NO302300B1 - Procedure for reducing wear on surfaces subjected to frictional forces - Google Patents

Description

The present invention relates to a method for reducing wear on surfaces that are exposed to frictional forces, particularly between moving surfaces.

The primary purpose of lubricants is separation of the moving surfaces to minimize friction and wear. Several distinct systems are commonly recognized within the lubricant field. In fluid film lubrication, the load is thus supported entirely by pressure in the separating fluid film. This film pressure is often generated by relative motion of the involved surfaces, which pumps lubricant into a converging, wedge-shaped zone. The hydrodynamic behavior of such bearings is completely dependent on the viscous behavior of the lubricant. Both load-supporting oil film pressure and power loss are functions of lubricant viscosity in combination with geometry and shear rate imposed by bearing operating conditions.

As difficulties under operating conditions increase, eventually a point will eventually be reached where the load can no longer be carried out completely with oil film support. Spots or sharps on the matched surfaces must then cut with the lubricant in load support and the lubricating system changes from full-film to mixed-film and then to full limit high load, low speed, low viscosity lubricant, misalignment, high surface roughness and adequate supply of lubricant. With boundary lubrication, chemical additives in the lubricating composition and chemical metallurgical agents and mechanical factors involving the two scrubbing surfaces will determine the degree of wear and the degree of friction.

Under boundary conditions of lubrication, metal contact through the oil film results in connections between sharps and subsequent metal stripping on a microscopic scale. As the load increases, these contacts occur more often and result in softer deformation, higher temperatures and welding where eventual burning occurs on a gross and destructive scale. Hypoid gears, since they impose severe slip conditions in combination with high contact pressure, are particularly susceptible to this type of damage. Other lubricant films that are normally present become ineffective under the intense heat that leads to very high surface temperatures.

To combat welding under such extreme conditions, extreme pressure lubricants were developed. Such lubricants contain additives that react at the high contact temperatures and form high-melting inorganic lubricant films on the metal surfaces that prevent massive welding and breakdown. In general, these additives consist of sulfur, chlorine, phosphorus, and lead compounds that react either by producing low shear layers to minimize metal stripping or by serving as fluxing agents to contaminate the metal surface and prevent welding. Since all the extreme pressure additives are affected by chemical action, ie the formation of covalent bonds, their use is generally avoided to eliminate possible corrosion difficulties.

Dry slip, involving solid-to-solid contact does not infrequently exist, even when sufficient fluid film lubricant is produced. Dry slipping can e.g. takes place during start-up of a machine, with misalignment or insufficient clearance during run-in, during a change of direction, and during any delay or interruption in the supply of the lubricant fluid. When conventional oils or greases cannot be used due to extreme temperatures, high vacuum, radiation or contamination, thin coatings of the dry lubricant have been applied to reduce the higher friction and more extensive wear that otherwise results from scrubbing the structural materials against each other.

Thus, the goal of the lubricant is the elimination of this wear and minimize the friction that otherwise occurs in dry sliding. Although this can be accomplished by complete separation of the scrubbing surfaces as with a full film of fluid lubricant, generally such complete separation is not possible under all working conditions. As a result, surface chemical effects have become relevant in boundary lubricants to reduce the friction and wear that occurs in boundary lubricants. Thus, anti-wear agents have been added to the liquid lubricants which produce a surface film on the sliding parts by either a chemical or physical adsorption mechanism, where films reduce friction under boundary lubrication conditions.

A wide variety of compounds have been used to improve lubricants under boundary film conditions. Thus, there have been used compounds containing oxygen, such as fatty acids, esters and ketones, compounds containing sulfur or combinations of oxygen and sulfur, organic chlorine compounds, such as chlorinated waxes, organic sulfur compounds, such as sulfur-treated fats and sulfur-treated olefin compounds containing both chlorine and sulphur, organic phosphorus compounds such as tricresyl phosphate, thiophosphates and phosphites and also organic lead compounds.

When boundary lubrication conditions are mild, polar additives having a polar group at one end of the molecule and a solubilizing group at the other—usually a long-chain hydrocarbon to affect solubility in the lubricating oil—have been used to produce an adherent adsorbed film over metallic surfaces. A class of heterocyclic compounds useful as additives that produce friction modification and improved fuel economy is described in WO 87/0596 and has the general formula:

where Z is PR or PRA, where A is 0 or S and R is H, alkyl, alkenyl, hydrocarbylacyl, hydrocarbylphenolate or -(CH2)mQ, where m is 1 to about 12, and O is O-alkyl or N-alkyl, X is independently E, COOH; NH 2 , CONH 2 , NENH 2 , OR, COR, NHR, OH, SH or CN where R is the same as defined above; p is 0 to 2; e is 0 to 2 where e+p is 2 to approx. 4; T is NH 2 , NHR where R is the same as defined above, SH, OH or tautomers, hydrocarbyl acyl or hydrocarbyl phenolate; and Y is

or CH2NH2, where

A is the same as defined above. Such adsorbed films of additives have until now only been successful under relatively mild boundary lubrication conditions, primarily because the thickness of such films is very low and usually in the order of one nanometer. Under more severe conditions in the boundary lubrication, substances such as tricresyl phosphate or zinc dialkyldithiophosphates have been found to be necessary. In extreme scrubbing conditions with severe metal-to-metal contact, active sulphur, chlorine and lead compounds have been found to be essential. However, such additives react chemically to form low shear surface layers such as lead sulphide ion chloride or ion sulphide. This surface layer thus prevents destructive welding, extensive metal transfer and severe surface degradation. However, such chemical reactivity with the surfaces of the sliding parts is not generally desirable and is only undertaken when no other alternatives are available.

As indicated, the polar type of compound which forms an adherent adsorbed film over the moving surfaces is most preferred. The thickness of such films which has been possible by the use of heretofore known additives in the lubricating compositions has produced insufficient thickness of adsorbed film to function under any conditions other than mild conditions.

It is an aim of the present invention to produce a lubricating system whereby adherent adsorbed films with polar material are produced up to 1000 times thicker than what has been possible until now.

The present invention relates to a radical advantage in lubrication by producing a system where multi-molecular layers are adsorbed on the surfaces to be protected and thus enable relatively thick protective films to be built up on the surfaces which are subject to frictional wear. It has now been found that certain molecules have the property of forming such multi-molecular layers when brought into contact with the surface which, when incorporated into a carrier which is continuous or periodically brought into contact with at least part of the surface to be protected . The molecules that have been found to have this property are essentially simple or condensed unsaturated ring systems comprising at least one six-membered unsaturated heterocyclic ring comprising at least one heterocyclic part which functions as a hydrogen acceptor, the molecule also comprises at least one hydrogen donor part. The molecules may comprise other five- or six-membered unsaturated rings which, together with said six-membered unsaturated heterocyclic ring, form a condensed ring system.

More specifically, the invention relates to a method for reducing wear on a surface that is exposed to frictional forces and comprises forming and maintaining on this surface a protective layer, characterized in that the protective layer, which is a multi-molecular layer, is formed on the surface which is to be protected by contacting at least a portion of this surface with a mixture comprising a carrier and dissolving and/or dispersing therein an effective amount of a heteropolar compound comprising at least one fully unsaturated heterocyclic six-membered ring wherein at least one unsaturated heteroatom part acts as a hydrogen acceptor and where the compound also comprises at least one hydrogen donor part, and in which the heteropolar compound has no substituent which alone or together with another substituent or substituents creates such steric hindrance and/or makes the molecule so basic or acidic or changes it steric geometry of the molecule so that it prevents interaction of the hydrogen donor and the acceptor parts of a molecule in the heteropolar compound with the hydrogen donor or the acceptor parts of another molecule in said heteropolar compound or any other substituent which alone or together with another substituent or substituents has the effect of solubilizing the heteropolar compound in a selected carrier in to the extent that migration of the heteropolar compound to the boundary region between the carrier and the carrier environment is prevented.

The multi-molecular layers in the lubrication system are built up by initial adsorption of a layer of molecules on the surface to be protected followed by adsorption of further molecules onto the initial layers to form a second layer and still further adsorption to form further layers until films of up to about. 1 micrometer thick are formed. Without being bound by theory, it is believed that the presence of both hydrogen donor and hydrogen acceptor parts in the heteropolar molecules enables this adsorption to take place.

Although unsubstituted heteropolar molecules are preferred, substituents may be present on the heteropolar molecules provided they do not individually or collectively hinder interaction of the hydrogen donor and acceptor moieties as by steric hindrance. Thus, e.g. hydrocarbon substituents such as alkyl groups, preferably not containing more than four carbon atoms, preferably not more than two carbon atoms. When the substituent is ortho to any of the heteroatom or hydroxyl group, the steric hindrance effect is likely to be greater than when said substituent is in the meta or para position to any of the heteroatom or a hydroxyl group. Alkene and alkyne substituents, carboxyl containing and amine containing substituents will all affect the activity of the heteropolar molecules and should be avoided.

Formation of the multi-molecular layer of heteropolar molecules can take place by incorporating the heteropolar compound into a carrier which is brought into contact with the surface to be lubricated. It has been found that the heteropolar molecules move through the carrier to the surface to be coated and build up on that surface to form multi-molecular layers. The carrier may be a liquid such as an oil or fat or may even be aqueous. Solid carriers are also feasible, such as polyamide plastic such as those used to build up worn machine parts such as drive shafts and the like. Incorporation of a heteropolar compound into plastic material enables a multi-molecular layer of heteropolar molecules to form, not only on the surface of the plastic by movement through the plastic material, but also by transfer from the plastic surface to another surface that screws against the plastic surface.

It has also been found that the heteropolar molecules move laterally across the surfaces as they are adsorbed below the contact boundaries of the surface with the support material. Contact of the carrier with the entire surface to be protected is therefore not necessary to form a lubricating layer of heteropolar molecules over the entire surface to be protected. It is also not necessary to have continuous contact between the carrier and the surface to be treated, but periodic contact is also effective. The multimolecular layer is naturally not formed instantly, but is built up over a period of time. Relative movement of carrier and surface to be protected accelerates formation and maintenance of the multi-molecular layer of heteropolar molecules on the surface to be protected.

The heteropolar molecules move through the carrier to the interface of the carrier with the surrounding environment. Unsubstituted heteropolar heterocyclic unsaturated simple or fused ring systems having the aforementioned hydrogen donor and acceptor moieties have this motion property. Any substituent in such heteropolar molecules should not exhibit such a solubility effect on the heteropolar molecules that they lose their ability to move through the support to the interfaces in the support's environment. Since a main application is in oils and fats, it is essential that the molecules do not exhibit such a solubility effect that they fail to move. Where they must be added to oils and fats, each substituted grouping should consequently not "oversolubilize" the molecule. Therefore, hydrocarbon substituents should preferably contain no more than 4 carbon atoms, preferably no more than 2 carbon atoms.

The carrier may be a liquid such as a lubricating oil or hydrocarbon fuel for an internal combustion engine or aqueous system, or the carrier may be a grease or semi-solid (non-Newtonian fluid) such as a lubricating grease. or grease-like lubricant. The carrier can also be a solid substance such as a plastic composition, e.g. a polyamide that is used in the repair or reconstruction of corroding surfaces. In the case of liquids, the content of heteropolar compound can be from 0.5% to 4% by weight based on the combined weight of carrier and additive and in the case of non-Newtonian fluids can be from 3% to 10% by weight based on total weight of carrier plus additive. It is advantageous in the case of liquid that the heteropolar compound content is higher than 1$, e.g. from 1.1$ to 4 weight-# based on combined weight of carrier and additive. The concentration required in a solid carrier will depend on the type of solid carrier involved. In the cases of polyamides, somewhat more additive is generally required than is required in a semi-solid for equivalent results. This is the case for a "polyamide" which is 10 weight pounds based on the total weight of "polyamide" and additive. However, quantities greater than 1056 are preferred, e.g. 10.156 to 2096.

The preferred hydrogen acceptor moiety is one involving nitrogen as the heteroatom in the form of a -N= moiety. The preferred hydrogen donor moiety is a hydroxyl group. Both such parts occur in the preferred heteropolar compound of the invention which is 8-hydroxyquinoline:

The condensed ring system in the heteropolar compounds useful in the invention may contain up to four -N= moieties, with preferably up to two such moieties being incorporated as ring-forming atoms in any ring. Other unsubstituted heteropolar compounds useful in the method of the present invention include:

As previously indicated, the preferred heteropolar compounds are unsubstituted materials. Substituents should not create steric hindrance that prevents interaction of the hydrogen donor and hydrogen acceptor moieties. Thus, the purpose of a methyl group ortho to the -N=-hydrogen acceptor moiety in 8-hydroxyquinoline is to form the compound:

does not materially affect the activity of this heteropolar molecule in forming an adsorption film on the metal surfaces. The number and size of the substituents that can be tolerated in the heteropolar molecule depends on the number and position of the hydrogen donor and acceptor parts in the molecule. In general, the substituent groups should not exceed four atoms in number (eg in the case of the hydrocarbyl butyl group), preferably no more than two atoms and most preferably only one carbon atom.

A good indication of whether steric hindrance is likely to cause problems is given by the measurements of the adsorption free energy of the compound in question. If the free energy of adsorption as measured on a copper surface is essentially in the range of 3 to 6 Kcal/mol, steric hindrance is not likely to be a problem.

The invention will further be illustrated with reference to the following examples which are purely illustrative. In each of the examples, the heteropolar compound is 8-hydroxyquinoline.

Example 1

0.5 wt # heteropolar compound was incorporated into SAE 30 engine oil which was then used in a test bed fully instrumented Whirlepeel system. The following results were obtained as shown in Tables 1 to 5.

The following examples show EP effect when adding heteropolar compound to different lubricants.

Example 2

Extremtrvkk (EP) effect

Shell four-ball machine

Lubricant medium: Lithium grease with 3 wt-56 heteropolar compound.

"Without heteropolar welding at 2 7 -2.2<7>N

With heteropolar without welding at 3<7>N

Example 3

Falex lubricant testers

To LP Test 241/69 with 0.5 wt-56 heteropolar compound.

Note: Jaw load 1000N increases for a period of* one

minute.

Example 4

Composite materials

"Polyamide" stock with 1056 M0S2 composite additive, compared to a similar stock containing 1056 heteropolar compound. The bearing having the heteropolar compound in "polyamide" lowered the friction by 3056 compared to the bearing containing M0S2 additive.

Example 5

Dust reduction. ion

When the heteropolar compound is incorporated into the lubricant in a rear axle differential, the noise decreased by 2dB. When the heteropolar compound was incorporated into the lubricant in a Vauxhall Astra engine, the noise decreased from 86dB to 80dB.

Claims (22)

1. Method for reducing wear on a surface that is exposed to frictional forces and comprises forming and maintaining on this surface a protective layer, characterized in that the protective layer, which is a multi-molecular layer, is formed on the surface to be protected by contacting at least a portion of this surface with a mixture comprising a carrier and dissolving and/or dispersing therein an effective amount of a heteropolar compound comprising at least one fully unsaturated heterocyclic six-membered ring in which at least one unsaturated heteroatom moiety acts as a hydrogen acceptor and where the compound also comprises at least one hydrogen donor part, and in which the heteropolar compound has no substituent which alone or together with another substituent or substituents creates such steric hindrance and/or makes the molecule so basic or acidic or changes the steric geometry of the molecule so that it prevents interaction of the hydrogen donor and the acceptor parts of e t molecule in the heteropolar compound with hydrogen donor or the acceptor parts in another molecule in said heteropolar compound or any other substituent which itself or together with another substituent or substituents has the effect of making the heteropolar compound soluble in a selected carrier to the extent that migration of the heteropolar connection to the boundary region between the carrier and the carrier environment is prevented. 2. Method according to claim 1, characterized in that at least part of the surface to be protected is continuously brought into contact with the mixture. 3. Method according to claim 1, characterized in that at least part of the surface to be protected is periodically brought into contact with the mixture. 4. Method according to claims 1 to 3, characterized in that the heteropolar compound comprises up to three condensed unsaturated rings, and one of these rings is the fully unsaturated heterocyclic six-membered ring. 5. Method according to claim 4, characterized in that one of the condensed rings is a five-membered unsaturated heterocyclic ring. 6. Method according to claim 4, characterized in that all the condensed rings are six-membered unsaturated rings. 7. Method according to any one of claims 1 to 6, characterized in that the carrier is a liquid. 8. Method according to claim 7, characterized in that the liquid is a lubricating oil. 9. Method according to claim 8, characterized in that the lubricating oil comprises at least one unsaturated hydrocarbon. 10. Method according to claim 9, characterized in that the heteropolar compound is present in the range from 0.5% to 4% by weight based on the total weight of carrier and additive. 11. Method according to claim 7, characterized in that the carrier is an aqueous liquid substance. 12. Method according to claim 7, characterized in that the carrier is a liquid hydrocarbon fuel for an internal combustion engine. 13. Method according to any one of claims 1 to 6, characterized in that the carrier is a lubricating fat or lard-like material. 14 . Method according to claim 13, characterized in that the heteropolar compound is present in the range from 3% to 10% by weight based on the total weight of carrier and additive. 15. Method according to any one of claims 1 to 6, characterized in that the carrier is a solid plastic material. 16. Method according to claim 15, characterized in that the solid plastic material is a polyamide. 17. Method according to claim 16, characterized in that the heteropolar compound is present in the range from 10.1$ to 209a based on the total weight of carrier and additive. 18. Method according to any one of the preceding claims, characterized in that the heterocyclic part which functions as a hydrogen acceptor is an -N— part. 19. Method according to any one of the preceding claims, characterized in that the heteropolar compound contains up to four -N= moieties. 20. Method according to any one of the preceding claims, characterized in that the hydrogen donor part is an —OE group. 21. Method according to any one of the preceding claims, characterized in that the heteropolar compound is 8-hydroxyquinoline. 22. Method according to any one of claims 1 to 20, characterized in that the heteropolar compound is selected from 2,3-dihydroxypyridine, 4,6-dihydroxypyrinidine, 2-pteridinol, 2-methyl-8-quinolinol, 2,4-quinolindiol, 2,3-dihydroxyquinoxaline, 2,4-pteridinediol, 6-purinol, 3-phenanthridinol, 2-phenanthrolinol and 2-phenazinol.