GB2137117A - Fluorocarbon Coating Process - Google Patents

Abstract

The invention described is the coating in situ of polytetrafluoroethylene (PTFE) on interacting mechanical parts. A paste of PTFE and mineral oil is prepared and diluted with further mineral oil. The mechanical parts are operated until the parts are burnished with PTFE.

Description

SPECIFICATION Fluorocarbon Coating Process During conventional lubrication when two metal parts, with oil between, begin to slide over each other there is an initial period of high friction called "stiction". This occurs because the sliding velocity is too low to form a supporting oil film. As the sliding speed increases, the viscosity of the oil prevents it from squeezing out. This is called hydrodynamic lubrication because the sliding parts are actually "surfing" on a wedge of viscous fluid.

Ideally, in this kind of lubrication there is no metal-to-metal contact between the sliders. But, as the sliding velocity drops again, the "surfing action" stops as the oil squeezes out, leaving the sliding surfaces to grind over each other in metal to-metal contact. Such action occurs, for example, in an internal combustion engine and there is constant danger from metal-to-metal contact. If the sliding surfaces could be made perfectly smooth, the problem would be simple; but there is no such thing as a perfectly smooth surface. Bearing surfaces, the carefully honed bores of engine cylinders, precision gear-teeth, all of these smooth surfaces when seen under high magnification consist of a mountainous surface of irregularities.

When two such irregular surfaces make contact, the tips of these "mountains" are what make the actual contact. It is known that two metal blocks, sitting one on top of the other, make actual contact over less than 1/10,000 of their total surface. Because of the smallness of the real contact areas, pressure at the "mountain peaks" is very high, enough to cause melting and welding of some of the peaks. With sliding metals, it is the force required to make and break these millions of microscopic welds during the sliding that produces the resistance called friction. The harder the metal surfaces press together, the more the mountain peaks deform, and the greater the area of real contact, thus further increasing friction.

For a long time, lubrication engineers have been aware that considerable reductions in friction and wear could be brought about if the valleys between the peaks could be filled with a solid lubricant, such as rubbing soap over sandpaper.

Polytetrafluoroethylene (PTFE) is one of the most remarkable high polymers created by chemical synthesis, a material with a combination of useful chemical and physical properties which at present cannot be found in any one single material of construction, natural or man-made, and is an excellent solid lubricant, especially when both sliding surfaces are coated. With such a coating, a transfer film is established in which the formation of micro-welds occurs not between peaks of hard metal but between the peaks of soft PTFE causing the friction force to become much lower. Also, the valleys are filled and the surface area increases. Thus, even a light oil film can become effective again, whereas previously the valleys acted like rain grooves in a highway, allowing the light oil to drain away without providing lubrication.

The film that is formed in the engine reduces the pressure required to make the engine run. This reduced friction may allow increased power and/or reduced fuel consumption. Perhaps most important is the obvious reduction in engine wear as more load bearing surface comes into play loaded with a PTFE resin film.

The increasing amount of harmful sulfur in today's crude oil is becoming extremely serious.

Refiners cannot remove enough the meet the criteria for "low sulfur" fuel, or they simply forego the time-consuming fuel quality specifications.

When fuel sulfur combines with moisture, air and the heat of combustion, sulfuric acid is formed.

This acid accumulates, suspended in oil in the engine lubricating system, and chemically will attack cylinder lines, piston rings, exhaust valve guides, thereby deteriorating any metal in the engine. Sulfuric acid is the cause of gum, varnish and carbon in an engine. Unlike other types of engine wear, fuel sulfur damage gives no advance warning and premature engine failure may occur.

High humidity, poor engine adjustment, or increased moisture condensation further increases the daily accumulation of sulfuric acid.

The resistance of PTFE to chemical attack by most highly corrosive chemical is well-known.

Even at elevated temperatures for long periods of time, fuming sulfuric and nitric acids, hydrofluoric acid, strong alkaline solutions have no effect on the polymer.

It is known to apply polytetrafluoroethylene to surfaces of mechanical parts which interact with one another. There are various techniques of applying the coating of PTFE to mechanical parts, for example, a PTFE lining may be preformed and then be inserted into, for example, a cylinder as a lining. PTFE may be baked onto individual parts of an engine. Particular engine parts may be made of PTFE. Alternatively, individual parts of an engine may be sprayed or powder-coated with PTFE and subsequently baked.

According to the present invention, there is provided a method of coating moveable interacting mechanical parts in situ with polytetrafluoroethylene, which method comprises mixing finely divided polytetrafluoroethylene powder with a mineral oil carrier to form a paste and subsequently diluting the polytetrafluoroethylene paste with further mineral oil carrier and applying the diluted material to the mechanical parts in situ and operating them for up to 10 hours in the diluted material.

The present invention is particularly suited to internal combustion engines, gear boxes, etc. The internal combustion engine may be a diesel, petrol, L.P.G., two-stroke, or rotary engine. The process of the present invention is such that the fluorocarbon coating can be applied while the engine is fully assembled. The coating of polytetrafluoroethylene occurs during the running of the engine or gear box. The coating occurs, it is believed, by heat generated through the interaction of mechanical parts, such that the polytetrafluoroethylene is baked or burnished onto the parts of the engine.

To ensure the proper mixing of the polytetra fiuoroethylene powder, the fluorocarbon powder is first wetted thoroughly with mineral oils.

Preferably, this is carried out by using 2025% of mineral oil by volume to 7580% fluorocarbon powder, such that a paste is formed by slow blending. The paste is a premix of the polytetrafluoroethylene/m ineral oil suspension.

Due to the high concentration of the powder in the paste form, the paste is preferably further diluted to a range of 220% by volume of paste to 8098% mineral oil, forming a dispersion.

The mineral oil to be used as the carrier for the polytetrafluoroethylene is preferably one which would normally be used in the internal combustion engine to be treated by the process of the invention. Thus, for a petrol engine, for example, a motor oil with a viscosity of SAE No.

30. In selecting the oil, the important criteria is that the specific gravity of the oil must be sufficient to retain the polytetrafluoroethylene particles in a suspension when used. Thus, the oil must be capable of carrying the particles of poly tetrafluoroethylene around, for example, an internal combustion engine.

The particle size of the polytetrafluoroethylene is not critical. However, should the treating process be for an internal combustion engine, it is preferred that the particle sizes of the polytetrafluoroethylene particles should average less than 1 micron in order that they might pass through the oil filter. When applying the process to other items, it is essential that the particle sizes be small enough to ensure that they are retained in suspension in the carrier so that all parts of the mechanical parts would be coated.

It is possible to use the present invention to coat the mechanical parts of, for example, a gear differential or gear box. In this instance, the viscosity of the oil used would be SAE No. 90.

Because of the higher viscosity of the oil, the wetting procedure for the polytetrafluoroethylene particles is not so critical and a paste as previously described would be prepared and then subsequently diluted in oil to be used in the gear box i.e. SAE 90 oil.

The present invention may be used for other mechanical parts to coat them in situ, for example, hydraulic pumps etc.

Prior to coating the interacting mechanical parts with oil, it is necessary that they are cleaned and that the metal surface is prepared to ensure the maximum attachment of polytetrafluoroethylene to the mechanical parts. Thus, the mechanical parts may be pretreated with solvents such as flushing oil to remove any unwanted or extraneous matter from the mechanical parts prior to the process of the present invention. If a new engine is to be used, it is preferred that the engine be "run-in" prior to the coating process, to ensure that the interacting mechanical parts are suitably bedded and smooth.

While not wishing to be bound by the theory of the present invention, it is believed that the polytetrafluoroethylene is mechanically locked onto the surface of the metal by the normal operation of an engine or gear box. The fine particles of PTFE are first loosely attached to the interacting parts and gradually burnished once the parts are operational onto the metal surface due to the temperature and pressure between the interacting parts to form a smooth surface. As the melting point of polytetrafluoroethylene is higher than the flash point of the lubricating oil, the film of PTFE will remain in its normal solid form, providing the melting point of PTFE is not exceeded. Should the melting point be exceeded, the polytetrafluoroethylene will vapourise and be transferred to a cooler part of the engine where the solid PTFE will form once again.Because of the basic design of mechanical parts, i.e. circular shape of the cylinder or journal bearings, the particles of PTFE will lodge on and burnish into the minute grooves and crevices of those parts to form a semi-permanent or permanent smooth surface and will remain in situ until the melting point of PTFE has been exceeded.

The invention will be further described with reference to the accompanying examples.

EXAMPLE 1 A finely divided polytetrafluoroethylene powder having an apparent density of about 500 g/l and an inherent density of 2.3, with a particle size which averages less than 1 micron was wetted with a base lubricant of SAE viscosity No.

30 motor oil. The oil had a specific gravity at 150 of 0.895 and a flash point of 2340C. Its Kinematic viscosity at 400C was 88 centistokes, and at 1 0000, 11 centistokes. The viscosity index of the oil was 108. The blending took place slowly with a ratio of 20% by volume of oil and 80% by volume of powder to form a paste. After blending, the paste was diluted with a further SAE viscosity No. 30 motor oil in the volume ratio of 10% paste to 90% motor oil to form a uniform dispersion.

The dispersion was poured into an internal combustion engine which had previously been run in and flushed with standard flushing oil. The engine was run for 10 hours or until the oil was contaminated and the dispersion of motor oil/polytetrafluoroethylene was subsequently removed. The engine was then refilled with normal motoroil and used. After treatment, it was found that the fuel consumption of the engine was reduced because the frictional drag of the internal moving parts was reduced. In addition, during subsequent running it was found that the engine had a lower oil consumption, because of increase of the compression ratio. In addition, the operating temperature of the engine was slighly reduced, it is believed because of reduction in friction between the interacting parts of the engine.

EXAMPLE 2 A blended polytetrafluoroethylene/motor oil paste was prepared as indicated in Example 1.

This paste was subsequently diluted with a gear oil having SAE viscosity No. 90 such that one part by volume of paste with 50 parts by volume SAE 90 gear oil was prepared. The oil was inserted in a differential gear box of a car and the car was run for 10 hours. The dispersion was removed and replaced by SAE viscosity No. 90 gear oil.

Although the paste was prepared using SAE No.

30 oil, gear oil may be used. In addition, renewal of the dispersion after running the gear box is not necessarily required until the oil is contaminated in the normal way of operation.

Claims (6)

1. A method of coating moveable interacting mechanical parts in situ with polytetrafluoroethylene, which method comprises mixing finely divided polytetrafluoroethylene powder with a mineral oil carrier to form a paste and subsequently diluting the polytetrafluoroethylene paste with a further mineral oil carrier and applying the diluted material to the mechanical parts in situ and operating them for up to 10 hours in the diluted material.

2. A method as claimed in claim 1 wherein the mechanical parts are an internal combustion engine.

3. A method as claimed in claim 1 wherein the mechanical parts are a gear box or a differential gear box.

4. A method as claimed in any one of the preceding claims wherein the mineral oil used to form the paste has a viscosity of SAE No. 30.

5. A method as claimed in any one of the preceding claims wherein the polytetrafluoroethylene has an average particle size of less than 1 micron.

6. A method as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.