GB2090284A - Solid Lubricant Particles Coated with a Buoyant Liquid - Google Patents

Abstract

A lubricant, which can be used as an additive to lubricating oil, is made up of solid lubricant particles, such as polytetrafluoroethylene, coated with a buoyant medium having a lower specific gravity than a carrier medium for the particles. The carrier medium may be tricresyl phosphate and the particles can be pre-wetted with an aliphatic naphtha or kerosene.

Description

SPECIFICATION Lubricant This invention relates generally to lubricants and more particularly has reference to a lubricant containing a dispersion of solid lubricant particles and to a method of making such a lubricant. A number of these lubricants are already known. For example, U.S. Patent No. 4,224,173 describes an eight step method for making lubricant oil containing polytetrafluoroethylene particles and a fluorochemical surfactant.

U.S. Patent No.2,510,112 describes an aqueous dispersion of colloidal polymerised polytetrafluoroethylene in a fluorinated hydrocarbon oil.

U.S. Patent No. 3,194,762 describes a product having resin particles suspended in an oil base.

U.S. Patent Nos. 3,159,557, 3,432,431, 3,493,513,3,505,229, 3,630,901 and 3,40,859 describe greases containing polytetrafluoroethylene particles.

U.S. Patent No. 3,723,317 describes a grease wherein triazene is combined with polytetrafluoroethylene to thicken a fluorinated polyether base oil.

U.S. Patent No. 4,029,870 describes unsintered polytetrafluoroethylene which has been irradiated.

U.S. Patent No. 3,933,656 describes submicron polytetrafluoroethylene particles.

U.S. Patent No.4,127,491 describes an aqueous dispersion of polytetrafluoroethylene particles.

The benefits of solid particle lubricant additives have been recognised for some time. Tests indicate varying but consistent improvements in engine efficiency through the use of molybdenum disulphide and graphite. The effects of solid particles as a cushion between sliding metal parts having been established, the natural tendency is to develop improved or advanced products.

Polytetrafluoroethylene has been introduced as a solid particle additive that exhibits the same cushioning effects as molybdenum disulphate and graphite, but with the advantage of being a cleaner material to work with and a better or lower friction lubricant.

However, there are several problems associated with the use of polytetraflurorethylene particle additives.

The preparation of a stable dispersion through chemical stabilisation of polytetrafluoroethylene is a complex and exacting science. One such stabilisation technique is described in U.S. Patent No.4,127,491.

Moreover, the dispersion achieved by the chemical stabilisation method is short-lived. Upon standing for short periods of time, the particles settle and develop what could be called a "hard settle", i.e., the particles cannot be redispersed.

Added to the "hard settling" problem are the in-service problems of short-lived effectiveness.

The apparent problem with dispersions achieved by the chemical stabilisation method is that the surface active materials and film forming polymers become ineffective after a brief period of use.

According to one aspect of the present invention a lubricant comprises solid lubricant particles in a carrier medium, the particles being coated with a buoyant medium having a lower specific gravity than the carrier medium.

The solid lubricant particles are preferably polytetrafluoroethylene particles, although perfluonnated alkoxy or fluorinated ethylene propylene may be used. The particles may be in powder form or may be sintered or ground to a size in the range of about 0.5 microns to about 20 microns, with about 5 microns being a preferred size.

The carrier medium is preferably a high specific gravity oil and the buoyant medium a low specific gravity oil and may have a low miscibility in the carrier medium. The concentration of the buoyant medium may be at least at the miscibility point and comprise a 10 wt to a 70 wt oil.

The particles may be pre-wet with aliphatic naphtha and the carrier medium may be tricresyl phosphate. The concentration of solid particles is preferably in the range of about 2 to about 15 per cent by volume of the lubricant.

The lubricant of the invention may be mixed with a major part by volume of a lubricating oil so that the lubricant of the invention acts as an additive. The major part of lubricating oil is preferably 95 to 97 per cent by volume of the mixture.

If desired the lubricant may be mixed with a propellant in a pressurised container to enable it to be used to give a spray-on coating. The concentration of propellant is preferably about 95 percent.

According to another aspect of the invention a method of producing a lubricant by reducing the apparent specific gravity of particles comprises coating the particles with a material having a relatively low specific gravity.

The particles may be pre-wet with a wetting agent such as an aliphatic naphtha.

The lubricant is preferably made by a method of making a stable dispersion comprising the steps of combining solid lubricant particles with a buoyant medium to form a first combination, subjecting the first combination to an atmosphere drawn to substantially vacuum, mixing the first combination at high speed in said atmosphere, combining the mixed first combination with a carrier medium to form a second combination, subjecting the second combination to an atmosphere drawn to substantially vacuum, and shearing the second combination at high speed in said atmosphere.

In order that the invention may be readily understood a lubricant in accordance therewith and a method of making it will now be described with reference to the following example.

The lubricant is primarily useful as an oil additive in engines but can also be used as a spray-on coating to reduce ice formation and drag on surfaces.

The lubricant of the present invention has ground and sintered polytetrafluoroethylene particles physically dispersed in a carrier medium.

Such a dispersion was heretofore thought to be impossible because of the high specific gravity of polytetrafluoroethylene.

The theory of creating the suspension in the present invention is relatively straightforward.

Heavy particles, such as polytetrafluoroethylene particles, are coated with a relatively low specific gravity buoyant medium, thus lowering the specific gravity of the particles. The coated particles are then floated in a relatively high specific gravity carrier medium. The resulting dispersion will stand for months and will not become solid or difficult to redisperse.

Ground polytetrafluoroethylene particles are used because of their durability and because of their inertness and electrostatic neutrality. The latter characteristics keep the particles from agglomerating.

The use of sintered polytetrafluoroethylene particles reduces the possibility of low boiling polytetrafluoroethylene particles being introduced to the combustion process of an engine. Sintered particles also have smoother surfaces and a more uniform geometry than the non-sintered particles used in the prior art.

The polytetrafluoroethylene particles used in the present invention are generally larger than the particles used in the prior art. The maximum particle size is determined by the intended use of the lubricant. For engine use, the particles must be small enough to pass through the engine's oil filter. Preferably, the particles have a diameter of below 7 microns for about 90% of the particles.

Particles at the upper ends of the useful ranges are more difficult to keep dispersed.

Polytetrafluoroethylene particles manufactured by LNP Corporation of Philadelphia, Pa., under the designation TL 102 are particularly suited to the present invention.

Preferably, the particles make up about 2 per cent to about 25 per cent of the volume of the lubricant.

Efforts to calculate the buoyant effect of the low specific gravity medium on the basis of surface area vs. particle mass prove to be no more accurate than the empirically derived method of adding more low specific gravity medium than is necessary and allowing it to rise to the top of the dispersion when it is mixed with the high specific gravity medium. It is important to use a low specific gravity medium that has low miscibility in the high specific gravity medium, and to start by adding to the miscibility point.

Tricresyl phosphate is particularly useful as the high specific gravity medium of the present invention. Tricresyl phosphate has been used for many years as a high pressure lubricant additive in greases, oils and gasoline. In addition to its lubricant properties, tricresyl phosphate tends to attach to scarred places in a cylinder wall, for example, and prevents further abrasion in that area. This is an extremely beneficial phenomenon and tests of NASA have shown oil life extended to 20,000 miles through the use of tricresyl phosphate additives.

Example: Shell Oil Company's aviation grade 10 to 70 wt oil is the preferred low specific gravity medium used in the present invention.

That oil was selected primarily because of its low specific gravity and high quality.

Agglomeration can be further prevented in the present invention by pre-wetting the polytetrafluoroethylene particles. Preferably, particles are pre-wet with aliphatic naphtha.

Aliphatic naphtha is particularly useful because it wets out instantly, prohibits any agglomeration, breaks up any agglomeration that may already be present, and does not break down in oil.

Alternatively, the polytetrafluoroethylene particles can be pre-wet with any other compatible solvent such as kerosene.

A method for making the lubricant of the present invention can now be described.

The polytetrafluoroethylene particles are ground and sintered. The resulting powder is then pre-wet.

The low specific gravity oil is then added to the wetted powder. That mixture is then placed in a vacuum drawn at least 29.8 inches at standard barometric pressure of 29.92 inches. While the vacuum is being drawn, the mixture is blended at high speed. Preferably, the high speed mixing is at least 4,000 rpm. The mixing can be conveniently carried out in a standard dispersion mixer.

50 gallon quantities of the mixture will usually require 30 minutes of mixing and vacuum.

Tricresyl phosphate is then added and the resultant mixture is sheared and vacuumed for 15 minutes.

The end product is a stable dispersion. The excess low specific gravity oil will migrate to the top of the dispersion because it is lighter than the oil coated particles and the tricresyl phosphate.

A sample formula would be as follows:- polytetrafluoroethylene 3 grams aliphatic naphtha 3 grams Shell aviation grade 50 weight oil 1.8 fluid ounces tricresyl phosphate 2.0 fluid ounces A lubricant made according to that formula is particularly useful as an oil additive in automobile engines.

When the lubricant of the present invention is used as an oil additive, it is preferred to use a quantity of additive which is about 2 per cent to about 20 per cent by volume of the oil capacity of the engine. However, other ratios can also be used. Acceptable results have been achieved with quantities of additive which were in excess of 10 per cent by volume of the oil capacity of the engine.

When the lubricant of the present invention is used as a spray-on coating, it is preferred that the lubricant and a propellant be combined in a pressurised valved container in a ratio of about 1:19 by volume. However, that ratio can be changed without departing from the present invention.

It is not necessary that the lubricant be applied to surfaces by spraying. The lubricant can be applied to a surface in any manner such as rubbing, painting or dripping or in any other conventional manner.

Claims (46)

Claims

1. A lubricant comprising solid lubricant particles in a carrier medium, said particles being coated with a buoyant medium having lower specific gravity than the carrier medium.

2. A lubricant as claimed in claim 1 wherein the solid lubricant particles are dispersed in the carrier medium.

3. A lubricant as claimed in claim 1 or 2 wherein the solid lubricant particles comprise polytetrafluoroethylene particles.

4. A lubricant as claimed in any preceding claim wherein the buoyant medium comprises low specific gravity oil.

5. A lubricant as claimed in any preceding claim wherein the carrier medium comprises high specific gravity oil.

6. A lubricant as claimed in claim 3 wherein the solid lubricant particles comprise sintered polytetrafluoroethylene particles.

7. A lubricant as claimed in claim 3 wherein the solid lubricant particles comprise ground polytetrafluoroethylene particles.

8. A lubricant as claimed in claim 7 wherein the polytetrafluoroethylene particles are ground to a size in the range of about .5 microns to about 20 microns.

9. A lubricant as claimed in claim 8 wherein the polytetrafluoroethylene particles are ground to a size of about 5 microns.

10. A lubricant as claimed in any of claims 1 to 5 wherein the solid lubricant particles comprise ground and sintered polytetrafluoroethylene particles.

11. A lubricant as claimed in any preceding claim wherein the buoyant medium has a low miscibility in the carrier medium.

12. A lubricant as claimed in any preceding claim wherein the concentration of buoyant medium in the carrier medium is at least at the miscibility point.

13. A lubricant as claimed in any preceding claim wherein the buoyant medium comprises a 10 wit. to 70 wt. oil.

14. A lubricant as claimed in any preceding claim wherein the carrier medium comprises tricresyl phosphate.

15. A lubricant as claimed in any preceding claim wherein the solid lubricant particles are prewet with a wetting agent.

16. A lubricant as claimed in claim 15 wherein the wetting agent comprises aliphatic naphtha.

17. A lubricant as claimed in claim 15 wherein the wetting agent comprises kerosene.

18. A lubricant as claimed in any preceding claim wherein the concentration of solid lubricant particles in the lubricant is in the range of about 2 per cent to about 15 per cent by volume.

19. A lubricant comprising a lubricating oil mixed with lubricant of any preceding claim to form a mixture.

20. A lubricant as claimed in Claim 19 wherein the lubricating oil comprises a major part by volume of the mixture.

21. A lubricant as claimed in Claim 19 or Claim 20 wherein the lubricating oil is in the range of 95 to 97 per cent by volume of the mixture.

22. A lubricant as claimed in any preceding claim wherein said combination of particles, carrier medium and buoyant medium is further combined with a propellant under pressure in a valved container.

23. A lubricant as claimed in claim 22 wherein the concentration of propellant in said container is about 95 per cent by volume.

24. A lubricant as claimed in claim 14 wherein the particles are coated with oil having a lower specific gravity than tricresyl phosphate.

25. A method of producing a lubricant by reducing the apparent specific gravity of particles comprising coating the particles with a material having a relatively low specific gravity.

26. A method as claimed in claim 25 wherein said particles comprise polytetrafluoroethylene particles and said coating material comprises a low specific gravity oil.

27. A method as claimed in claim 26 and includes polytetrafluoroethylene material with aliphatic naphtha.

28. A method of any one of Claims 25, 26 or 27 wherein the polytetrafluoroethylene material is powder form.

29. A method as claimed in any of claims 25, 26 or 27 wherein the polytetrafluoroethylene material comprises polytetrafluoroethylene particles ground to a size in the range of about 2 microns to about 20 microns.

30. A method as claimed in any one of claims 25, 26 or 27 wherein the polytetrafluoroethylene material comprises sintered polytetrafluoroethylene particles.

31. A method of making a lubricant as a stable dispersion comprising the steps of combining solid lubricant particles with a buoyant medium to form a first combination, subjecting the first combination to an atmosphere drawn to substantially vacuum, mixing the first combination at high speed in said atmosphere, combining the mixed first combination with a carrier medium to form a second combination, subjecting the second combination to an atmosphere drawn to substantially vacuum, and shearing the second combination at high speed in said atmosphere.

32. A method as claimed in claim 31 wherein the particles comprise polytetrafluoroethylene particles.

33. A method as claimed in claim 31 or claim 32 wherein the buoyant medium comprises low specific gravity oil.

34. A method as claimed in any one of claims 31, 32 or 33 wherein the carrier medium comprises high specific gravity oil.

35. A method as claimed in any one of claims 31 to 34 wherein the particles are ground to a size in the range of about .5 microns to about 20 microns.

36. A method as claimed in any one of claims 31 to 35 wherein the particles are pre-wet with a wetting agent.

37. A method as claimed in claim 35 wherein the particles are pre-wet with aliphatic naphtha.

38. A method as claimed in any one of claims 31 to 37 wherein said atmospheres are drawn to at least about 29.8 inches vacuum at a standard barometric pressure of about 29.92 inches.

39. A method as claimed in any one of claims 31 to 38 wherein said mixing comprises mixing the first combination at a minimum speed of about 4,000 rpm.

40. A method as claimed in any one of claims 31 to 39 wherein said mixing comprises mixing in a dispersion mixer.

41. A method as claimed in any one of claims 31 to 41 wherein said first combination is subjected to said mixing for about 30 minutes.

42. A method as claimed in any one of claims 31 to 41 wherein said second combination is subjected to said shearing for about 15 minutes.

43. A method of making a lubricant comprising mixing a lubricant manufactured according to the method of any one of claims 25 to 42 as an additive to a lubricating oil in which the lubricating oil forms a major part by volume of the mixture.

44. A method as claimed in claim 43 in which the lubricating oil comprises 95 to 97 per cent by volume of the mixture.

45. A method of making a lubricant substantially as hereinbefore described.

46. A lubricant substantially as hereinbefore described and with reference to the examples.