EP0420868A1

EP0420868A1 - Method of for reducing wear on surfaces subjected to frictional forces.

Info

Publication number: EP0420868A1
Application number: EP19890906387
Authority: EP
Inventors: Josef Fodor; Jack Schofield
Original assignee: British Technology Group Ltd
Current assignee: BTG International Ltd
Priority date: 1988-05-18
Filing date: 1989-05-17
Publication date: 1991-04-10
Anticipated expiration: 2009-05-17
Also published as: AU3697789A; NO904941D0; NO302300B1; IE891588L; HU209491B; HUT56389A; NZ229188A; HU893619D0; IE61949B1; AU622912B2; ES2017252A6; GB8811696D0; DK274090A; WO1989011518A3; DE68909236T2; CA1337292C; WO1989011518A2; EP0420868B1; DK274090D0; NO904941L

Abstract

On a mis au point un procédé et des compositions permettant de réduire l'usure sur des surfaces soumises à des forces de frottement. On peut appliquer lesdites compositions lubrifiantes dans un support pouvant être de nature organique ou inorganique. Dans ce système, des couches multimoléculaires sont adsorbées sur les surfaces à protéger, permettant ainsi l'accumulation de films protecteurs relativement épais sur les surfaces soumises à l'usure par frottement. Les molécules ayant cette propriété sont essentiellement des systèmes d'anneaux non saturés simples ou condensés qui comprennent au moins un anneau hétérocyclique non saturé à six éléments comprenant au moins une fraction hétérocyclique agissant en tant que fraction recevant et donnant de l'hydrogène. Si des substituants sont présents, ils ne devraient pas créer d'empêchement stérique et/ou rendre la molécule basique ou acide au point de modifier la géométrie stérique de la molécule et d'empêcher l'interaction des groupes actifs.A method and compositions have been developed for reducing wear on surfaces subjected to frictional forces. Said lubricant compositions can be applied in a support which may be of an organic or inorganic nature. In this system, multimolecular layers are adsorbed on the surfaces to be protected, thus allowing the accumulation of relatively thick protective films on the surfaces subjected to wear by friction. The molecules having this property are essentially simple or condensed unsaturated ring systems which comprise at least one six-membered unsaturated heterocyclic ring comprising at least one heterocyclic moiety acting as a receiving and giving moiety of hydrogen. If substituents are present, they should not create steric hindrance and / or make the molecule basic or acidic to the point of modifying the steric geometry of the molecule and preventing interaction of the active groups.

Description

DESCRIPTION "METHOD OF AND COMPOSITIONS FOR REDUCING WEAR ON SURFACES SUBJECTED TO FRICTIONAL FORCES"

The present invention relates to a method of, and compositions for, reducing wear on surfaces subjected to frictional forces, particularly between moving surfaces .

The primary purpose of lubrication is separation of moving surfaces to minimise friction and wear. Several distinct regimes are commonly recognised in the field of lubrication. Thus in fluid film lubrication the load is supported entirely by pressures within the separating fluid film. This film pressure is frequently generated by relative motion of the surfaces involved, which pumps the lubricant into a converging, wedge-shaped zone. The hydro dynamic behaviour of such bearings is completely dependent on the viscous behaviour of the lubricant. Both the load-supporting oil film pressure and the power loss are functions of lubricant viscosity in combination with the geometry and shear rate imposed by the bearing operating conditions.

As the severity of operating conditioas increases, a point is eventually reached where the load can no longer be carried completely by oil-film support. High spots, or asperities of the mating surfaces must then shear with the lubricant in load support and the lubrication regime shifts from full-film to mixed-film and then to complete boundary high load, low speed, low viscosity lubricant, misalignment, high surface roughness or an inadequate supply of lubricant. With boundary lubrication, chemical additives in the lubricating composition and chemical metallurgical, and mechanical factors involving the two rubbing surfaces will determine the extent of wear and the degree of friction.

Under boundary conditions of lubrication, metal contact through the oil film results in junctions of asperities and subsequent metal tearing on a microscopic scale. As loads increase these contacts become more frequent and result in more plastic deformation, higher temperatures and welding with seizure eventually occurring on a gross and devastating scale. Hypoid gears, since they impose severe sliding conditions in combination with high contact stress are particularly susceptible to this type of damage. The organic lubricant film normally present becomes ineffective under the intense heat which leads to very high surface temperatures.

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

Dry sliding, which involves solid-to-solid contact, not infrequently exists, even when adequate fluid film lubrication is provided. Dry sliding can occur for example, in starting up of a machine, with misalignment or inadequate clearance during run-in, during reversal of direction, and during any delays or interruptions in supply of the lubricating fluid. Where conventional oils and greases cannot be used because of extreme temperatures, high vacuum, radiation, or contamination, thin coatings of dry lubricants have been applied to reduce the higher friction and more extensive wear which otherwise obtain on rubbing the structural materials against each other. Thus the goal of lubrication is elimination of this wear and minimising of friction which would otherwise be encountered in dry sliding. Whilst this can be accomplished by complete separation of the rubbing surfaces as by a full film of fluid lubricant, generally such complete separation is not possible under all working conditions and as a result, surface chemical effects have been brought into play in boundary lubrication to reduce friction and wear which does occur in boundary lubrication. Thus anti-wear agents have been added to liquid lubricants which produce a surface film on the sliding parts by either a chemical or physical adsorption mechanism, the film reducing friction and wear under boundary lubrication conditions.

A wide variety of compounds have been used for improving lubrication under boundary film conditions. Thus compounds containing oxygen such as fatty acids, esters and ketones, compounds containing sulphur or combinations of oxygen and sulphur, organic chlorine compounds such as chlorinated waxes, organic sulphur compounds such as sulphurised fats and sulphurised olefines compounds containing both chlorine and sulphur, organic phosphorous compounds such as tricresyl phosphate, thiophosphates and phosphites and also organic lead compounds have been used. When the conditions in boundary lubrication are mild, polar additives having a polar group at one end of the molecule and a solubilising group at the other - usually a long chain hydrocarbon to effect solubilisation in the lubricating oil, have been used to provide an adherent adsorped film over metallic surfaces. A class of heterocyclic compounds useful as additives which provide friction modification and improved fuel economy are disclosed in WO 87/0596 and have the general formula:

wherein Z is S , NR, PR or PRA, wherein A is O or S and R is H, alkyl, alkenyl, hydrocarbyl acyl, hydrocarbyl phenolate or -(CH₂)_mQ, where m is 1 to about 12, and Q is O-alkyl or N-alkyl, X is independently H, COOH, NH₂, CONH₂, NHNH₂, OR, COR, NHR, OH, SH, or CN wherein R is the same as defined above; p is 0 to 2; e is 0 to 2 wherein e+p is 2 to about 4; T is NH₂, NHR wherein R is the same as defined above, SH, OH or their tautomers, hydrocarbyl

acyl or hydrocarbyl phenolate; and Y is CN, CO₂H or CH₂NH₂ wherein A is the same as defined above. Such adsorped films of additive have hitherto only been successful under relatively mild boundary lubrication conditions, primarily because the thickness of such films is very low and usually of the order of one nanometer. Under more severe conditions of boundary lubrication, substances like tricresyl phosphate or zinc dialkyl dithiophosphates have been found necessary and in extreme rubbing conditions where severe metal-to-metal contact would otherwise be encountered, active sulphur, chlorine and lead compounds have been found essential. Such additives however react chemically to form low shear strength surface layers such as lead sulphide ion chloride or ion sulphide. This surface layer then prevents destructive welding, excessive metal transfer and severe surface breakdown. Such chemical reactivity with the surfaces of the sliding parts however is not in general desirable and is only undertaken when no other alternative is available.

As indicated, the polar type of compound which forms an adherent adsorped film over the moving surfaces is much to be preferred, but the thicknesses of such films which have been possible by the use of hitherto known additives in lubricating compositions have produced insufficient thicknesses of adsorped film to function under any conditions other than mild conditions.

It is an object of the present invention to provide a lubricating regime whereby adherent adsorbed films of polar material are provided up to 1,000 times thicker than have hitherto been possible.

The present invention concerns a radical advance in lubrication by providing a regime in which multimolecular layers are adsorbed onto the surfaces to be protected thus enabling comparatively thick protective films to be built up on sufaces subject to frictional wear. It has now been found that certain molecules have the property of forming such multimolecular layers when contacted with the surface as by incorporation in a carrier which is continuously or intermittently brought into contact with at least a portion of the surface to be protected. The molecules which have been found to have this property are essentially single or condensed unsaturated ring systems which comprise at least one six-membered unsaturated heterocyclic ring comprising at least one heterocyclic moiety which acts as a hydrogen acceptor, the molecule also comprising at least one hydrogen donor moiety. The molecules may comprise other five or six-membered unsaturated rings which together with the said six-membered unsaturated heterocyclic ring form a condensed ring system. The multimolecular layers of the lubricating regime of the present invention are built up by initial adsorption of a layer of molecules onto the surface to be protected followed by adsorption of further molecules onto the initial layers to form a second layer and yet further adsorption to form more layers until films up to about 1 micrometer thick are formed. Without wishing to be bound by theory it is believed that presence of both hydrogen donor and hydrogen acceptor moieties in the heteropolar molecules enables this adsorption to take place.

Whilst unsubstituted heteropolar molecules are preferred substituents may be present on the heteropolar molecules provided they do not singly or collectively prevent interaction of the hydrogen donor and acceptor moieties as by steric hindrance. Thus, for example, hydrocarbon substituents such as alkyl groups should not contain more than four carbon atoms, preferably not more than two carbon atoms. When the substituent is ortho to either the heteroatom or the hydroxyl group the steric hindrance effect is likely to be greater than when said substituent is in the meta or para position to either the heteroatom or a hydroxyl group. Alkene and alkyne substituents, carboxyl containing and amine containing substituents will all effect the activity of the heteropolar molecules and should be avoided. In one embodiment of the invention therefore, a method is provided of reducing wear on a surface which is subject to frictional forces which comprises forming and maintaining on said surface a protective layer composed of two or more molecular layers of a heteropolar compound comprising at least one unsaturated heterocyclic six-membered ring in which at least one heteroatom moiety acts as a hydrogen acceptor and in which said compound also comprises at least one hydrogen donor moiety, said heteropolar compound having no substituent which by itself or together with another substituent or substituents creates such steric hindrance and/or renders the molecule so basic or acidic or so alters the steric geometry of the molecule as to prevent interaction of the hydrogen donor and acceptor moieties of one molecule of heteropolar compound with the hydrogen donor and acceptor moieties of another molecule of said heteropolar compound.

Formation of the multimolecular layer of heteropolar molecules may be effected by incorporating the heteropolar compound in a carrier which is brought into contact with the surface to be lubricated. It has been found that the heteropolar molecules migrate through the carrier onto the surface to be lubricated and build up on that surface to form multimolecular layers. The carrier may be a liquid such as an oil or grease or may even be aqueous. Solid carriers are also feasible such as polyamide plastics such as those used to build up worn machinery parts such as drive shafts and the like. Incorporation of a heteropolar compound in the plastic material enables a multimolecular layer of heteropolar molecules to form not only on the surface of the plastic by migration through the plastic material but also by transfer from the surface of the plastic to another surface which rubs against the surface of the plastic.

It has also been found that the heteropolar molecules migrate laterally over the surface on which they are adsorbed beyond the boundaries of contact of that surface with the carrier material. Contact of the carrier with the whole of the surface to be protected is not therefore necessary in order to form a lubricating layer of heteropolar molecules over all the surface to be protected. Nor is it necessary to have continuous contact between carrier and surface to be treated, but intermittent contact is also effective. The multimolecular layer is not of course formed instantaneously but builds up over a period of time. Relative movement of carrier and surface to be protected accelerate the formation and maintainance of the multimolecular layer of heteropolar molecules on the surface to be protected. The heteropolar molecules migrate through the carrier to the interfaces of the carrier with the surrounding environment. Unsubstituted heteropolar heterocyclic unsaturated single or condensed ring systems having the aforementioned hydrogen donor and acceptor moieties have this property of migration. Any substituents in such heteropolar molecules should not exert such a solubilizing effect on the heteropolar molecules that they lose their ability to migrate through the carrier to the interfaces of the carrier's environment. Since a major application of the compounds of the invention is in oils and greases it is essential that the molecules should not exert such a solubilizing effect that they fail to migrate. Consequently, where they are to be added to oils and greases any substituted groupings should not "over solubilize" the molecule. Therefore hydrocarbon substituents should not contain more than 4 carbon atoms, preferably not more than 2 carbon atoms.

According to another embodiment of the present invention there is provided a composition having anti-surface wear properties comprising a carrier and dissolved and/or dispersed therein an effective amount of a compound having anti-surface wear properties characterised in that said compound is a heteropolar compound comprising at least one wholly unsaturated heterocyclic six-membered ring in which at least one unsubstituted heteroatom moiety acts as a hydrogen acceptor and in which said compound also comprises at least one hydrogen donor moiety and in which said heteropolar compound has no substituent which by itself or together with another substituent or substituents creates such steric hindrance and/or renders the molecule so basic or acidic or so alters the steric geometry of the molecule as to prevent interaction of the hydrogen donor and acceptor moieties of one molecule of the heteropolar compound with the hydrogen donor and acceptor moieties of another molecule of the heteropolar compound nor any substituent which by itself or together with another substituent or substituents has the effect of solubilizing said heteropolar compound in the selected carrier to the extent that migration of the heteropolar compound to the interfaces of the carrier with the carrier environment is prevented.

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 material (non-Newtonian fluid) such as a lubricating grease or grease-like lubricant. The carrier may also be a solid such as a plastics composite, e.g. a polyamide used in repairing or rebuilding beading surfaces. In the case of liquid the content of heteropolar compound may be from 0.5% to 4% by weight based on the total weight of carrier and additive and in the case of greases or non-Newtonian fluids may be from 3% to 10% by weight based on the total weight of carrier plus additive. Preferably, in the case of a liquid the content of heteropolar compound is greater than 1%, e.g. from 1.1% to 4% by weight based on the total weight of carrier and additive. The concentration necessary in a solid carrier will depend on the type of solid carrier involved. In the case of polyamides somewhat more additive is in general necessary than that required in a semi-solid for equivalent results. This is the case of a 'Polyamid' bearing 10% by weight based on the total weight of 'Polyamid" and additive was found satisfactory. However amounts of greater than 10% e.g. 10.1% to 20% are preferred.

The preferred hydrogen acceptor moiety is one involving nitrogen as the heteroatom in the form of an -N= moiety. The preferred hydrogen donor moiety is a hydroxyl group. Both such moieties 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 one ring. Other unsubstituted heteropolar compounds useful in the method and compositions of the present invention include:

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

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

A good indication of whether steric hindrance is likely to cause problems is given by measuring the adsorption-free energy of the compound in question. If the adsorption-free energy as measured on a copper surface is substantially in the range of 3 to 6 Kcal/mol then steric hindrance is unlikely to be a problem.

The invention will be further illustrated by reference to the following Examples which are purely illustrative. In each of the examples the heteropolar compound was 8-hydroxyquinoline. EXAMPLE 1

0.5% by weight of heteropolar compound was incorporated in 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.

Table 3

WEAR RATES

WITHOUT HETEROPOLAR 0.006g Fe/hour

WITH HETEROPOLAR 0.003g Fe/hour

Table 4

FRICTION

WITHOUT HETEROPOLAR 170.5 kPa FRICTION MIDDLE PRESSURE

WITH HETEROPOLAR 162.5 kPa " " " Table 5 EMISSION (Bosch Units

(a) Ground Revs (Tick-over speed)

(b) Full Throttle (Revs) 1.5 MAX PERMISSIBLE VALUE

The following Examples show the EP effect of adding the heteropolar compound to different lubricants.

EXAMPLE 2

EXTREME PRESSURE (EP) EFFECT

Shell Four Ball Machine

Lubricating Medium: Lithium grease, with 3% by weight of heteropolar compound.

WITHOUT HETEROPOLAR welding at 2⁷ - 2.2⁷N

WITH HETEROPOLAR no welding at 3⁷N

EXAMPLE 3

FALEX LUBRICANTS TESTER

To. L.P. Test 241/69 with 0.5% by weight of heteropolar compound.

Note: Jaw Load 1000N increments for a period of one minute. EXAMPLE 4

COMPOSITE MATERIALS

'Polyamid' Bearing with 10% of M_oS₂ composite additive, compared with a similar bearing containing 10% of heteropolar compound. The bearing having the heteropolar compound in the 'Polyamid' lowered the friction by 30% as compound to the bearing containing the M_oS₂ additive.

EXAMPLE 5 NOISE REDUCTION

When the heteropolar compound was incorporated in the lubricant in a back axle differential gearing the noise reduced by 2dB. When the heteropolar compound was incorporated in the lubricant in a Vauxhall Astra engine the noise decreased by 86dB to 80dB. ......................................................

Claims

CLAIMS 1. A composition having anti-surface wear properties comprising a carrier and dissolved and/or dispersed therein an effective amount of a compound having anti-surface wear properties characterised in that said compound is a heteropolar compound comprising at least one wholly unsaturated heterocyclic six-membered ring in which at least one unsubstituted heteroatom moiety acts as a hydrogen acceptor and in which said compound also comprises at least one hydrogen donor moiety and in which said heteropolar compound has no substituent which by itself or together with another substituent or substituents creates such steric hindrance and/or renders the molecule so basic or acidic or so alters the steric geometry of the molecule as to prevent interaction of the hydrogen donor and acceptor moieties of one molecule of the heteropolar compound with the hydrogen donor and acceptor moieties of another molecule of the heteropolar compound nor any substituent which by itself or together with another substituent or substituents has the effect of solubilizing said heteropolar compound in the selected carrier to the extent that migration of the heteropolar compound to the interfaces of the carrier with the carrier environment is prevented.

2. A composition as claimed in claim 1, in which the heteropolar compound comprises up to three condensed unsaturated rings, one of which rings is said wholly unsaturated heterocyclic six-membered ring.

3. A composition as claimed in claim 2, in which one of said condensed rings is a five-membered unsaturated heterocyclic ring.

4. A composition as claimed in claim 2, in which all the condensed rings are six-membered unsaturated rings.

5. A composition as claimed in any of claims 1 to 4, in which the carrier is a liquid.

6. A composition as claimed in claim 5, in which the liquid is a lubricating oil.

7. A composition as claimed in claim 6, in which the lubricating oil comprises at least one unsaturated hydrocarbon.

8. A composition as claimed in claim 7, wherein the heteropolar compound is present in the range 1.1% to 4% by weight based on the total weight of carrier and additive.

9. A composition as claimed in claim 5, in which the carrier is an aqueous liquid.

10. A composition as claimed in claim 5, in which the carrier is a liquid hydrocarbon fuel for an internal combustion engine.

11. A composition as claimed in any of claims 1 to 4, in which the carrier is a lubricating grease or grease-like material.

12. A composition as claimed in claim 11, wherein the heteropolar compound is present in the range 3% to 10% by weight based on the total weight of carrier and additive.

13. A composition as claimed in any of claims 1 to 4. in which the carrier is a solid plastics material.

14. A composition as claimed in claim 13, in which the solid plastics material is a polyamide.

15. A composition as claimed in claim 14, wherein the heteropolar compound is present in the range 10.1% to 20% based on the total weight of carrier and additive.

16. A composition as claimed in any of the preceding claims, in which the heterocyclic moiety which acts as a hydrogen acceptor is an -N= moiety.

17. A composition as claimed in claim 16, in which the heteropolar compound contains up to four -N= moieties.

18. A composition as claimed in any of the preceding claims, in which the hydrogen donor moiety is an -OH group.

19. A composition as claimed in any of the preceding claims, in which the heteropolar compound is 8-hydroxyquinoline.

20. A composition as claimed in any of claims 1 to 18, in which the heteropolar compound is selected from 2,3-dihydroxypyridine, 4,6-dihydroxypyrinidine, 2-pteridinol, 2-methyl 8-quinolinol, 2,4-quinolindiol, 2,3-dihydroxyquinoxalin, 2,4-pteridinediol, 6-purinol, 3-phenanthridinol, 2-phenanthrolinol and 2-phenazinol.

21. A method of reducing wear on a surface which is subject to frictional forces which comprises forming and maintaining on said surface a protective layer composed of two or more molecular layers of a heteropolar compound comprising at least one unsaturated heterocyclic six-membered ring in which at least one heteroatom moiety acts as a hydrogen acceptor and in which said compound also comprises at least one hydrogen donor moiety, said heteropolar compound having no substituent which by itself or together with another substituent or substituents creates such steric hindrance and/or renders the molecule so basic or acidic or so alters the steric geometry of the molecule as to prevent interaction of the hydrogen donor and acceptor moieties of one molecule of heteropolar compound with the hydrogen donor and acceptor moieties of another molecule of said heteropolar compound.

22. A method as claimed in claim 21, in which the protective layer is formed on the surface to be protected by contacting at least a portion of the said surface with a composition as claimed in any of claims 1 to 20.

23. A method as claimed in claim 22. in which at least a portion of the surface to be protected is continuously contacted with said composition.

24. A method as claimed in claim 22, in which at least a portion of the surface to be protected is intermittently contacted with said composition. .......................................................