2) Determination of Abrasion Loss

Firction Plate A was prepared by coating a coating composition on a surface of a steel plate in the same manner as in the above (1) Determination of Friction Coefficient. Friction Plate B (outer diameter, 120 mm; inner diameter, 100 mm; and thickness, 1.8 mm) was prepared from a quenched and tempered steel plate (made of JIS SK5M steel; hardness, HV 460) having a surface roughness of 0.5 µRa. Friction test was performed by using Friction Plate A in combination with Friction Plate B.
To be more specific, Friction Plate A was pressed with a load of 500 k.g.f. onto Friction Plate B, while rotating Friction Plate B at about r.p.min in an oil used for differential-limiting differential gears, and its abrasion loss (thickness of wearing) was measured after 200 h.
It would be apparent from Table 2 that coating compositions for lubrication according to the invention have higher friction coefficients than the prior art composition (Comparative Example 1), and their friction coefficients can be varied over a wide range. In addition, their abrasion losses are extremely small. On the other hand, the prior composition (Comparative Example 1) has an excessively low friction coefficient and exhibits a large abrasion loss.
In addition, in the case of Comparative Example 1, the coated film wore to the extent that part of the base steel was exposed, whereas in the case where the coating compositions according to the present invention were used, the coated films remained almost completely.
In the test using the thrust tester performed in (2) Determination of Abrasion Loss, coating compositions according to the invention formed films having a statical friction coefficient (µs) smaller than the dynamic friction coefficient (µd). It was also found that the coated films according to the invention effectively suppress the generation of noises owing to the stick slip phenomenon. On the other hand, in the case of the prior composition (Comparative Example 1), the µs of the coated film was almost equal to, or a little smaller than, its µd. In addition, its steel base was exposed during the test since the abrasion resistance of the coated film was not sufficiently high, and noises were generated owing to the stick slip phenomenon.
In Table 1 trade names and manufacturers of the components used in Comparative Example 1 and Examples 1 to 21 are shown.

Claims

1. A coating composition for lubrication comprising 20 to 80% by weight of a synthetic resin selected from the group consisting of epoxy resins, phenyl resins, amino resins, polyester-alkyd resins, polyurethane resins, vinyl resins and polyamide-imide resins, 20 to 80% by weight of a solid lubricant selected from the group consisting of sulfides, fluorides, graphite, melamine-cyanuric acid addition products, and boron nitride and 5 to 30% by weight of a friction coefficient adjuster
characterized in that
the friction coefficient adjuster is carbon fibers.

2. The composition of Claim 1 wherein said synthetic resin is a mixture of an epoxy resin and an amino resin in a weight ratio of from 5:1 to 1:1.

3. The composition of Claim 1 or 2 wherein said lubricant is a mixture of a sulfide and a fluoride in a weight ratio of from 2:1 to 1:2.

4. The composition of any of Claims 1 to 3 wherein said carbon fibers have a length of 10µm to 1mm and a diameter of 10 to 50µm.