Background of the invention Certain self-lubricating polymer systems are known in the art for use in machine components such as bearings.

US 5,180,761 discloses a self-lubricating composite material used for the fabrication of bearing members. The composite material comprises 100 parts by weight of polymeric materials, 1-15 parts by weight of liquid lubricants, of which 0.1-15 parts by weight are polar compounds, and 4-100 parts by weight of filler or solid lubricants that have been treated with 0.2-3 parts by weight of titanates or silane compounds per 100 parts by weight of fillers or solid lubricants.

US 4,357,249 discloses structural members, in particular journal-type bearings, produced by molding under pressure at elevated temperatures a blend of 70-95% by weight of a linear ultra high molecular weight ethylene polymer and 5-30% by weight of a normally solid lubricant selected from waxes, fats and mixtures thereof alone or with a grease. The bearings are said to have a surface which is not oily to the touch. Lubricant is slowly released at temperatures developed in ordinary use.

There remains a need for self-lubricating polymers that have other physical characteristics than the self-lubricating bearings, etc. described above, and which therefore are suitable for use in preparing different types of self-lubricating products that are not currently available. Bearings and other machine parts made from the known self-lubricating materials such as those referred to above are designed with the aim of providing low friction between such parts that move relative to each other. They are not designed, however, with the aim of providing low friction between one such part and other surfaces such as a human or animal body tissue or a liquid such as water. In particular, a need remains for producing flexible self-lubricating polymers that can be formed into e. g. tubes,

hoses, films, sheets, tapes, etc. Such self-lubricating products would be advantageous for friction-reducing applications such as in the medical sector, e. g. as catheters, to replace mylar foil and intertape greasing in superconducting cable constructions, etc.

In addition, such self-lubricating polymer products in the form of e. g. a film, foil or tape could be applied to the surface of a variety of different products in order to provide them with self-lubricating properties. In addition to a variety of industrial applications, contemplated uses include skis as well as lubrication of surfaces to reduce friction upon contact with water, e. g. on boats, ships, surfboards and wind-surfboards.

Brief disclosure of the invention In its broadest aspect, the present invention relates to a method by which the friction properties of a polymer can be adjusted so as to result in a desired decreased or increased friction coefficient.

The invention thus provides, in one embodiment, self-lubricating polymers, suitable for a wide variety of applications, produced by including a modifying agent (lubricating agent) in the molten mass of the polymer. Although it will typically be desired to decrease the friction coefficient of a polymer, the invention makes it possible to either decrease or increase the friction coefficient of a polymer, depending on the nature of the particular combination of modifying agent and polymer.

Thus, in one aspect, the invention relates to a self-lubricating polymeric material comprising at least one crystalline or semi-crystalline synthetic polymer having incorporated therein at least one lubricating agent, wherein the lubricating agent migrates towards exposed surfaces of the polymer to result in self-lubrication of said surfaces.

Another aspect of the invention relates to a method for producing a self-lubricating polymeric material, comprising providing a molten mixture comprising at least one crystalline or semi-crystalline synthetic polymer and at least one lubricating agent, forming the molten mixture into a material having a desired shape, and allowing the material to cool.

In a further aspect, the invention relates to shaped articles comprising a self-lubricating polymeric material as described herein.

In a still further aspect, the invention relates to a method for producing a polymeric material with an increased friction coefficient, comprising providing a molten mixture comprising at least one synthetic polymer selected from amorphous polymers and polymers having a crystallinity of at the most about 20%, and at least one modifying agent serving to increase friction of the polymer, forming the molten mixture into a material having a desired shape, and allowing the material to cool.

Detailed description of the invention The mechanism behind the self-lubrication of the invention Any polymer in its fully molten state becomes isotropic, i. e. it loses its structural order.

Crystalline polymer for instance goes through different stages during the melting process, a softening stage which results in a change of free volume, a melt stage where the crystals start to disorganise, and finally an isotropic stage where the polymer becomes fully liquid. During the cooling process, the polymer tends to regain its organised state in the opposite order, i. e. from isotropic to crystallisation and finally solidification. If we imagine that during the melting process another polymer having glass transition temperature (Tg) corresponding to room temperature (RT) is mixed into molten polymer in a liquid state, when the mixture of the two polymers starts to cool down, the RT Tg polymer will be expelled out during crystallisation of the host polymer. The RT Tg polymer will then tend to slowly migrate to the surface, thus yielding a lubricated polymer. Having in mind that all polymers crystallise as rapidly as possible to gain their lowest energy state, but that full crystallisation can take hours, weeks, months or even years, a slow crystallisation, also called the annealing process, will take place over a period of time depending on a number of different factors. While not wishing to be bound by any particular theory, it is believed that the lasting lubrication that can be obtained in accordance with the present invention, and the possibilities to"fine-tune"the lubrication of a given polymer, result from that fact that during the annealing stage, the polymer that migrates toward the surface (the RT Tg polymer in the example above) keeps on migrating for an extended period of time, e. g. weeks, months or years, so as to yield a long-lasting lubrication behaviour.

Polymers suitable for obtaining a self-lubricated product As indicated above, the present invention is based on the use of at least one crystalline or semi-crystalline polymer. In this context, the term"crystalline polymer"refers to a polymer with a crystallinity of at least 90%, and the term"semi-crystalline polymer"refers to a polymer with a crystallinity of less than 90% but generally more than about 20%.

Crystallinity may be determined by known methods such as differential scanning calorimetry (DSC). This group of polymers includes, but is not limited to, polyolefins, including polyolefin homopolymers and copolymers, e. g. isotactic, atactic and syndiotactic polypropylene homopolymers as well as random or block copolymers thereof with another alpha-olefin, e. g. with ethylene, 1-butene, 4-methylpentene, 1-hexene, etc., and linear or non-linear polyethylenes of different densities such as high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). Examples of other suitable polymers are copolymers of ethylene/vinylacetate, propylene/vinylacetate, ethylene/acrylate and propylene/acrylate.

The percentage of the vinyl or acrylate monomer in such polymers will typically vary from about 5% to about 50% by weight, more typically from about 10% to about 30% by weight of the weight of the ethylene or propylene component. Other olefin-based copolymers, typically block copolymers, such as EPDM (ethylene-propylene-diene-monomer) may also be used, as can e. g. styrene/butadiene and styrene/acrylonitrile block copolymers.

Lubricating agents Lubricating agents suitable for use in the present invention will typically be oligomers or polymers having a room temperature glass transition (RT Tg). In this context, the term "room temperature glass transition temperature"refers to polymers that are liquid at room temperature, i. e. at 20°C. Non-limiting examples of such lubricating agents include paraffinic, naphtenic and aromatic oils, silicone oils, fluorine oils, silico-hydrocarbon oils, fluoro-hydrocarbon oils, silico-fluorine oils, and silico-fluoro-hydrocarbon oils. Also suitable as lubricating agents are polyethylene glycol, polypropylene glycol, fatty acids and esters thereof with polyethylene glycol or polypropylene glycol, fatty alcohols and ethers thereof with polyethylene glycol or polypropylene glycol, as well as glycerol and modified fatty acids, modified fatty acid esters and derivatives thereof. Non-limiting examples of fatty

acids that may be used include soybean oil, castor oil and epoxidised or esterified derivatives thereof.

Amount of lubricant It will be understood in light of the above discussion that the amount of lubricant to be incorporated into any given host polymer can vary within wide limits depending on factors such as the nature the polymer, the nature of the lubricant and the specific properties desired in the finished product to be produced from the polymer. Generally, the amount of lubricant will be in the range of from about 5% to about 50% by weight based on the weight of the polymer. Typically, the amount of lubricant will be at least about 10%, e. g. at least about 15%, and up to about 30%, e. g. up to about 25%. Although the amount of lubricant can vary considerably, persons skilled in the art will be able to select an appropriate amount of lubricant in any given case based on knowledge of the polymer and lubricant as well as the desired properties of the product, supplemented by routine tests to verify the performance of any given combination.

As mentioned above, in one embodiment, instead of decreasing the friction coefficient of a polymer, it can be increased. In this embodiment, one typically chooses a fully amorphous polymer or a polymer with a very low crystallinity, typically a crystallinity that does not exceed about 20%, more typically not more than about 15%, e. g. not more than about 10%. As mentioned above, the degree of crystallinity may be determined using methods such as DSC. The increased friction coefficient is obtained by incorporating one of the lubricating agents discussed above into the polymer, taking into account the polarity of the lubricating agent and the host polymer. If the polarity of the RT Tg polymer and the host polymer are close to each other, the two polymers will mix and the RT Tg polymer will remain in the host polymer, so that the friction coefficient of the host polymer will be increased. It is believed that in this case, the lubricating agent is solubilised in the low crystallinity host polymer, yielding a polymer surface with a higher friction coefficient than that of the pure polymer without the lubricant.

Non-limiting examples of suitable amorphous polymers or low crystallinity polymers include polyvinyl acetate (PVA), polyvinychloride (PVC), polyacrylate, polyacrylic acid and polyurethane. A number of low crystallinity copolymers can also be used in this regard, since copolymers general have a relatively low crystallinty. Examples include ethylene-

alpha olefin copolymer, propylene-alpha-olefin copolymer, styrene-butadiene block copolymer, styrene-acrylonitrile copolymer, etc.

Methods for mixing and product shaping For mixing the polymer and the lubricant, any mixing device equipped with a heating system and a feeding and discharging system can be used. Such equipment is well known and the art and can be used in the context of the present invention in a manner known per se. Examples include dough mixers, batch mixers such as a Bumbury mixer, Brabender type or Haake type, single screw extruder, twin screw extruder, Buss co- kneader, etc.

To shape the end product, any of above-mentioned devices equipped with a suitable die can be used. Such devices for the production of shaped articles, e. g. using methods such as extrusion or injection moulding, are also well known in the art and can be used in a manner known per se. Mixing and shaping will typically take place in a single run.

The end product can be anything that conventionally is able to be produced from the polymer (s) of the given mix by any suitable method such as extrusion, injection molding, etc. The product can, for example, be extruded in the form of a pipe or tube, a foil, film or sheet, a tape, a rod, a filament, a fibre, etc. When using injection molding techniques, products having a immense variety of different shapes can be produced by methods known in the art.

Mixture components A typical mixture recipe for producing self-lubricating polymers according to the present invention will often, in addition to the host polymer and the lubricant, comprise one or more additional components known per se in the art such as stabilising agents, anti-UV agents, antioxidants, plasticising agents, colorants and dyeing agents, antistatic agents, etc. The mixture will normally include at least one antioxidant and at least one anti-UV agent.

Applications The self-lubricating products of the invention can find use in any application where a low friction coefficient is needed and, in the case of products with an increased friction coefficient, also in applications where a high friction coefficient is needed or where a need otherwise exists to be able to"fine-tune"the friction properties of a polymer.

Among the fields where a low friction coefficient is needed are the medical sector. In this case, products prepared according to the invention can be used to manufacture self- lubricated catheters or any device that is designed to be inserted in a human or animal body, such as operation pipes that are to be inserted in human body through the nose or the mouth. It is also contemplated that the present invention will be applicable to the production of self-lubricating condoms.

For tube-in-tube or pipe-in-pipe insertion, this insertion can be eased if at least one of the tubes or pipes are suitably internally or externally lubricated according to the invention. A similar use is in reducing friction in pistons in general, including in syringes, thus allowing easier administration of injected medicaments.

For non-medical uses, there is practically no limit to the possibilities. One can for instance imagine a disposable lubricated film put on, e. g., the bottom surface of a pair of skis or a mono-ski or surf board. This would improve and ease snow-skiing or snowboarding, or water-skiing or surfing (although in the case of water-skiing or surfing a hydrophobic lubricant might be advantageous). Similarly, putting a lubricated film on the flipper of a sailboat could ease the sailing and increase speed by decreasing the friction coefficient between the surface of the boat and water.

A wide variety of industrial applications can also be envisaged. Commodity cable construction requires lubrication between the elements constituting the cable to ease internal cable motion. Extruding a self-lubricated, insulated wire would be ideal when assembling insulated wires together. The same is valid for multilayer constructions where the layers are subjected to movement relative to each other. An example of such multilayer constructions is a super-conducting cable comprising multilayer tapes. By applying the present invention to such constructions, it will be possible to produce cables in which motion of the tape layers relative to each other is eased during cable bending.

The invention will be further illustrated by the following non-limiting examples.

Examples Example 1 100 parts of medium density polyethylene, 1.5 parts of an antioxidant and 0.5 parts of a UV stabiliser are slowly fed into a single screw extruder equipped with a flat die. In the metering zone of the extruder 20 parts of naphtenic oil is added. The mixture is extruded through the flat die, after which the extruded band is rolled onto a take-up roll and stored in a dust-free package until use.

Example 2 100 parts of low density polyethylene, 1.5 parts of an antioxidant and 0.5 parts of a UV stabiliser are slowly fed into a single screw extruder equipped with a flat die. In the metering zone of the extruder 20 parts of paraffinic oil is added. The mixture is extruded through the flat die, after which the extruded band is rolled onto a take-up roll and stored in a dust-free package until use.

Example 3 100 parts of isotactic polypropylene and the appropriate additives such as an antioxidant and a UV stabiliser are slowly fed into a single screw extruder equipped with a flat die. In the metering zone of the extruder 20 parts of silicone oil is added. The mixture is extruded through the flat die, after which the extruded band is rolled onto a take-up roll and stored in a dust-free package until use.

Example 4 100 parts of ethylene vinylactate copolymer having 10% vinylacetate and appropriate additives such as an antioxidant and a UV stabiliser are slowly fed into a single screw extruder equipped with a pressure die. In the metering zone of the extruder 20 parts of polyethylene glycol having a molecular weight of 400 is added. The mixture is extruded through the pressure die, after which the extruded pipe is rolled onto a take-up roll and stored in a dust-free package until use.

Example 5 100 parts of ethylene vinylactate copolymer having 10% vinylacetate and appropriate additives such as an antioxidant and a UV stabiliser are slowly fed into a single screw extruder equipped with a pressure die. In the metering zone of the extruder 20 parts of soybean oil having a molecular weight of 2000 is added. The mixture is extruded through the pressure die, after which the extruded pipe is rolled onto a take-up roll and stored in a dust-free package until use.

Example 6 100 parts of ethylene vinylactate copolymer having 10% vinylacetate and appropriate additives such as an antioxidant and a UV stabiliser are slowly fed in a single screw extruder equipped with a pressure die. In the metering zone of the extruder 20 parts of epoxidised soybean oil having a molecular weight of 2000 is added. The mixture is extruded through the pressure die, after which the extruded pipe is rolled onto a take-up roll and stored in a dust-free package until use.

Example 7 100 parts of film grade polyethylene and appropriate additives such as an antioxidant and a UV stabiliser are slowly fed in a single screw extruder equipped with a film blowing tool.

In the metering zone of the extruder 20 parts of epoxidised soy bean oil having a molecular weight of 2000 is added. The mixture is extruded through the film blowing tool, after which the blown film is cut to size, rolled onto a take-up roll and stored in a dust-free package until use.