Title: Method for reducing friction

The invention is directed to a method for reducing friction between at least two bodies.

When two bodies slide against each other, the resistance against this sliding is termed friction. Friction is thus the force that resists motion when the surface of one object comes into contact with the surface of another. The level of friction, or the calculated friction coefficient upon measurement, depends on the operational conditions, the interacting surfaces of both objects and the environment. Friction can be a serious nuisance in devices that continuously move, since it constitutes a dissipation of energy. Most of this energy loss appears as heat, while a small proportion induces loss of material from the sliding surfaces, and this eventually leads to further waste, namely, to the wearing out of the whole mechanism.

In order to lower the friction between interacting surfaces of different objects to acceptable values lubricants are conventionally applied. There is considerable interest in developing new kinds of lubricants that produce low friction and maintain this property for long periods of times, thereby reducing maintenance expenses.

Conventional lubricants prevent contact of parts in relative motion, and thereby reduce friction and wear. Typically, oils that are derived from crude petroleum are used as lubricants. It is often desirable to add various additive chemical to lubricating oils. Such additives include viscosity-index improvers, pour-point depressants, antioxidants, anti-wear and friction-reducing additives, and dispersants. Normally, lubricants contain about 90 % base oil and less than 10 % additives. Lubricants do not necessarily have to be liquid. Non-liquid lubricants include grease, powders (such as dry graphite, polytetrafluorethylene (PTFE), molybdenum disulphide, talc, and boron nitride), PTFE tape, air cushion and others.

Although lubricants in general considerably lower the amount of friction, they do not always provide the desired level of friction reduction, even in the presence of additives. Accordingly, lubricant failure can occur. There is thus still a need in the art for methods that allow reducing friction between solid objects to a higher level and to novel types of lubricants.

Object of the invention is to fulfil this need and to provide a method by which the friction between two or more solid bodies can be reduced, which at the same time prevents, or at least minimises, wear.

It was found that this object can be met by using a specific combination of a polymeric coating with a liquid.

Accordingly, in a first aspect the invention is directed to a method for reducing friction between at least two solid bodies, comprising providing a surface of at least one of said at least two solid bodies with a covalently bound coating, wherein said surface is to be contacted with another one of said at least two solid bodies, and wherein said coating comprising at least one polymer; and providing a liquid onto said coating.

The inventors surprisingly found that the coating and the liquid work synergistically in lowering the friction between the two bodies when they are in moving contact with each other, i.e. the combination of the coating and the liquid yields a lower friction between two different objects than one would expect based on the sum of the friction lowering effects of the coating or the liquid alone.

The coating can bind to the surface of at least one solid body in a durable way. Accordingly, the lowered friction coefficient and desirable friction conditions can be maintained for a prolonged period of time.

Once the coating has been applied to the surface of at least one solid body the lowered friction characteristics can be induced in a durable way at any desired time by addition of the liquid. Additionally, the friction

characteristics may be reversed by removing the liquid {e.g. by evaporation, solvent extraction, etc.) at any desired time.

A coating can comprise different polymers which can interact with different liquids. Accordingly, a coating can be tailored to fit existing fluid environments {e.g. biomedical environment) to reduce friction.

Preferably, the coating has a strong chemical and/or physical interaction with the liquid. Without wishing to be bound by theory, it is believed that the combination of the coating and the liquid provides a low viscous, yet somewhat immobilised layer that improves lubricity. The coating comprises a polymer. Preferably, this polymer is an organic polymer. The polymer can suitably be a homopolymer or a copolymer, such as a block copolymer. Mixtures or blends of polymers and/or copolymers can also be used in the coating. The polymer can be of hydrophilic or hydrophobic nature. Preferably, a hydrophobic coating is combined with a hydrophobic liquid and a hydrophilic coating is combined with a hydrophilic liquid. As an example, when a more hydrophobic liquid is used, such as an oil or certain hydrocarbons or silicone oil, the polymer preferably has a hydrophobic nature. A large variety of suitable polymers can be used.

In order to covalently bind to the surface of the solid body, the polymer of the invention can have at least one functional group. Suitable functional groups can for instance be chosen from the group consisting of acids, acyl halogens, acrylates, alcohols, aldehydes, alkenes, alkynes, amines, azides, carboxylics, cyanides, epoxides, halogens, imines, isocyanates, ketones, silanes, thiols, vinyls, and vinylethers. Examples of suitable hydrophobic polymers include polysiloxane, perfluoropolyether and other fluoropolymers, polystyrene, polyoxypropylene, polyvinylacetate, polyoxybutylene, (hydrogenated) polyisoprene or polybutadiene, polyvinylchloride, polyalkylacrylates, polyalkylmethacrylates, polyacrylonitrile, polypropylene, polyethylene, polytetrahydrofuran (PTHF),

polymethacrylates, polyacrylates, polysulphones, polyvinylethers, and poly(propyleneoxide), and copolymers thereof.

Examples of suitable hydrophilic polymers include polyoxazoline, polyethylene glycol, polyvinylalcohol, polyvinylpyrrolidone, polyacrylamide, poly(meth)acrylic acid, polyethylene oxide-co-polypropylene oxide block copolymers, poly(vinylether), poly(N,N-dialkylacrylamide), polyacyl alkylene imine, polyhydroxyalkylacrylates such as (homo)polymers of hydroxyethyl methacrylate (HEMA), hydroxyethyl acrylate, and hydroxypropyl acrylate, polyols, and copolymeric mixtures of two or more of the above-mentioned polymers, natural polymers such as polysaccharides and polypeptides, and copolymers thereof, and optionally polyionic molecules such as polyallylammonium, polyethyleneimine, polyvinylbenzyltrimethylammonium, polyaniline, sulphonated polyaniline, polypyrrole, and polypyridinium, polythiophene-acetic acids, polystyrenesulphonic acids, polymers and copolymers of zwitterionic molecules, and polyelectrolytes.

Covalent binding of the polymer to the surface of the body may be realised as described in WO-A-2007/021180, which is hereby incorporated by reference. Thus, conventional wet-chemical techniques as well as techniques such as chemical vapour deposition (CVD), plasma deposition, plasma assisted grafting, or plasma polymerisation can be used.

In a preferred embodiment the polymer is attached to the surface of the body by a wet-chemical technique. The polymer material suitably comprises a relatively high concentration of pendant reactive moieties per monomer, such as at least 0.3 pendant reactive moieties per monomer, preferably at least 0.5 pendant reactive moieties per monomer, even more preferably at least one pendant reactive moiety per monomer. The pendant reactive moieties are preferably carboxyl moieties. Furthermore, the surface of the body to which the polymer is to be applied is preferably functionalised with amine groups. Even more preferably, the polymeric material is a poly(acrylic

acid) polymer or a derivative thereof, and the surface of the body to which the polymer is to be applied is functionalised with 3-aminopropyltrialkoxysilane.

It was found that the liquid can be retained better by the coating, when the coating comprises more than one polymer layer. The different polymer layers may be covalently bound to each other or not. The different polymer layers may comprise different or the same polymers.

Advantageously, the coating can have a thickness of less than 1 μm. Preferably, the coating has a thickness of less than 100 nm. Hence, in accordance with the invention a very small amount of coating material suffices for obtaining a satisfactory reduction in friction. In addition, it was found that relatively thin layers attach better to the surface of the solid body. Moreover, the advantage of having a thin film is that the solid body maintains its characteristic properties and only the surface properties are altered. Normally, the thickness of the coating is not less than 5 nm. In a preferred embodiment a poly(acrylic acid) polymer (or a derivative thereof), covalently linked to the surface, is modified by reaction with either a hydrophobic or a hydrophilic polymer containing either primary or secondary amine moieties. As an example, but not limited thereto, an amino-functionalised polyethylene glycol may be used for this purpose. In this way, a coating comprising two polymer layers is created. In another embodiment, post-modification of coated polymer layer(s) can be realised.

Preferably, the coating comprises poly(acrylic acid) polymer (PAA) or a derivative thereof. Such coatings give a remarkable reduction in friction when combined with water as liquid. It was found that in this combination the water is retained by the coating very well due to the strong absorptive properties of the poly(acrylic acid) polymer. The surface of the body with the combination of the coating and water does not moisten its user, nor does it feel wet when applying limiting amounts of water. These two characteristics — remaining wet without feeling wet — make this combination very attractive.

In the case of a hydrophilic polymer, the number average molecular weight of the polymer in the coating can suitably be at least 2 000 g/mol, preferably at least 5 000 g/mol, more preferably in the range of 10 000-2 000 000 g/mol. In the case of a hydrophobic polymer, the number average molecular weight of the polymer in the coating can suitably be at least 500 g/mol, preferably in the range of 2 000 - 50 000 g/mol, more preferably in the range of 5 000-10 000 g/mol.

The liquid is preferably hydrophobic or hydrophilic. The liquid can be a lubricant. Examples of liquids suitable in the method of the invention include water, aqueous media, organic fluids, silicone fluids, miscible combinations thereof or solutions comprising these liquids.

Examples of preferred combinations of polymer and liquid, which lead to significant reduction of friction include poly(acrylic acid) polymer (and/or one or more derivatives thereof) and water or an aqueous medium, poly(acrylic acid) polymer (and/or one or more derivatives thereof) and silicone oil, poly(acrylic acid) polymer (and/or one or more derivatives thereof) and C6-C20 alkane, polyethylene glycol (and/or one or more derivatives thereof) and water or an aqueous medium, polyethylene glycol (and/or one or more derivatives thereof) and silicone oil, polyethylene glycol (and/or one or more derivatives thereof) and C6-C20 alkane, polysiloxane (and/or one or more derivatives thereof) and water or an aqueous medium, polysiloxane (and/or one or more derivatives thereof) and silicone oil, polysiloxane (and/or one or more derivatives thereof) and C6-C20 alkane, polyolefin (and/or one or more derivatives thereof) and water or an aqueous medium, polyolefin (and/or one or more derivatives thereof) and silicone oil, polyolefin (and/or one or more derivatives thereof) and C6-C20 alkane. Suitable C6-C20 alkanes include octane, nonane, decane, undecane, dodecane, tetradecane, hexadecane, and octadecane. Suitable polyolefins include polyethylene and polypropylene.

Particularly preferred combinations of polymer and liquid are PAA (and/or one or more derivatives thereof) with water (or an aqueous medium),

polyethylene glycol (and/or one or more derivatives thereof) with water (or an aqueous medium), polyethylene glycol (and/or one or more derivatives thereof) with silicone oil, polysiloxane (and/or one or more derivatives thereof) with silicone oil, polysiloxane (and/or one or more derivatives thereof) with dodecane, polyethylene (and/or one or more derivatives thereof) with silicone oil, and polyethylene (and/or one or more derivatives thereof) with dodecane.

Good results have been obtained when the polyethylene glycol (and/or one or more derivatives thereof), the polysiloxane (and/or one or more derivatives thereof), or the polyolefm (and/or one or more derivatives thereof) is provided on top of a layer of PAA (and/or one or more derivatives thereof).

Thus, in a preferred embodiment the following combinations of coating and liquid are applied: a coating comprising a layer of polyethylene glycol (and/or one or more derivatives thereof) on top of a layer of poly (acrylic acid) polymer (and/or one or more derivatives thereof) and water or an aqueous medium; a coating comprising a layer of polyethylene glycol (and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and silicone oil; a coating comprising a layer of polyethylene glycol (and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and C6-C20 alkane; a coating comprising a layer of polysiloxane (and/or one or more derivatives thereof) on top of a layer of poly (acrylic acid) polymer (and/or one or more derivatives thereof) and water or an aqueous medium; a coating comprising a layer of polysiloxane (and/or one or more derivatives thereof) on top of a layer of poly (aery lie acid) polymer (and/or one or more derivatives thereof) and silicone oil; a coating comprising a layer of polysiloxane (and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and C6-C20 alkane; a coating comprising a layer of polyolefin (and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and water or an aqueous medium; a coating comprising a layer of polyolefin

(and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and silicone oil; and a coating comprising a layer of polyolefin (and/or one or more derivatives thereof) on top of a layer of poly(acrylic acid) polymer (and/or one or more derivatives thereof) and C6-C20 alkane.

The amount of liquid to be combined with the coating of the invention can vary. It is preferred to use an amount which more or less equals the maximum loading capacity of the coating with the respective liquid. Usually, the amount of liquid combined with the coating will be in the range of 0.05-10 g per gram of dry coating, preferably 0.1-5 g per gram of dry coating more preferably 0.2-1 g per gram of dry coating.

The at least two solid bodies can be of the same or different material. Suitably, the at least two solid bodies comprise one or more materials selected from the group consisting of plastics, metals, alloys, glass, textiles, metal oxide, ceramics, wood, living tissue (skin) and mucosal membranes. Excellent results have been obtained using two solid bodies made of coated glass and silicone rubber, respectively, and two solid bodies made of coated textiles and silicone rubber, respectively.

The surface of one or more of the at least two solid bodies may be surface modified in order to covalently bind the polymer to the solid body throμgh a suitable linker molecule. Typically, the surface of the bodies can be modified with organic compounds which can suitably be chosen from the group consisting of acids, acyl halogens, acrylates, alcohols, aldehydes, alkenes, alkynes, amines, azides, carboxylics, cyanides, epoxides, halogens, imines, isocyanates, ketones, silanes, thiols, vinyl, and vinylethers. Suitable linker molecules include aminopropyltrialkoxysilane, aminopropyltrichlorosilane and heterofunctional silane-based coupling agents.

The method of the invention can for instance be used in sportswear to reduce friction between textiles and skin, for ski runs with artificial snow to reduce friction between the artificial snow and the ski equipment, blankets

and garments like pyamas for decubitus patients to reduce friction with the skin, catheters, guide wires, endoscopes and other medical devices to reduce friction with the living tissue as well as for other applications.

Examples

Example 1

In this Example, a method of modifying a glass surface by covalent bonding thereto of, subsequently, an aminopropyltrimethoxysilane linker and a poly(acrylic acid) polymer is described.

Pre-cleaned glass substrates were submerged in a fresh coating solution of 5 % 3-aminopropyltrimethoxysilane in isopropylalcohol and sonicated for 20 min in an ultrasonic bath. The substrates were thoroughly washed (3x) with water followed by drying for 1 hour in an oven at 60 ⁰ C. The thus functionalised substrates were dipped in a solution of poly(acrylic acid) polymer (Mw 1 080 000; Mn 135 000) in water (0.5 wt.%) and dried in an oven at 100 ⁰ C under reduced pressure (< 100 mbar) for 4 hours. The substrates were thoroughly washed (3x) with water to remove physisorbed and not chemically bonded poly(acrylic acid) polymer. Contact angle measurements of water on the coating surface showed an advancing contact angle of 70-80 degrees, whereas that of the pre-cleaned glass substrate was found to be in between 20 and 40 degrees depending on the cleaning method.

Example 2

In this Example, a method of modifying a glass surface by covalent bonding thereto of, subsequently, an aminopropyltrimethoxysilane linker, a poly(acrylic acid) polymer and amino functionalised polyethylene glycol (PEG 5 000; Mw 5 400; Mn 5 000) is described.

Using the poly(acrylic acid) polymer modified glass substrate prepared as described in Example 1, the glass substrate was further dipped in a solution of 1 % of monofunctionalised amino-PEG 5 000 in water and dried in an oven at 120 ⁰ C under reduced pressure (< 100 mbar) for 1 hour. The substrates were thoroughly washed (3x) with water to remove physisorbed and not chemically bonded PEG 5 000. Contact angle measurements of water on the coating surface showed an advancing contact angle of less than 20 degrees.

Example 3 In this Example a tribological assessment is done using the coated glass surfaces from Examples 1 and 2.

Tribological assessment was done using a reciprocating pin-on-plate test setup. This configuration was realised on the commercially available tribometer PLINT TE67, for which a dedicated glass support unit was constructed.

Operational conditions:

All experiments were done at room temperature and in air. The humidity and the temperature of the laboratory varied between 20-40 % RH and 19-22 ⁰ C respectively. Following the FIFA 08/05 - 01 test method "Determination of Skin/Surface Friction", in which Silicon Skin L7350 is selected as best skin substitute, this Silicone rubber is used for the tribological assessment.

Other conditions: Track length was set to 40 mm for all experiments. Ten strokes were done per experiment (five from left to right and five from right to left). The maximum velocity during the track was 1, 10 or 62 mm/s. The normal load was applied by the mass of the specimen holder or by an additional dead weight to a resulting normal force of 29 and 98 kPa respectively.

Tests were performed both with and without demineralised water as liquid. The first condition is referred to as with water, the second as dry conditions. The results are shown in Figures 1 and 2 for the dry condition and Figures 3 and 4 for the "with water" condition. The used term glass means the pre-cleaned glass substrate without coating whereas the terms +PAA and +PEG stands for the glass substrate with a coating of PAA (Example 1) and PAA plus a layer of PEG 5000 (Example 2) respectively.

The coefficient of friction is calculated based on the friction force measured and normal force applied: μ = Fw / Fn

Figure 1 shows the maximum coefficient of friction, fmax, as a function of the sliding velocity and plate surface, at 98 kPa contact pressure and at dry condition.

Figure 2 shows the maximum coefficient of friction, fmax, as a function of the sliding velocity and plate surface, at 29 kPa contact pressure and at dry condition.

Figure 3 shows the maximum coefficient of friction, fmax, as a function of the sliding velocity and plate surface, at 98 kPa contact pressure in the presence of water.

Figure 4 shows the maximum coefficient of friction, fmax, as a function of the sliding velocity and plate surface, at 29 kPa contact pressure in the presence of water.

From the Figures 1-4 it can be observed that in the absence of liquid the calculated maximum coefficients of friction are all above 0.7 no matter the diverse circumstances. In the presence of water, the calculated maximal

coefficients of friction are somewhat lower for the glass substrates without coating, as may be expected upon addition of water, although water is not known to be an effective lubricant as such. However, for glass substrates with either PAA or PEG coating, the calculated coefficients are substantially lower; an unexpected large reduction is observed. These results show a large synergistic performance of the coating polymer in cooperation with the liquid. Especially in the situation wherein a normal pressure of 29 kPa is applied, a maximal synergistic performance is obtained, no matter the maximum velocity during the track.

Example 4

In this Example a tribological assessment is done using the coated glass surfaces from Example 2.

Tribological assessment is done in parallel as described in Example 3 using different liquids, i.e. water, silicone oil and dodecane.

The maximum velocity during the track was 10 mm/s. The normal load was applied by the mass of the specimen holder resulting in a normal force of 29 kPa. The results are shown in Figure 5. Again, a coating as such has no (major) influence on the obtained coefficient of friction. Addition of a liquid alone often has, but not in the situation with silicone oil, a certain reduction of the coefficient of friction. However, the combination of the coating with the liquid results in a synergistic reduction of the coefficient of friction. This clearly shows that the observed reduction in the coefficient of friction cannot be explained by the presence of the liquid as such, neither by the presence of the coating as such, but is due to the specific combination of the coating and the liquid lubricants which results in a reduction in a synergistic way.

Example 5

In this Example a commercial available amino-containing silicone polymer is integrated in the coating on top of the poly(acrylic acid) polymer on glass substrate. The PAA-coated glass substrate is covered with toluene solution containing the amino-containing silicone polymer Wacker 1650 whereafter the toluene is evaporated by heating at 120 °C under N2 flow. Subsequently, the coated glass substrate is washed with toluene and water at room temperature respectively whereafter dried at 60 ⁰ C.

Tribological assessment is done in parallel as described in Example 3 using different liquids, i.e. water, silicone oil and dodecane.

The maximum velocity during the track was 10 mm/s. The normal load was applied by the mass of the specimen holder resulting in a normal force of 29 kPa.

As in Example 4, different lubricants are tested for synergistic impact on coefficient of friction. As can be observed in Figure 6, use of silicone polymer as coating will lead to reduction of coefficient of friction no matter the nature of lubricant used. Whereas the use of water and dodecane as lubricant will lead to friction reduction using uncoated glass substrate, a synergistic effect is observed with the PAA-silicone polymer coated glass substrate. Using silicone oil as lubricant, this synergistic effect on friction reduction is also observed whereas the impact on bare glass substrate is nil.

Example 6 In this Example glass substrate is coated with poly(acrylic acid) and subsequent with either amino-functionalised polyethylene (PE) or octadecylamine. Preparation of the PE top layer is realised according the procedure described in example 5 for silicone top layer except that the toluene solution is heated to prevent precipitation of the poorly soluble PE polymer. Construction of the octadecane toplayer requires tetrahydrofuran (THF)

instead of toluene. The PAA modified glass substrate is dipped in octadecane containing THF whereafter the substrate is heated.

Tribological assessment is done as described in Example 5.

In Figure 7 the obtained coefficients of friction are shown using water as liquid. Coefficients of friction from PE coated glass substrate using silicone oil and dodecane as liquids are also given.

As can been noted, coating of glass with PE top layer results in obtained coefficients of friction which are substantially reduced compared with that of the glass substrate without coating: a synergistic impact of the combination of coating with liquid is again shown since the friction reduction of uncoated glass is substantial higher. This synergistic effect is optimally shown in the situation of PE coating and silicone oil liquid: without coating the application of silicone oil as liquid shows no reduction of coefficient of friction at all, while with coating, a substantial reduction is observed.