Pi fine powder as lubricant additives

Abstract

The invention relates to PI particles as lubricant additives.

Description

[0001] The invention relates to the use of PI particles as lubricant additives, in particular lubricating grease additives.

[0002] Lubricants, especially lubricating greases and lubricating oils, are well-known and widely used. Lubricating greases, sometimes also referred to as "fats," are solid to semi-liquid substances created by dispersing a thickener in a liquid lubricant (base fluid). Other additives that impart special properties may be included. In this context, reference can be made to publication DE 10 2012 215145, for example.

[0003] The basic consistency of a lubricating grease is determined by the combination of base fluid and thickener. The base fluid is usually a base oil commonly used in the lubricant industry, such as mineral oil, synthetic oil, or vegetable oil.

[0004] Alkali metal or alkaline earth metal soaps are used as thickeners. Non-soap-based thickeners, such as bentonite (clay-based) or polyurea, are also known.

[0005] Lubricants, especially greases and lubricating oils, can be produced in batch processes or by continuous processes.

[0006] Lubricants and their manufacturing processes are known from the state of the art and are commercially available in numerous compositions.

[0007] The lubrication of machine elements (e.g. gears or bearings) presents a particular challenge with regard to wear and friction. For this reason, different additives are used in lubricating oils and greases to improve or meet these two main properties, among others. The tribological contact is also influenced by the friction partners. This can be inorganic (e.g. steel / steel), organic (e.g. plastic / plastic) or mixed contact (e.g. steel / plastic). The additives used must therefore be tailored to this tribological pairing. In other words: different tribological pairings (metal / metal, metal / plastic, plastic / plastic, etc.) require different lubricant properties. To date, optimized lubricants are not available for all pairings.

[0008] Publication JP 2002 / 363589 discloses PTFE powder as a thickener. Commercially available products include Algoflon® and "Xeon" PTFE waxes.

[0009] Publications GB 2192896 B, US 4,787,993, and JP S63 / 172794 disclose PEEK or PI powders as additives for lubricants. However, only the effect on metal-to-metal friction pairs was investigated. Commercially available products are not known.

[0010] PTFE contains fluorine and is used in relatively large quantities in lubricants (15-40%). At high temperatures caused by friction, toxic fluorine compounds may be formed. Furthermore, PFOA (perfluorooctanoic acid) and other PFAS (per- and polyfluorinated alkyl compounds), which have since been banned, are minor components that are / were present in small quantities in PTFE. PFOA-free PTFE is now available, but a ban on PTFE is also being discussed in the EU (German Federation for the Environment and Nature Conservation, "Practical, durable, and toxic"); https: / / www.bmuv.de / FAQ / which-pfas-have-been-banned-so-far; https: / / echa.europa.eu / restrictions under consideration / - / substance-rev / 72301 / term; https: / / echa.europa.eu / documents / 10162 / f605d4b5-7c17-7414-8823-b49b9fd43aea ) . No information is available on the tribological behavior in metal / plastic or plastic / plastic friction pairings.

[0011] The current lubricants are therefore unsatisfactory. The challenge is therefore to provide lubricants that offer advantages over the current lubricants.

[0012] This problem is solved by the subject matter of the patent claims.

[0013] It was surprisingly found that with PI powders, i.e. polyimide powders, of a certain grain size (approximately 5 µm) a significant reduction of the wear and / or friction coefficient can be achieved for the aforementioned contact pairings.

[0014] Examples of suitable PI powders are known to the person skilled in the art, for example from the publication DE 10 2005 034 969 A1 .

[0015] Preferably, the PI powder is or contains a polyimide-polyimide block copolymer composed of the following structural units: where: n, m = each 2 to 20, preferably 8 to 15, and x = 1 to 300, preferably 75 to 200, particularly preferably 100 to 175, where further: R 1 and R 3 have, independently of one another, the following meanings with X = and R 2 and R 4 have, independently of one another, the following meanings: with Z= H, CH 3 , CF 3 , OCH 3 with X = with n = 1 to 12, preferably 4 to 9.

[0016] In a further preferred embodiment, the PI powder contains polyimides with terminal dicarboxylic acid anhydride groups as blocks for the production of block copolymers, constructed as follows: where functional groups R 1 and R 2 are as defined in paragraph 15 and n = 2-20, preferably 5 to 15, particularly preferably 8 to 12.

[0017] In a further preferred embodiment, the PI powder contains a polyimide-polyamidecarboxylic acid block copolymer, constructed as follows: where: n, m = each 2 to 20, preferably 8 to 15, particularly preferably 9 to 12, and x = 1 to 300, preferably 75 to 200, particularly preferably 100 to 175, R 1 , R 2 , R 3 and R 4 are defined as in paragraph 15.

[0018] It was surprisingly found that by adding small amounts, especially in the range of 0.5-2 wt.% of PI fine powder with an average particle size of approximately 5 µm, the tribological properties of lubricants or greases can be improved.

[0019] It was surprisingly found that a lubricant containing PI particles (approximately 5 µm) has advantages over a conventional lubricant preparation in the plastic / metal system both at room temperature (RT) and at elevated temperature of 100°C.

[0020] PI-modified lubricants according to the invention can be used in a variety of applications. A primary application is seen in the automotive sector. Another area of ​​application is the lubrication of machine components (e.g., gears or bearings) in various industries.

[0021] Ultrafine PI powders are produced directly during the precipitation process. The resulting ultrafine PI powders are dispersed in lubricating greases and lubricants using commercially available dispersing equipment, such as planetary mixers.

[0022] PI is a thermally very stable polymer and exhibits significantly lower creep behavior than PTFE. Surprisingly, it was found that, although PI proves to be spherically stable under pressure and thus does not adapt to the friction geometry, the wear of the lubricated surfaces is reduced compared to the unfilled grease. Furthermore, it can be assumed that PTFE could "grind down" over time under high pressure loads and become ineffective. Thus, a longer service life could be achieved according to the invention.

[0023] Figure 1 shows the particle size distribution in a first PI powder, measured by laser diffraction particle size analysis. The D 10 value is 1.3 µm, the D 50 value is 4.3 µm, the D 90 value is 9.8 µm, and the D 100 value is 23.0 µm.

[0024] Figure 2 shows a device for performing an HFRR method.

[0025] Figures 3a and 3b each show the results of an HFRR procedure.

[0026] In a first aspect, the invention relates to a lubricant, preferably a lubricating grease, which contains PI particles with a particle size D 50 of up to 15 µm (ie a PI powder or PI ultrafine powder), formulated for lubrication between at least two surfaces.

[0027] For the purposes of the invention, a lubricating grease can be understood as a lubricant, lubricant, or lubricating material. These terms can be used synonymously.

[0028] For the purposes of the invention, a lubricating grease is a specific embodiment of a lubricant. In a preferred embodiment, the lubricant according to the invention is a lubricating grease.

[0029] In a preferred embodiment, the lubricant or lubricating grease according to the invention is / are not to be regarded as lubricating oil.

[0030] In a preferred embodiment, polyimides (PI) also include polyetherimides (PEI) and / or polyamideimides (PAI).

[0031] In a preferred embodiment, at least two surfaces, particularly preferably all surfaces, border on one another, ie they have a common contact surface.

[0032] In a preferred embodiment, at least one of the surfaces is non-metallic.

[0033] For the purposes of the invention, "non-metallic" preferably means that the corresponding surface contains at most 5 wt.% of metals, based on the total weight of the surface, and particularly preferably contains 0 wt.% of metals, based on the total weight of the surface.

[0034] Preferred metals are steel, aluminum, zinc, copper, gold, silver, platinum and alloys containing these metals.

[0035] In a preferred embodiment, at least one of the non-metallic surfaces contains or consists of plastic and at least one of the remaining surfaces contains or consists of metal; or at least two of the non-metallic surfaces contain or consist of plastic and at least two of the remaining surfaces contain or consist of metal; or all of the non-metallic surfaces contain or consist of plastic and all of the remaining surfaces contain or consist of metal; or exactly two surfaces are present, wherein one of the surfaces contains or consists of plastic and the other of the surfaces contains or consists of metal.

[0036] Suitable plastics include, for example, thermoplastics, thermosets, and elastomers known to those skilled in the art. Preferably, at least one of the non-metallic surfaces contains or consists of polyetheretherketone (PEEK), and / or polyoxymethylenes (POM), and / or UHMWPE (ultra-high molecular weight polyethylene, preferably with a molecular weight of 3.5 to 7.5 million g / mol), and / or PA66, and / or PA6G (cast polyamide 6), and / or PA12G (cast polyamide 12), and / or PPAs. PPAs are "semi-aromatic" nylon-type polyamides, i.e., either aromatic diamine or aromatic diacid.

[0037] In a further preferred embodiment, the particle size D50 is determined by laser diffraction particle size analysis. The particle size D50 is preferably determined according to ISO 13320:2020. One possible system for determining the particle size D50 is "LB (laser diffraction) wet, aqueous system with dispersant, ultrasound, Malvern Mastersizer 3000."

[0038] The lubricant used according to the invention usually contains a base oil which is preferably selected from the group consisting of mineral oils, synthetic oils, vegetable oils and any combination of two or more of these oils.

[0039] In a preferred embodiment, the lubricant contains the base oil in an amount of about 65 wt.% to about 90 wt.%, based on the total weight of the lubricant.

[0040] For the purposes of the present invention, the terms "base oil" and "base oil" can be used synonymously.

[0041] Suitable base oils are known to those skilled in the art and are commercially available.

[0042] Any known materials commonly used in the manufacture of lubricants can be used as base oils. Examples include mineral oils, lubricating oils of vegetable or animal origin, or synthetic oils. The latter include, in particular, hydrocarbon oils, including halogenated hydrocarbon oils, as well as non-hydrocarbon oils such as polyalkylene glycols, polyether oils, ester oils such as oils based on phosphoric acid esters, and silicone oils. Compound oils, such as mixtures of mineral oils and synthetic oils, can also be used as base oils. Mixtures of different base oils can also be used.

[0043] Examples of base oils include lubricating oils made from mineral oils, synthetic oils, and natural oils. Mineral lubricating oils include mineral oils that have been purified through a suitable combination of purification operations, such as vacuum distillation, solvent deasphalting, solvent extraction, hydrogenolysis, solvent dewaxing, sulfuric acid treatment, clay treatment, and / or hydrorefining.

[0044] Other examples of base oils are synthetic lubricating oils. These include hydrocarbon oils, aromatic oils, ester oils, and ether oils. Hydrocarbon oils include poly-alpha-olefins, such as n-paraffins, isoparaffins, polybutene, polyisobutylene, 1-decene oligomers, 1-decene / ethylene co-oligomers, and hydrogenation products thereof. Aromatic oils include alkylbenzenes, e.g., mono-, di-, or polyalkylbenzenes; and alkylnaphthalenes, such as mono-, di-, or polyalkylnaphthalenes.The ester oils include di-, tri-, or tetraesters of carboxylic acids, such as dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, ditridecyl glutarate, methyl acetyllicinolate, trioctyl trimesitate, tridecyl trimesitate, and tetraoctyl pyrromelitate; or they include polyol esters of carboxylic acids, such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate; or they include complex esters, which are oligoesters between a polyhydroxy alcohol and a mixed mono- or dibasic fatty acid. The ether oils include polyglycols, such as polyethylene glycol, polypropylene glycol, polyethylene glycol monoether, and polypropylene glycol monoether. as well as phenyl ethers, such as alkylated diphenyl, triphenyl and polyphenyl ethers of varying degrees of alkylation, or dialkyl ethers.Furthermore, phosphoric acid esters such as tricresyl phosphate, silicone oils and perfluoroalkyl ether oils (fluorine oils) can be used as synthetic oils.

[0045] The base oils used in the invention are not subject to any specific restrictions, and any base oil commonly used in lubricating oils and greases can be used. Base oils typically have a kinematic viscosity of 1 to 4000 mm² / s at 40°C (determined using a glass capillary tube viscometer).

[0046] The base oils used in the present invention can be used either individually or in the form of a mixture thereof. The kinetic viscosity of the base oils to be used is usually adjusted to the preferred range specified above.

[0047] Typical viscosity indices of base oils are in the range of 90-110 for mineral oils, 120-200 for synthetic hydrocarbon oils, 150-300 for polyalkylene glycols, 100-180 for ester oils, 190-500 for silicone oils and 50 to 400 for perfluoroalkyl ether oils (PFPE oils).

[0048] Petroleum fractions containing paraffinic, naphthenic and / or aromatic hydrocarbons are preferably used.

[0049] Synthetic oils, in particular poly-alphaolefins, polyalkylene glycols, polyalkylene glycol ethers, dialkyl ethers, acetals, natural ester oils, perfluoropolyether oils and silicone oils are also preferred.

[0050] To achieve an extended lubrication life, it is particularly preferred to use at least 10% by weight, based on the weight of the base oil, of an ester oil (in particular a polyol ester oil).

[0051] In a further preferred embodiment, the base oil contains at least one substance according to at least one of groups I to V according to the classification of American Petroleum Institute (API) from 1993: Group I: "Base stocks containing less than 90 percent saturates and / or more than 0.03 percent sulfur and a viscosity index of at least 80 and a maximum of 120." Group II: "Base stocks containing at least 90 percent saturates and a maximum of 0.03 percent sulfur, and a viscosity index of at least 80 and less than 120." Group III: "Base stocks containing at least 90 percent saturates and a maximum of 0.03 percent sulfur, and a viscosity index of at least 120." Group IV: Synthetic oils made from polyalphaolefins (PAO) with a viscosity index in the range of 125 to 200. Group V: Any other type of base oil, e.g., naphthenic oils and esters.

[0052] The lubricant used according to the invention usually contains a thickener, in particular a soap thickener.

[0053] For the purposes of the present invention, the terms "thickener" and "thickening agent" can be used synonymously. A soap thickener is a specific form of a thickener.

[0054] Suitable thickeners are known to the person skilled in the art and are commercially available.

[0055] Examples of suitable thickeners are urea compounds, amide compounds, imide compounds, condensed aromatic hydrocarbons such as indanthrene dyes, fluoropolymers, polyolefins, metal soaps, metal complex soaps, and inorganic substances such as silica gels, bentonites, carbon blacks, graphites or aerosils.

[0056] Preferred thickeners are metal soaps or metal complex soaps derived from carboxylic acids, sulfonic acids, phosphoric acids, with mono- or polyvalent metal cations, in particular with aluminum, alkali metal (preferably lithium), alkaline earth metal, titanium, and / or zirconium cations, or finely divided organic polymeric compounds, such as polytetrafluoroethylene, polyethylene or waxes.

[0057] Other preferred soap thickeners are lithium 12-hydroxystearate, lithium soaps, lithium complex soaps, aluminum soaps, aluminum complex soaps, calcium soaps and calcium complex soaps.

[0058] The soap thickener is particularly preferably a lithium soap or a lithium complex soap.

[0059] The lubricant according to the invention preferably contains about 2% by weight to about 20% by weight of a thickener or a mixture of thickeners, in each case based on the total weight of the lubricant.

[0060] The lubricant according to the invention particularly preferably contains about 2% to about 20% by weight of a lithium soap or a lithium complex soap or a combination thereof, in each case based on the total weight of the lubricant.

[0061] In a further preferred embodiment, the lubricant contains a thickener, in particular a soap thickener selected from the group consisting of lithium soaps, lithium complex soaps, aluminum soaps, aluminum complex soaps, calcium soaps, calcium complex soaps and any combinations thereof.

[0062] In a particularly preferred embodiment, the lubricant contains a soap thickener which contains or consists of a lithium soap or a lithium complex soap.

[0063] The lubricant used according to the invention usually contains an additive, in particular selected from the group consisting of antioxidants, metal deactivators, corrosion inhibitors, extreme pressure additives, wear protection additives, solid lubricants, adhesion additives, dyes and any combinations thereof.

[0064] In a further preferred embodiment, the lubricant contains the additive in an amount of more than 0 wt.% up to about 15 wt.%, based on the total weight of the lubricant.

[0065] The lubricant according to the invention may contain one or more further optional additives.

[0066] Suitable additional optional additives are known to the person skilled in the art and are commercially available.

[0067] Useful additives include antioxidants such as amines, phenols, sulfur compounds, and zinc dithiophosphate; rust inhibitors such as petroleum sulfonates, dinonyl naphthalene sulfonate, and sorbitan esters; lubricity-enhancing additives such as fatty acids and vegetable oils; metal deactivators such as benzotriazole and sodium sulfite; extreme pressure additives such as chlorine-, sulfur-, or phosphorus-containing inorganic compounds, zinc dithiophosphate, and organomolybdenum compounds; viscosity index improvers such as polymethacrylate, polyisobutylene, and polystyrene; and solid lubricants such as polytetrafluoroethylene ("PTFE"), bentonites, tungsten disulfide, molybdenum disulfide, or other layered phosphates, oxides, sulfides, and sulfates.

[0068] The lubricant according to the invention preferably contains up to about 15% by weight of one or more additives, each based on the total weight of the lubricant.

[0069] In a further preferred embodiment, the lubricant contains the PI particles in an amount of more than 0 wt.% up to about 15 wt.%, based on the total weight of the lubricant, and / or the PI particles have a particle size D 50 of up to 7 µm.

[0070] In a further preferred embodiment the lubricant contains the PI particles in an amount of more than 0 wt.% or of about 0.25 wt.%, up to about 14 wt.% or up to about 13 wt.% or up to about 12 wt.% or up to about 11 wt.% or up to about 10 wt.% or up to about 9 wt.% or up to about 8 wt.% or up to about 7 wt.% or up to about 6 wt.% or up to about 5 wt.% or up to about 4 wt.% or up to about 3 wt.-%, in each case based on the total weight of the lubricant; and / or the PI particles have a particle size D 50 of up to 14 µm or up to 13 µm or up to 12 µm or up to 11 µm or up to 10 µm or up to 9 µm or up to 8 µm or up to 7 µm or up to 6 µm or up to 5.5 µm or up to 5.25 µm; and / or the PI particles have a particle size D 50 of at least 0.1 µm or at least 0.5 µm or at least 1.0 µm or at least 1.5 µm or at least 2.0 µm or at least 2.5 µm or at least 3.0 µm. .

[0071] In a further preferred embodiment the PI particles have a particle size D 50 of up to 7 µm and the lubricant contains the PI particles in an amount of more than 0 wt.% up to about 1 wt.%, based on the total weight of the lubricant; or the PI particles have a particle size D 50 of up to 12 µm and the lubricant contains the PI particles in an amount of more than about 1 wt.% up to about 3 wt.%, based on the total weight of the lubricant.

[0072] In a further preferred embodiment, the lubricant does not contain the PI particles in an amount of more than 0.8 wt% to 1.2 wt%, based on the total weight of the lubricant.

[0073] In a further preferred embodiment, the PI particles have a particle size D 10 of up to 3.0 µm, and / or a particle size D 50 of up to 6.0 µm, and / or a particle size D 90 of up to 13.0 µm and / or a particle size D 100 of up to 47.0 µm.

[0074] In a further preferred embodiment, the particle size D 10 , D 50 , D 90 and / or D 100 is determined by means of laser diffraction particle size analysis.

[0075] In a further preferred embodiment, the particle size D 10 , D 50 , D 90 and / or D 100 is determined in accordance with ISO 13320:2020.

[0076] In a further preferred embodiment, the lubricant contains tetrafluoroethylene and / or polytetrafluoroethylene (PTFE) in an amount of at most about 1.0 wt.%, in each case based on the total weight of the lubricant, particularly preferably in an amount of 0 wt.%, in each case based on the total weight of the lubricant.

[0077] In a further preferred embodiment, the lubricant contains compounds that release toxic fluorine compounds at high temperatures generated by friction, in an amount of at most about 1.0 wt.%, based on the total weight of the lubricant, particularly preferably in an amount of 0 wt.%, based on the total weight of the lubricant. When a temperature is considered high depends on the specific application of the lubricant and can be determined by the person skilled in the art.

[0078] In a further preferred embodiment, the determination of the friction coefficient is carried out by an MTM tribometer, preferably carried out according to the experimental part of the present application.

[0079] Corresponding methods are known to the person skilled in the art, for example from the publications DE 10 2012 215145 and WO 2012 / 123192 A1.

[0080] In a further preferred embodiment, the wear or wear coefficient is determined via the HFRR test, preferably carried out according to the experimental part of the present application.

[0081] Corresponding methods are known to the person skilled in the art, for example from the publication WO 2012 / 123192 A1.

[0082] In a further aspect, the invention relates to a use of a lubricant according to the invention for lubricating between at least two surfaces, at least one of which is non-metallic, and which are located in or on a vehicle and / or in or on a machine element.

[0083] In a preferred embodiment, the vehicle is an automobile and / or the machine element is a gear or bearing.

[0084] In a further aspect, the invention relates to a use of PI particles having a particle size D 50 of up to 15 µm or up to 8 µm for improving tribological properties (in particular with regard to the coefficient of friction, the wear and / or the required lubrication between mutually interacting surfaces in relative motion) of a lubricant according to the invention for lubricating between at least two surfaces, of which at least one is non-metallic.

[0085] In a preferred embodiment, the number of (required) PI particles is determined by their proportion in the lubricant, preferably by their proportion in the lubricant in wt.%, based on the total weight of the lubricant.

[0086] The lubricants according to the invention can be used in a wide variety of applications. For example, the lubricant according to the invention can be a rolling bearing grease, high-temperature grease, plain bearing grease, lubricant for food applications, fitting grease, assembly grease, contact lubricant, fluid grease (especially for gears), high-speed grease, extreme pressure grease, grease for plastics lubrication, or long-life grease.

[0087] The lubricants according to the invention are particularly suitable for use in or for vehicles, gears, bearings, constant velocity joints, rolling bearings and transmissions. Experimental data

[0088] The following examples serve to illustrate the invention but are not to be construed as limiting.

[0089] To produce the lubricants according to the invention, a lithium grease (manufacturer: AXEL Christiernsson) was mixed with the respective proportion of PI particles at 2000 rpm for 5 minutes without any additional additives. The lubricant was then drained and used for the tests.

[0090] Figures 3a and 3b show, among other things, that PI particles have approximately the same effect on the friction coefficient as PTFE particles (5 µm), but without exhibiting their negative properties according to paragraph

[0010] .

[0091] Figures 3a and 3b also show, among other things, that PI particles reduce the wear coefficient compared to empty grease.

[0092] The results of the HFRR procedure shown in Figures 3a and 3b were determined using the device according to Figure 2and at the following parameters: Temperature: 80 °C; Contact pressure (GPa): 0.9; Stroke (mm): 15; Frequency (Hz): 15; Test cylinder: AISI 52100; and test disc: AISI 52100.

Claims

1. A lubricant containing PI particles with a particle size D 50 of up to 15 µm.

2. The lubricant according to claim 1, wherein the particle size D 50 determined by laser diffraction particle size analysis, preferably according to ISO 13320:2020.

3. The lubricant according to claim 1 or 2, wherein the lubricant contains a thickener, preferably a soap thickener, preferably selected from the group consisting of lithium soaps, lithium complex soaps, aluminum soaps, aluminum complex soaps, calcium soaps, calcium complex soaps and combinations thereof.

4. The lubricant according to claim 3, wherein the lubricant contains a soap thickener which contains or consists of a lithium soap or a lithium complex soap.

5. The lubricant according to claim 3 or 4, wherein the lubricant contains the thickener, preferably soap thickener, in an amount of about 2 wt.% to about 20 wt.%, based on the total weight of the lubricant.

6. The lubricant according to one or more of the preceding claims, wherein the lubricant contains a base oil, preferably selected from the group consisting of mineral oils, synthetic oils, vegetable oils and combinations thereof.

7. The lubricant of claim 6, wherein the lubricant contains the base oil in an amount of about 65 wt% to about 90 wt%, based on the total weight of the lubricant.

8. The lubricant according to one or more of the preceding claims, wherein the lubricant contains an additive, preferably selected from the group consisting of antioxidants, metal deactivators, corrosion inhibitors, extreme pressure additives, anti-wear additives, solid lubricants, adhesion additives, dyes and combinations thereof.

9. The lubricant of claim 8, wherein the lubricant contains the additive in an amount of more than 0 wt.% up to about 15 wt.%, based on the total weight of the lubricant.

10. The lubricant according to one or more of the preceding claims, wherein - the PI particles have a particle size D 50 of 5±4 µm, 5±3 µm, 5±2 µm or 5±1 µm; or - the lubricant contains the PI particles in an amount of more than 0 wt.% up to about 15 wt.%, based on the total weight of the lubricant, and / or the PI particles have a particle size D 50of up to 12 µm; or - the lubricant contains the PI particles in an amount of more than 0 wt.% up to about 7 wt.%, based on the total weight of the lubricant, and / or the PI particles have a particle size D 50 from 2 µm to 8 µm; or - the lubricant contains the PI particles in an amount of about 0.25 wt.% to about 3.5 wt.%, based on the total weight of the lubricant, and / or the PI particles have a particle size D 50 from 3 µm up to 6 µm; or - the lubricant is a lubricating grease, which preferably contains a soap thickener which contains or consists of a lithium soap or a lithium complex soap, and / or the lubricating grease contains the PI particles preferably in an amount of about 0.25 wt.% up to about 3.0 wt.%, based on the total weight of the lubricating grease, and / or the PI particles preferably have a particle size D 50 from 3 µm to 5.5 µm.

11. Use of a lubricant as defined in one or more of the preceding claims for lubricating between at least two surfaces, which are preferably located in or on a vehicle and / or in or on a machine element.

12. The use according to claim 11, wherein the vehicle is an automobile and / or the machine element is a gear or bearing.

13. The use according to claim 11 or 12, wherein - at least one surface is non-metallic; or - all surfaces contain or consist of metal; or - all surfaces contain or consist of plastic; or - all surfaces are non-metallic and contain or consist of plastic; or - all surfaces contain or consist of metal and do not contain plastic; or - at least one of the surfaces is non-metallic and contains or consists of plastic and at least one of the remaining surfaces contains or consists of metal and does not contain plastic; or - at least two of the surfaces are non-metallic and contain or consist of plastic and at least two of the remaining surfaces contain or consist of metal and do not contain plastic;or - there are exactly two surfaces, one of which contains or consists of plastic and the other of which contains or consists of metal.; 14. The use according to claim 11, 12 or 13, wherein the plastic contains or consists of polyetheretherketone (PEEK), and / or contains or consists of polyoxymethylenes (POM), and / or contains or consists of UHMWPE (ultra-high molecular weight polyethylene), and / or contains or consists of PA66, and / or contains or consists of PA6G (cast polyamide 6), and / or contains or consists of PA12G (cast polyamide 12), and / or contains or consists of PPAs.

15. Use of PI particles with a particle size D 50of up to 15 µm, preferably up to 8 µm or up to 6 µm, for improving tribological properties of a lubricant as defined in one or more of claims 1 to 10, for lubrication between at least two surfaces as defined in one or more of claims 11 to 14.

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