GB2541168A

GB2541168A - Process for preparing a coating

Info

Publication number: GB2541168A
Application number: GB1513166.7A
Authority: GB
Inventors: Nedelcu Ileana; Allen Warrick
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2015-07-27
Filing date: 2015-07-27
Publication date: 2017-02-15
Also published as: GB201513166D0

Abstract

The invention relates to a process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component. In addition, the invention relates to (i) a mechanical component (e.g. a steel rolling element bearing) obtainable by the present process, and (ii) the use of a protective coatings as prepared in the present process for reducing friction between the mechanical component and a further mechanical component that is in dynamic contact with the mechanical component.

Description

PROCESS FOR PREPARING A COATING FIELD OF THE INVENTION

The present invention relates to a process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material; a mechanical component obtainable by the present process; and (ii) the use of a protective coatings as prepared in the present process for reducing friction between the mechanical component and a further mechanical component that is in sliding contact with the mechanical component.

BACKGROUND OF THE INVENTION

Bearing arrangements consist of bearings, as well as shafts, housings and seals. A bearing is a device that permits constrained relative motion between two components. Bearings may be used in many different types of machinery to retain and support rotating components such as, for example, a wheel on a vehicle, a vane on a windmill or a drum in a washing machine. A rolling element bearing comprises inner and outer rings and a plurality of rolling elements (balls or rollers). As the bearing rotates, the raceways of the inner ring and outer ring make contact with the rolling elements. This results in a wear path on both the rolling elements and the raceways. For a variety of reasons it sometimes happens that a bearing does not attain its calculated rating life. Rolling fatigue can, for instance, occur as a result of wear and fretting. A seal is used in association with a bearings to enhance the performance and life of bearings, by retaining lubricant keeping out contaminants. Seals are also prone to wear and fretting as a result of the friction to which they are exposed. Similar type of friction problems occur in general when two surfaces are in sliding contact with each other, for example when plain bearings are in sliding contact or when a bushing is in sliding contact with another surface.

The minimization of wear and fretting is therefore important to extend the fatigue life of bearings and seals. One way of extending the fatigue life of a bearing is to apply lubricating oils and greases which, by means of an oil film, reduce the friction and wear in the contact areas of the respective bearing components. A disadvantage of such lubrication is the fact that the oil film is not present from the start of the use of the bearing and that formation of the layer only occurs when a particular temperature has been reached.

Object of the present invention is to provide a mechanical component derived from a non-elastomeric material which displays low friction when in dynamic contact with a further component.

SUMMARY OF THE INVENTION

It has now been found that this object can be realized when use is made of a particular coating which is applied on a mechanical component such as a bearing ring, a seal or a bushing.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

In accordance with the present invention a new approach is provided to combat the effects of wear and fretting on mechanical components such as bearings and bushings, and which deals with the disadvantages discussed above. In particular, the present invention provides a mechanical component for use in bearing applications, wherein a tribological surface of the component is provided with a coating in which polyurethanes have been formed. Hence, a protective coating is already present before operating the bearing, resulting in an improved fatigue life of the bearing and/or seal.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention a coating is prepared on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material. The non-elastomeric material to be used is suitably selected from the group consisting of ceramic materials, chrome steels, stainless steels and carbon alloy steels. Examples of suitable ceramic materials include zirconia and aluminium oxides, silicon carbide, boron nitride, and in particular silicon nitrides-Examples of suitable stainless steels include martensitic grades such as 410 and 41 OH, austenitic grades such as 304 and 316, ferritic grades such as 41 OS, duplex grades such as 2205 and precipitation hardened grades such as 17-4 PH. Examples of carbon alloy steels include 1060 and A3 6. Preferably, the non-elastomeric material comprises steel.

In step (a), an aqueous polymer dispersion is applied onto at least part of the tribological surface to form a coating. The aqueous polymer dispersion can be applied onto the whole tribological surface of the mechanical component. An advantage of the present invention is that it is also possible to apply the aqueous polymer dispersion only on that part of the tribological surface where the coating is needed, instead that the complete mechanical component is covered with a coating which is usually the case when use is made of for instance a vapour deposition technique.

In accordance with the present invention, in step (a) the aqueous polymer dispersion is applied onto at least part of the tribological surface may means of a dip coating, contact coating, roller coating or spray coating process. The aqueous polymer dispersion is preferably applied onto at least part of the tribological surface may means of spray, spin or dip coating process or a combination of these.

Suitably, the aqueous polymer dispersion is applied onto at least part of the tribological surface at a temperature in the range of from 5-60 °C. Step (a) can be carried out at a low pressure or a high pressure. Preferably, the aqueous polymer dispersion is applied onto at least part of the tribological surface at a temperature in the range of from 10-40 °C.

The aqueous polymer dispersion comprises an isocyanate which contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes. The isocyanate-reactive polyol comprises a polyol which contains one or more isocyanate-reactive groups. The polyol may also contain other reactive groups. Suitable polyols to be used in accordance with the present invention include 1,4-cyclohexyldimethanol, ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1 ,4-butanediol, 1 ,6-hexanediol, furan dimethanol, cyclohexane dimethanol, glycerol, trimethylolpropan, dimethylol propanoic acid (DMPA), dimethylol butanoic acid (DMBA), polypropylene glycols, poly(propylene oxide/ethylene oxide) copolymers, polytetrahydrofuran, poly butadiene, hydrogenated polybutadiene, poysiloxane, polyamide polyesters, isocyanate-reactive polyoxyethylene compounds, polyester, polyether, polyether ester, polycaprolactone, polythioether, polycarbonate, polyethercarbonate, polyacetal and polyolefin polyols. Polyether polyols which may be used include products obtained by the polymerisation of a cyclic oxide, for example ethylene oxide, propylene oxide or tetrahydrofuran or by the addition of one or more such oxides to polyfunctional initiators, for example water, methylene glycol, ethylene glycol, propylene glycol, diethylene glycol, cyclohexane dimethanol, glycerol, trimethylopropane, pentaerythritol or Bisphenol A. Preferred polyether polyols include polyoxypropylene diols and triols, poly (oxyethylene-oxypropylene) diols and triols prepared by the simultaneous or sequential addition of ethylene and propylene oxides to appropriate initiators and polytetramethylene ether glycols obtained by the polymerisation of tetrahydrofuran.

Preferred polyols are acrylic polyols. Acrylic polyols are polyols which are obtained by radical copolymerisation of acrylic monomers (ternary or quaternary copolymers), such as acrylic or methacrylic acids and esters. The acrylic polyols may be obtained from the copolymerization of conventional acrylic monomers, such as ethyl acrylates (EA) or butyl acrylates (BA), acrylic acid (AA), methyl methacrylate (MMA), or styrene (ST) with hydroxylated acrylic monomers such as 2-hydroxy ethyl acrylates (HEA) or 4-hydroxybutyl acrylates (HBA). The preferred acrylic polyols to be used is accordance with the present invention include hydroxyl functional polyacrylic dispersions such a Bayer’s A2695 and A2058.

In step (a), a mixture of two or more isocyantane-reactive polyols can be applied. For example, a mixture of one or more triols and one or more diols can be used. The weight average molecular weight of the isocyanate-reactive polyol is suitably in the range of from 500-6000 Daltons, preferably in the range of from 500-3000 Daltons

The isocyanate to be used in accordance with the present invention contains two or more isocyanate groups per molecule. Hence, the isocyanate comprises a di-isocyanate or a polyisocyanate. Preferably, the isocyanate comprises a polyisocyanate. The polyisocyanates may be aliphatic, cycloaliphatic, araliphatic, aromatic and/or polyisocyanates modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine, urethdione or isocyanurate residues. Examples of suitable polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane-1,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, p-xylylene diisocyanate, α,α'-tetramethylxylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanates, 2,4'-diphenylmethane diisocyanate, 3(4)-isocyanatomethyl-l-methyl cyclohexyl isocyanate, and 1,5-naphthylene diisocyanate. Preferred polyisocyanates are isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, toluenediisocyanate and 4,4'-diphenylmethane diisocyanate.

Suitably, a mixture of two or more of any of the above-mentioned polyisocyanates can be used. It is also possible to use a mixture of an isocyanate which contains two or more isocyanate groups per molecular and one or more isocyanates that contain one isocyanate group per molecule.

The isocyanate that contains two or more isocyanate groups per molecule is suitably present in the aqueous polymer dispersion an amount in the range of from 15-60 wt%, preferably in an amount in the range of from 20-40 wt%, based on the total weight of the aqueous polymer dispersion

The aqueous polymer dispersion contains water in an amount in the range of from 10-60 wt%, preferably in the range of from 15-30 w1%, based on the total weight of the aqueous polymer dispersion.

The aqueous polymer dispersion may contain in addition to the water a co-solvent which acts as a solvent with respect to both the isocyanate-recative polyol and the isocyanate.

Hence, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and na isocyanate-reactive polyol to form polyurethanes, and a co-solvent; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

In one embodiment, the solvent is a polar organic solvent. The polar solvent may be a polar aliphatic solvent or polar aromatic solvent, such as an alcohol, a ketone, ester, acetate, glycol ethers, aprotic amide, aprotic sulfoxide, or aprotic amine. Examples of useful solvents include methyl ethyl ketone, methyl isobutyl ketone, m-amyl acetate, ethylene glycol butyl ether-acetate, propylene glycol monomethyl ether acetate, xylene, n-methylpyrrolidone, or blends of aromatic hydrocarbons. In another embodiment, the solvent is water or a mixture of water with small amounts of aqueous co-solvents. Suitable alcohols include ethanol, isopropanol, n-butanol, and n-propanol. Suitable acetates include hexyl acetate, octyl acetate, and glycol ether acetates such as propylene glycol monomethyl ether acetate. Suitable ketones include methyl propyl ketone, methyl isobutyl ketone, and methyl hexyl ketone. Glycol ethers and glycol ether acetates are especially preferred. Further, the solvent may include non-polar aromatic and/or aliphatic solvents.

The co-solvent is suitably present in the aqueous polymer dispersion in an amount up to 40 wt%, preferably in an amount of less than 20 wt%, based on the total weight of the aqueous polymer dispersion.

Preferably, the aqueous polymer dispersion comprises in addition particles of a filler material.

The present invention therefore also relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, and particles of a filler material; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

Such particles suitably have a an average particle size of less than 10 micron, preferably less than 50 micron, more preferably less than 100 nm, and most preferably in the range of from 5-50 nm. These particles will be dispersed in the matrix of the polyurethane formed and will improve the wear resistance of the coating formed on the tribological surface.

The filler material of which the particles are made may be inorganic or organic.

Preferably, the filler material is an inorganic filler material. Examples of suitable inorganic nanoparticles include metal oxides such aluminium oxide, silicon oxide, zinc oxide, iron oxide, titanium dioxide, tin oxide, indium oxide, zirconium dioxide, cerium oxide and mixtures thereof. Other suitable examples of filler materials include metal powders such as copper powders and nickel powders, and silicon carbides. Preferably, the filler material is a silicon oxide. Examples of suitable organic filler materials include for instance carbon nanoparticles.

The polyurethanes formed in step (b) of the process according to the present invention may be homopolymers of urethane or copolymers of urethane. Examples of suitable copolymers of urethane include copolymers of urethane and an ether, copolymers of urethane and a carbonate, and copolymers of urethane and an acrylate. Preferably, the copolymer of urethane is a copolymer of urethane and an acrylate.

In accordance with the present invention the coating as obtained in step (b) suitably contains polyurethanes which contain a low surface tension group which is selected from the group consisting of a fatty acid chain having at least 6 carbon atoms, fluor-containing groups and silicon-containing groups

The silicon atom which is present in the silicon-containing group may be incorporated into the silicon-containing group by means of C-Si bond units, C-O-Si bond units or Si-O-Si bond units. The fluor-containing groups is suitably a fluorocarbon. Such a fluorocarbon suitably contains at least 4 fluorine atoms.

The low surface tension groups present in the polyurethanes will provide a very attractive low friction performance of the coating on the tribological surface of the mechanical component.

Preferably, the polyurethanes in the coating contain one or more fluor-containing groups and/or one or more silicon-containing groups. Preferably, the polyurethanes in the coating contain one or more flour-containing groups and one or more silicon-containing groups.

The fluor-containing groups in the polyurethanes may suitably be derived from the isocyanate-reactive polyol. Suitably, the isocyanate-reactive polyols may therefore contain one or more fluor-containing groups and/or one or more silicon-containing groups.

Preferably, the isocyanate-reactive polyols contain one or more fluor-containing groups and one or more silicon-containing groups.

In another embodiment of the present invention the fluor-containing groups in the polyurethanes may suitably be derived from the isocyanate that contains two or more isocyanate groups per molecule. Suitably, the isocyanates may therefore contain one or more fluor-containing groups and/or one or more silicon-containing groups. Preferably, the isocyanates contain one or more fluor-containing groups and one or more silicon-containing groups.

In a preferred embodiment, the fluor-containing groups that are present in the polyurethanes are derived from a compound which is present in the aqueous polymer dispersion and which compound contains one or more fluor-containing groups.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, and a compound which contains one or more fluor-containing groups; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

Preferably, the silicon-containing groups that are present in the polyurethanes are derived from a compound which is present in the aqueous polymer dispersion and which compound contains one or more silicon-containing groups.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, a compound which contains one or more silicon-containing groups; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

The present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, a compound which contains one or more fluor-containing groups and a compound contains one or more silicon-containing groups; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

The silicon-containing compound which is preferably present in the aqueous polymer dispersion is suitably a siloxane compound, preferably a polysiloxane. Examples of suitable silicon-containing compounds include trimethyl hydroxypropyl silane, heptamethyl hydroxypropyl trisiloxane and hydroxy functional silicone oils like polydimethylsiloxane carbonal terminatedSuitable siloxanes include cyclic siloxanes, polyether siloxanes, acrylic siloxanes, and fluorosiloxanes. Suitable polysiloxanes include acrylated polysiloxanes and a substituted polydimethylsiloxanes such as for example polymethylphenylsiloxane.

Suitable fluorocarbons for purposes of the present invention include perfluorohexan-l-ol, perfluoroethanol, and the reaction product of perfluoroacetic acid with epoxies like Glydexx NIO.

The aqueous polymer dispersion may in addition contain a photo-initiator which will contribute to the forming of polyurethanes in the coating during the curing treatment in step (b) when the curing treatment is a UV treatment.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, and a photo-initatior; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

Isobutyl benzoin ether is a suitable example of a commonly used free radical photoinitiator. A triarylsulfonium hexaflouroantimonate salt is a suitable example of commonly used as a cationic photo-initiator.

Typical photo-initiators to be used in accordance with the present invention may include, for example either a Type I photo-initiator or type II photo-initiator with a synergist.

Examples of type I photoinitiators include α-hydroxy alkylphenols such a 1-hydroxycyclohexyl phenyl ethyl ketone), α-amino alkyl phenones and acyl phosphine oxides (for instance ‘BAPO’ phenylbis(2,4,6-trimethylbenzoyl)phosphorous oxide. Examples of Type II initiators include benzophenones such as 4-dimethylamino benzophenone, thioxanthones such as 2-isopropyl thioxanthone, with synergists that include, for example, ethyl-4-(dimethylamino) benzoate (known as EBD).

The UV treatment will cause the photo-initiators to generate free radicals and cationic particles which initiate the formation of cross-linking bonds among the isocyanate-reactive polyols and the isocyanate but also for instance a siloxane when present. Suitably, a mixture of different known photo-initiators can be used in accordance with the present invention. A photo-initiator will suitably be present in an amount in the range of from 0.1-10 wt%, based on the total weight of the aqueous polymer dispersion. A catalyst can also suitably be present in the aqueous polymer dispersion to be used in accordance with the present invention. The catalyst may catalyze a reaction between the isocyanate-reactive polyol and the isocyanate. In addition, the catalyst may also catalyze other reactions between polymerizable components that are present in the aqueous polymer dispersion.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, and a catalyst; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

Such polymerizable components may include the compound that contains one or more fluor-containing groups and/or one or more silicon-containing groups. Examples of suitable catalysts include catalysts selected from the group consisting of tin catalysts, acid catalysts, acid phosphates, aromatic acids, and combinations thereof. Specific examples of suitable tin catalysts include dibutyltin diacetate (DBTDA) and dibutyltin dilaurate (DBTDL). Specific examples of suitable acid catalysts include sulfonic acids including dodecylbenzene sulfonic acid (DDBSA), dinonylnapthalene sulfonic acid (DNNSA), dinonylnapthalene disulfonic acid (DNNDSA); and p-toluene sulfonamine (PTSA).

In step (b), at least part of the coating as obtained in step (a) is subjected to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component. The curing can be established using heat or UV light.

In case the curing treatment in step (b) is a thermal treatment a temperature is applied in the range of from 50-200 °C, preferably in the range of from 80-150 °C. The thermal treatment can be carried out in manners known to the skilled person. Suitable thermal treatments include the use of a heat source directed at the coating to establish the formation of polyurethanes in the coating on the tribological surface of the mechanical component. For instance, one or more heat sources can be located in the close vicinity of a spraying or other coating device by way of which the aquesous polymer dispersion is applied onto the tribological surface.

In case a UV treatment is used as the curing treatment, UV light is used having a wavelength in the range of from 250-450 nm.

The UV treatment can be carried out in manners known to the skilled person. Suitable UV treatments include the use of UV lamps that radiate light waves of the desired wavelength into the coating to establish the formation of the polyurethanes in the coating on the tribological surface of the mechanical component. For instance, one or more UV lamps can be located in the close vicinity of a spraying or other coating device by way of which the aquesous polymer dispersion is applied onto the tribological surface.

The aqueous polymer dispersion may suitably contain one or more additives such as a flow agent, a wetting agent and a slip or wetting agent.

Accordingly, the present invention relates to process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes, and a flow agent, a wetting agent and/or a slip or wetting agent; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component. A flow agent serves to enable good levelling of the coating, increase its uniformity and can also reduce the tendency for the surface to scratch. Examples of suitable flow agents include poly ethersiloxanes copolymers, polysiloxanes, and polyacrylates. Flow agents can suitably be present in the aqueous polymer dispersion in an amount in the range of from 0.1-1 wt%, based on the total weight of the aqueous polymer dispersion. A wetting agent serves to improve interaction of the coating onto the substrate by modification of its surface energy at that interface and serves to increase adhesion to the substrate. Sometimes they may also offer improvement in flow properties. Examples of suitable wetting agents include anionic surfactants, cationic surfactants, non-ionic surfactants and poly ether siloxane copolymers. Wetting agents can suitably be present in the aqueous polymer dispersion in an amount in the range of from 0.1-1 wt%, based on the total weight of the aqueous polymer dispersion. A slip, glide or anti-friction agent serves to lower the friction of the coating to another surface. Examples of suitable slip or friction agents include siloxanes, poly ether siloxane coploymers and organically modified siloxanes. Slip, glide or anti-friction agents can suitably be present in the aqueous polymer dispersion in an amount in the range of from 0.1-12 wt%, based on the total weight of the aqueous polymer dispersion.

The coating to be prepared with the process in accordance with the present invention has suitably a thickness in the range of from 1-100 micron, preferably in the range of from 5-30 microns.

In accordance with the present invention the mechanical component is preferably a bearing component such as a plain bearing, a seal, a bushing or any other component that is in dynamic contact with another mechanical component. When the mechanical component is a bearing ring of a rolling element bearing, the coating is provided on the raceway of the bearing ring.

Examples

Example 1 10 ml of an acrylic polyol (Bayhydrol A2695, available from Bayer AG) was added to 2 ml acetone (99%) and mixed together for several minutes by stirring. 3.0 ml of a nano-silica (Tego Nanopol C764, available from Evonik Industries AG) was then added and mixed in thoroughly. 1.3 ml of the hydroxy polydimethylsiloxane dispersion Tego Protect N5100 (available from Evonik Industries AG) was then added and mixed thoroughly into the other components for several minutes. 5 ml of a polyisocyanate (Desmodur N3600, available from Bayer AG) was then added and mixed, after which 1.0 ml of deionized water was finally added and thoroughly mixed into the formulation. The mixture was then left to stand for about 20 minutes to allow air bubbles to leave the formulation.

The formulation was then applied as a thin layer on a flat steel substrate at approximately 75 microns thickness and left to dry gradually at room temperature over 24 hours, followed by treatment at 120°C for 3 hours in a fan assisted oven. In this way a thin, translucent coating was produced on the steel.

Dynamic friction in dry conditions was measured using a tribometer [CETR UMT-3] at 25°C against a flat nylon 6,6 test ring at approximately 9MPa contact pressure for a series of increasing rotational speeds at 0.16mm/s, 0.25mm/s, 0.49mm/s, 0.86 mm/s, 1.6 mm/s, 2.5 mm/s, 4.9 mm/s, 8.6 mm/s, 16 mm/s, 25 mm/s, 49 mm/s and 99 mm/s. Temperature was maintained at 25 °C.

The coated steel surface as prepared above was tested and a minimum dynamic friction coefficient of 0.09 was recorded at 8.6 mm/s with a maximum of 0.11 at 0.16 mm/s.

Example 2 5 ml of a polyisocyanate (Desmodur N3600, available from Bayer AG) was thoroughly mixed by stirring with 10 ml of an acrylic polyol (Bayhydrol A2695, available from Bayer AG). 1.3 ml of a hydroxy polydimethylsiloxane dispersion (Tego Protect N5100, available from Evonik Industries AG) was then added and again mixed thoroughly into the other components for several minutes. To this mixture 1 ml deionized water was added and mixed in by stirring to reduce the viscosity. The mixture was then left to stand for 20 minutes

Dynamic friction in dry conditions was measured using a tribometer [CETR UMT-3] at 25°C against a flat nylon 6,6 test ring at approximately 9MPa contact pressure for a series of increasing rotational speeds at 0.16mm/s, 0.25mm/s, 0.49mm/s, 0.86 mm/s, 1.6 mm/s, 2.5 mm/s, 4.9 mm/s, 8.6 mm/s, 16 mm/s, 25 mm/s, 49 mm/s and 99 mm/s. Temperature was maintained at 25°C.

The coated steel surface as prepared in this Example was testes and a minimum dynamic friction coefficient of 0.08 was recorded at 8.6 mm/s with a maximum of 0.10 at 99 mm/s.

Example 3 5 ml of a polyisocyanate (Desmodur N3600, available from Bayer AG) was thoroughly mixed by stirring with 10 ml of an acrylic polyol (Bayhydrol A2695, available from Bayer AG). 1.3 ml of a hydroxy polydimethylsiloxane dispersion (Tego Protect N5100, available from Evonik Industries AG) was then added and again mixed thoroughly into the other components for several minutes. To this mixture 1 ml deionized water was added and mixed in by stirring to reduce the viscosity. The mixture was then left to stand for 20 minutes

The formulation was also applied onto a flat steel test ring in a similar way and polymerised using the same conditions.

The dynamic friction of the two surfaces was then tested using a tribometer [CETR UMT-3] at 25°C at approximately a range of contact pressures from 0.9MPa to 9MPa with rotational speed maintained at 99 mm/s. Temperature was maintained at 25°C.

The coated steel surface as prepared in this Example was tested and a minimum dynamic friction coefficient of 0.05 was recorded at 9MPa with a maximum of 0.07 at 0.9MPa.

Example 4 A thin coating of the polyurethane acrylate dispersion coating as made in Example 1 was made on a steel ring having a 5 cm outer diameter and 3 cm internal diameter. A Mini Traction Machine (MTM2, PCS Instruments) was used to determine the traction friction coefficient of the polyurethane acrylate dispersion coating using a ball on disc arrangement. An uncoated steel ball was used with a contact pressure of 750MPa, a slide-roll ratio of approximately 5%, maintained at 90°C under oil lubricated conditions (Tribol 1510 used). The rolling speed was increased from 50 to 700 mm/s

An average traction coefficient of less than 0.01 was achieved across 100 to 600 mm/s This was repeated using a newly prepared coating on a ring, which gave similar results. A reference test of an uncoated steel ring against the ball was carried out for comparative reasons, using identical test conditions, and gave a traction friction coefficient between 0.03 and 0.04 under the same speed range.

From the results in the above Examples it will be clear that the present invention provides mechanical components which are derived from a non-elastomeric material showing attractive low friction performances. The results of Examples 1-3 show attractive low-friction performance under conditions of sliding contact, while the results of Example 4 show that a beneficial effect can also be achieved under conditions of rolling contact.

Claims

1. A process for preparing a coating on at least part of a tribological surface of a mechanical component which is derived from a non-elastomeric material, which process comprises the steps of: (a) applying an aqueous polymer dispersion onto at least part of the tribological surface to form a coating, which aqueous polymer dispersion comprises an isocyanate that contains two or more isocyanate groups per molecule and an isocyanate-reactive polyol to form polyurethanes; and (b) subjecting at least part of the coating as obtained in step (a) to a curing treatment during which the polyurethanes are formed in the coating on the at least part of the tribological surface of the mechanical component.

2. The process according to claim 1, wherein the coating as obtained in step (b) contains polyurethanes which contain a low surface tension group which is selected from the group consisting of a fluor-containing groups and a silicon-containing groups.

3. The process according to claim 1 or 2, wherein the coating as obtained in step (b) contains polyurethanes which contain a fluor-containing group and/or a silicon-containing group.

4. The process according to claim 3, wherein the coating as obtained in step (b) contains polyurethanes which contain a fluor-containing group and a silicon-containing group.

5. The process according to any one claims 1-4, wherein the polyol and/or isocyanate contain(s) a fluor-containing group and/or a silicon-containing group.

6. The process according to any one of claims 1-5, wherein the aqueous polymer dispersion further comprises a compound that contains a fluor-containing group and/or a compound that contains a silicon-containing group.

7. The process according to claim 6, wherein the aqueous polymer dispersion further comprises a compound that contains a fluor-containing group and a compound that contains a silicon-containing group.

8. The process according to any one of claims 1-7, wherein the silicon-containing group is derived from a siloxane, preferably a polysiloxane.

9. The process according to any one of claims 1-8, wherein at least part of the polyurethane-based polymers are copolymers of urethane and an acrylate.

10. The process according to any one of claims 1-9, wherein the aqueous polymer dispersion further comprises particles of a filler material which particles have an average particle size of less than 100 nm.

11. The process according to any one of claims 1-10, wherein the curing treatment in step (b) is a thermal treatment which is carried out at a temperature in the range of from 50-250 °C.

12. The process according to any one of claims 1-10, wherein the curing treatment in step (b) is a UV treatment in which UV light is used having a wavelength in the range of 250-450 nm.

13. The process according to any one of claims 1-12, wherein the coating obtained has a thickness in the range of from 1-100 microns.

14. A mechanical component obtainable by a process according to any one of claims 1-13

15. Use of a coating as prepared in a process according to any one of claims 1-13 for reducing friction between the mechanical component which is derived from a non-elastomeric material and a further mechanical component that is in sliding contact with the mechanical component which is derived from the non-elastomeric material.

16. Use of a coating as prepared in a process according to any one of claims 1-13 for reducing friction between the mechanical component which is derived from a non-elastomeric material and a further mechanical component that is in rolling contact with the mechanical component which is derived from the non-elastomeric material.