WO2020064619A1

WO2020064619A1 - Use of trialkoxysilane-based compounds for lubricants

Info

Publication number: WO2020064619A1
Application number: PCT/EP2019/075507
Authority: WO
Inventors: Benjamin JURETZKA; Stephan Wieber; Roland WILKENS; Michael Hagemann; Robert Kolb; Günter Schmitt; Can Metehan TURHAN; Philipp Albert; Ralf Riedel
Original assignee: Evonik Operations Gmbh
Priority date: 2018-09-24
Filing date: 2019-09-23
Publication date: 2020-04-02

Abstract

The present invention is directed to the use of trialkoxysilanes or trialkoxysilane-based compounds as film forming additives in lubricating oil compositions where they reduce friction, wear or both by building up a multilayer tribofilm. The present invention is also directed to the multilayer tribofilm comprising a tribofilm and a tribopolymer and an element comprising at least two components moveable with respect to one another, between the surfaces of which is present the multilayer tribofilm formed by the trialkoxysilanes or trialkoxysilane-based compounds according to the present invention.

Description

Use of Trialkoxysilane-based Compounds for Lubricants

The present invention is directed to the use of trialkoxysilanes or trialkoxysilane-based compounds as film forming additives in lubricating oil compositions where they reduce wear, friction or both by building up a multilayer tribofilm. The present invention is also directed to the multilayer tribofilm comprising a tribofilm and a tribopolymer and an element comprising at least two components moveable with respect to one another, between the surfaces of which is present the multilayer tribofilm formed by the trialkoxysilanes or the trialkoxysilane-based compounds according to the present invention.

State of the Art

Tribofilms, sometimes also referred to as boundary lubricant films, boundary lubricating films, tribo- boundary films or boundary films, are films that form on tribologically stressed surfaces. While there exists no universal definition of the term, it is mostly used to refer to solid surface films that result from a chemical reaction of lubricant components and/or tribological surfaces.

Tribofilms play an important role in reducing friction and wear in lubricated systems. They form as a result of complex mechanochemical interactions between surface materials and lubricants, and the study of tribofilm formation processes is a major field of tribology.

The efficiency of modern gearboxes, engines or hydraulic pumps depends not only on the characteristics of the machine parts but also greatly on the frictional properties of the lubricant used. For the development of such lubricants, it is of particular importance to have knowledge about the behavior of the lubricant components used in relation to film formation and friction, and the choice of suitable additives, which are able to lower the average fuel consumption of a vehicle by a few percent. In this context, particularly effective constituents of a lubricant may be base oils with particularly low viscosity and hence low inherent friction, and also organic friction modifiers.

As the dimensions of modern gearboxes and pump housings are getting smaller and smaller, they are cooled less efficiently, in addition both gearwheels and bearings have to bear higher loads, which in total results in higher demands for lubricants.

The reduction of power losses in transmissions and bearings is a strong trigger for research and investigations of friction modifying materials. Lower viscosity lubricants and higher power densities in tribological systems require new optimized lubricants to improve anti wear and load carrying capacities (B.H. K. Michaelis, M. Hinterstoi¾er, Influence factors on gearbox power loss, Industrial Lubrication and Tribology 201 1 , 63, 46 - 55; S.D. Evans, Delivering Axle Efficiency and Fuel Economy Through Optimized Fluid Design, SAE International 2014, 1 ). Therefore, extensive research has been performed on novel surface-active chemicals, which show promising behavior and provide unique physical properties upon lubrication (H. Spikes, Friction Modifier Additives, Tribology Letters 2015, 60). The overall goal of research on surface-active lubricant additives is to understand how to tailor these additives to obtain beneficial boundary or tribofilms.

Organosilanes/-siloxanes are classical film formers used via spin coating, dip coating or spray coating on a vast range of materials and also for many kinds of applications like anti corrosion, hydrophobic or scratch resistant coatings. Their degree of functionalization is very high and well established in order to achieve molecular and micro structures with tailored properties. Their key reaction mechanisms are the hydrolysis and subsequent condensation reactions to cross-link under formation of gel networks. The resulting microstructure is, however, strongly dependent on parameters like water ratio or pH value. A special property is that, upon drying or pyrolysis of such gels, polymeric, glassy or ceramic materials can be obtained. The stronger the cross-linking and the lower the organic content the higher is the mechanical strength of such materials. Therefore, it is of fundamental interest to study which kind of film structures and properties are obtained under tribological stress.

Yet, there is still no detailed understanding about the decomposition and deposition of such films and, more importantly, on how to tailor them.

US 7,867,960 discloses a method for forming an antiwear film on an internal engine component by using a tetra-functional hydrolyzable silane compound which forms a film. The document does not describe multilayer films formed by trialkoxysilane-based polymers and their ability to reduce friction and wear.

US 7,399,734 and US 6,887,835 both disclose non-phosphorous-containing antiwear, anti-fatigue and extreme pressure additives that are derived from polysiloxanes and their use in fuels and lubricants. The documents do not describe multilayer films formed by trialkoxysilane-based polymers and their ability to reduce friction and wear.

US 9,828,392, US 6,767,982, EP 2782952 and EP 2782953 are directed to compositions comprising olefinically functionalized siloxane oligomers which are derived from olefinically functionalized alkoxysilanes and optionally alkoxysilanes functionalized with saturated hydrocarbons (Hereafter also mentioned as trialkoxysilane-based compounds or short organosiloxane.).

Summary of the Invention

It was now surprisingly found that trialkoxysilane-based compounds can be used as film forming additives in lubricating oil compositions wherein they form a multilayer tribofilm which leads to reduced wear or friction or even both, reduced wear and reduced friction.

Detailed Description of the Invention An object of the present invention is directed to the use of trialkoxysilanes or trialkoxysilane-based compounds of general formula (I) as film forming additives in lubricating oil compositions, wherein the trialkoxysilane-based compounds form a multilayer tribofilm. This multilayer tribofilm leads to a reduced wear, reduced friction or both, reduced wear and reduced friction.

The tria!koxysi!ane-based compounds which can be used in accordance with the present invention are defined by general formula (I)

(I) wherein

R¹ independently of each other denotes H, Chb or C₂H5,

R² independently of each other denotes Ci-3-alkyl,

R³ independently of each other denotes Ci-ie-alkyl or C_2-4-alkenyl,

m denotes an integer of > 0, preferably of 0 to 20, and

n denotes an integer of > 0, preferably of 0 to 20,

with the proviso that (m+n) is > 2 or

m > 2 with n = 0 or

n > 2 with m = 0 or

n = 1 with m = 0. The trialkoxysilane-based compounds of general formula (I) usually represent a mixture of linear, cyclic and branched compounds with a corresponding oligomer distribution. Hence, the said general formula (I) gives only an exemplary or idealized example to show the complicated reality of the present corresponding organosiloxanes. Therefore, reference is made here to the more detailed explanations regarding the preparation, structure and properties of said trialkoxysilane- based compounds and their composition disclosed in US 9,828,392, EP 2 782 952 and

EP 2 782 953, in particular EP 2 782 953.

The term "Ci-3-alkyl" encompasses methyl, ethyl, n-propyl and iso-propyl.

The term "Ci-16-alkyl" encompasses straight chain or branched alkyl groups having 1 to 16 carbon atoms; e.g. methyl, ethyl, propyl and propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl or hexadecyl as well as their branched homologues.

The term "C_2-4-alkenyl" encompasses vinyl, allyl and butenyl, preferably vinyl. The trialkoxysilane-based compounds which can be further used in accordance with the present invention are preferably defined by general formula (I), wherein

R¹ independently of each other denote H or CH3,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl or vinyl and propyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0.

Further preferred trialkoxysilane-based compounds which can be used in accordance with the present invention are defined by general formula (I), wherein

R¹ independently of each other denote H or CFh or C2H5,

R² independently of each other denote CH3 or C2H5,

R³ independently of each other denote propyl,

m denotes an integer of > 0 and

n denotes an integer of > 1 ,

with the proviso that (m+n) is > 2, preferably n > 2 with m = 0, especially n = 3 to 6 with m = 0.

R¹ independently of each other denote CH3,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0.

The trialkoxysilane monomers which can be further used in accordance with the present invention are preferably defined by general formula (I), wherein

R¹ independently of each other denote H or CH3 or C2H5

R² independently of each other denote CH3 or C2H5, and

R³ independently of each other denote CH3, C2H5, C16H33 or vinyl,

with the proviso n = 1 with m = 0.

Another object of the present invention is directed to a lubricating oil composition, comprising:

(A) 80 to 99.9% by weight of a base oil; (B) 0.1 to 5% by weight of a trialkoxysilane or a trialkoxysilane-based compound of general formula (I); and

(C) 0 to 15% by weight of one or more further additives.

The content of each component (A), (B) and (C) is based on the total weight of the lubricating oil composition.

In a particular embodiment, the proportions of components (A), (B) and (C) add up to 100% by weight.

The lubricating oil compositions can be useful for various applications including driving systems (such as manual transmission fluids, differential gear oils, automatic transmission fluids and continuously variable transmission fluids, axle fluid formulations, dual clutch transmission fluids, and dedicated hybrid transmission fluids), hydraulic fluids (such as hydraulic oils for machinery, power steering oils, shock absorber oils, compressor oils), engine or motor oils (for gasoline engines and for diesel engines), industrial gear oil formulations (such as wind turbine), paper machine lubricant, machine tools lubricant, metalworking fluids, and transformer oils.

The lubricating oil compositions of the present invention can be conveniently prepared by blending or mixing the trialkoxysilanes or trialkoxysilane-based compounds (B) according to the present invention with a base oil (A) of lubricating viscosity and, optionally, one or more further additives (C).

The base oil used in the lubricating oil compositions of the present invention are generally tailored to the specific use, e.g. engine oil, gear oil, industrial oil, hydraulic fluids, etc. and comprises an oil of lubricating viscosity. Such oils include natural and synthetic oils, oil derived from hydrocracking, hydrogenation, and hydro-finishing, unrefined, refined, re-refined oils or mixtures thereof.

The base oil may also be defined as specified by the American Petroleum Institute (API) (see April 2008 version of "Appendix E-API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils", section 1.3 Sub-heading 1 .3. "Base Stock Categories").

The API currently defines five groups of lubricant base stocks (API 1509, Annex E - API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils, September 201 1 ). Groups I, II and III are mineral oils which are classified by the amount of saturates and sulphur they contain and by their viscosity indices; Group IV are polyalphaolefins; and Group V are all others, including e.g. ester oils. The table below illustrates these API classifications.

The kinematic viscosity at 100°C (KV100) of appropriate apolar base oils used to prepare an additive composition or lubricating composition in accordance with the present invention is preferably in the range of 3 mm²/s to 10 mm²/s, more preferably in the range of 4 mm²/s to 8 mm²/s, according to ASTM D445.

Further base oils which can be used in accordance with the present invention are Group ll-lll Fischer-Tropsch derived base oils.

Fischer-Tropsch derived base oils are known in the art. By the term "Fischer-Tropsch derived" is meant that a base oil is, or is derived from, a synthesis product of a Fischer-Tropsch process. A Fischer-Tropsch derived base oil may also be referred to as a GTL (Gas-To-Liquids) base oil. Suitable Fischer-Tropsch derived base oils that may be conveniently used as the base oil in the lubricating composition of the present invention are those as for example disclosed in EP 0 776 959, EP 0 668 342, WO 97/21788, WO 00/15736, WO 00/14188, WO 00/14187, WO 00/14183,

WO 00/14179, WO 00/081 15, WO 99/41332, EP 1 029 029, WO 01/18156, WO 01/57166 and WO 2013/189951 .

In the context of the present invention preferred base oils are API Group III oils and mixtures thereof.

The lubricating oil compositions according to the present invention may also contain, as component (C), further additives selected from the group consisting of VI improvers, dispersants, defoamers, detergents, antioxidants, pour point depressants, antiwear additives, extreme pressure additives, anticorrosion additives, dyes and mixtures thereof.

Suitable VI improvers include hydrogenated styrene-diene copolymers (HSDs, US41 16 917, US3772196 and US4788316), especially based on butadiene and isoprene, and also olefin copolymers (OCPs, K. Marsden: "Literature Review of OCP Viscosity Modifiers", Lubrication Science 1 (1988), 265), especially of the poly(ethylene-co-propylene) type, which may often also be present in N/O-functional form with dispersing action, or polyalkyl(meth)acrylates, which are usually present in N-functional form with advantageous additive properties ( boosters ) as dispersants, wear protection additives and/or friction modifiers (DE 1 520 696 to Rohm and Haas, WO 2006/007934 to RohMax Additives). Appropriate dispersants include poly(isobutylene) derivatives, for example

poly(isobutylene)succinimides (PIBSIs), including borated PIBSIs; and ethylene-propylene oligomers having N/O functionalities.

Dispersants (including borated dispersants) are preferably used in an amount of 0 to 5% by weight, based on the total amount of the lubricant composition.

Suitable defoamers are silicone oils, fluorosilicone oils, fluoroalkyl ethers, etc.

The defoaming agent is preferably used in an amount of 0.005 to 0.1 % by weight, based on the total amount of the lubricant composition.

The preferred detergents include metal-containing compounds, for example phenoxides;

salicylates; thiophosphonates, especially thiopyrophosphonates, thiophosphonates and phosphonates; sulfonates and carbonates. As metal, these compounds may contain especially calcium, magnesium and barium. These compounds may preferably be used in neutral or overbased form.

Detergents are preferably used in an amount of 0.2 to 1 % by weight, based on the total amount of the lubricant composition.

The suitable antioxidants include, for example, phenol-based antioxidants and amine-based antioxidants.

Phenol-based antioxidants include, for example, octadecyl-3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate; 4,4' -methylenebis(2,6-di-tert-butylphenol); 4,4' -bis(2,6-di-t- butylphenol); 4,4' -b is(2-methyl-6-t-butylphenol); 2,2' -methylenebis(4-ethyl-6-t-butylphenol); 2,2' - methylenebis( 4-methyl-6-t-butyl phenol); 4,4' -butyl idenebis(3-methyl-6-t-butylphenol); 4,4'- isopropylidenebis(2,6-di-t-butylphenol); 2,2'-methylenebis(4-methyl-6-nonylphenol); 2,2'- isobutylidenebis(4,6-dimethylphenol); 2,2'-methylenebis(4-methyl-6-cyclohexylphenol); 2,6-di-t- butyl-4-methylphenol; 2,6-di-t-butyl-4-ethyl-phenol; 2,4-dimethyl-6-t-butylphenol; 2,6-di-t-amyl-p- cresol; 2,6-di-t-butyi-4-(N,N'-dimethylaminomethylphenol); 4,4'thiobis(2-methyl-6-t-butylphenol); 4,4'-thiobis(3-methyl-6-t-butylphenol); 2,2'-thiobis(4-methyl-6-t-butylphenol); bis(3-methyl-4- hydroxy-5-t-butylbenzyl) sulfide; bis(3,5-di-t-butyl-4-hydroxybenzyl) sulfide; n-octyl-3-(4-hydroxy- 3,5-di-t-butylphenyl)propionate; n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate; 2,2'- thio[diethyl-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], etc. Of those, especially preferred are bis-phenol-based antioxidants and ester group containing phenol-based antioxidants.

The amine-based antioxidants include, for example, monoalkyldiphenylamines such as monooctyldiphenylamine, monononyldiphenylamine, etc.; dialkyldiphenylamines such as 4,4' - dibutyldiphenylamine, 4,4'-dipentyldiphe nylamine, 4,4'- dihexyldiphenylamine, 4,4'- diheptyldiphenylamine, 4,4'-dioctyldiphenylamine, 4,4'-dinonyldiphenylamine, etc.;

polyalkyldiphenylamines such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, tetranonyldiphenylamine, etc.; naphthylamines, concretely alpha- naphthylamine, phenyl-alpha-naphthylamine and further alkyl-substituted phenyl-alpha- naphthylamines such as butylphenyl-alpha-naphthylamine, pentylphenyl-alpha-naphthylamine, hexylphenyl-alpha-naphthylamine, heptylphenyl-alpha-naphthylamine, octylphenyl-alpha- naphthylamine, nonylphenyl-alpha-naphthylamine, etc. Of those, diphenylamines are preferred to naphthylamines, from the viewpoint of the antioxidation effect thereof.

Suitable antioxidants may further be selected from the group consisting of compounds containing sulfur and phosphorus in the form of "OOS triesters" = reaction products of dithiophosphoric acid with activated double bonds from olefins, cyclopentadiene, norbornadiene, a-pinene, polybutene, acrylic esters, maleic esters (ashless on combustion); organosulfur compounds, for example dialkyl sulfides, diaryl sulfides, polysulfides, modified thiols, thiophene derivatives, xanthates, thioglycols, thioaldehydes, sulfur-containing carboxylic acids; heterocyclic sulfur/nitrogen compounds, especially dialkyldimercaptothiadiazoles, 2-mercaptobenzimidazoles; zinc

bis(dialkyldithiocarbamate) and methylene bis(dialkyldithiocarbamate); organophosphorus compounds, for example triaryl and trialkyl phosphites; organocopper compounds and overbased calcium- and magnesium-based phenoxides and salicylates.

Antioxidants are used in an amount of 0 to 15% by weight, preferably 0.1 to 10% by weight, more preferably 0.5 to 5% by weight, based on the total amount of the lubricant composition.

The pour-point depressants include ethylene-vinyl acetate copolymers, chlorinated paraffin- naphthalene condensates, chlorinated paraffin-phenol condensates, polymethacrylates, polyalkylstyrenes, etc. Preferred are polymethacrylates having a mass-average molecular weight of from 5.000 to 200.000 g/mol.

The amount of the pour point depressant is preferably from 0.1 to 5% by weight, based on the total amount of the lubricant composition.

The preferred antiwear (AW) and extreme pressure (EP) additives include sulfur-containing compounds such as molybdenum dithiocarbamate, molybdenum dithiophosphate, disulfides, sulfurized olefins, sulfurized oils and fats, sulfurized esters, thiocarbonates, thiocarbamates, polysulfides, etc.; phosphorus-containing compounds such as phosphites, phosphates, for example trialkyl phosphates, triaryl phosphates, e.g. tricresyl phosphate, amine-neutralized mono- and dialkyl phosphates, ethoxylated mono- and dialkyl phosphates, phosphonates, phosphines, amine salts of those compounds, etc.; sulfur and phosphorus-containing anti-wear agents such as thiophosphites, thiophosphates, thiophosphonates, amine salts of those compounds, etc.

The antiwear agent may be present in an amount of 0 to 3% by weight, preferably 0.1 to 1.5% by weight, more preferably 0.5 to 0.9% by weight, based on the total amount of the lubricant composition. Some of the compounds listed above may fulfil multiple functions. Many sulfur- and phosphorus- containing AW and EP have the character of an antioxidant and corrosion inhibitor (here: metal passivator/deactivator). The above-detailed additives are described in detail, inter alia, in T. Mang, W. Dresel (eds.): "Lubricants and Lubrication", Wiley-VCH, Weinheim 2001 ; R. M. Mortier, S. T. Orszulik (eds.): "Chemistry and Technology of Lubricants"; Blackie Academic & Professional, London 1992; or J. Bartz: "Additive fur Schmierstoffe", Expert-Verlag, Renningen-Malmsheim 1994. Preferably, the total concentration of the one or more additives (c) is up to 20% by weight, more preferably 0.05% to 15% by weight, more preferably 5% to 15% by weight, based on the total weight of the lubricant composition.

It was found that the trialkoxysilanes or trialkoxysilane-based compounds of general formula (I) form a multi-layered film architecture, consisting of a polymeric layer on top of a glass-like coating layer which is strongly crosslinked and partly inorganic.

The multilayer tribofilm therefore comprises the following layers:

(a) a tribofilm A and

(b) a tribopolymer B.

As already outlined further above, tribofilms are films that form on tribologically stressed surfaces. The term is mostly used to refer to solid surface films that result from a chemical reaction of lubricant components and/or tribological surfaces. Tribofilms form, as a result of complex mechanochemical interactions, between surface materials and lubricants.

The architecture of the trialkoxysilane-based multilayer tribofilms can be illustrated by Scheme 1.

Scheme 1 : Schematic architecture of trialkoxysilane -based multilayer tribofilm.

composes and

[Multi-layered-film

The tribofilm A is adhesive, strongly crosslinked and is the consequence of an organic-into- inorganic transformation (i.e. is partly inorganic). The tribopolymer-layer B is weakly crosslinked and shows only weak adhesion on the surface.

The tribopolymer B results from hydrolysis and condensation reactions with water in the base oil. While the tribofilm A increases the friction, it also provides corrosion resistance. The tribopolymer- layer B exhibits a viscous behavior and its thickness is dependent on the molecular structure of the starting precursor. If the tribopolymer-layer B is derived from oligomeric organosiloxanes, then its thickness can overcome the adhesive tribofilm roughness, resulting in reduced friction and wear by more than 40%.

Consequently, the tribofilm A is characterized by being an adhesive, rough layer consisting of polysiloxanes with a high degree of Si-O-Si cross-linking. In this context, the term "high degree of Si-O-Si cross-linking" refers to a micro-structure that is characterized by an absence of unreacted Si-OChh and Si-OH groups.

That means that the tribofilm A is a strongly cross-linked film resulting from severe frictional decomposition of the trialkoxysilane-based compounds according to the present invention.

The polysiloxanes form a silicon oxycarbide (SiOC) network. The tribofilm A is the result from further decomposition and networking of the tribopolymer layer B by an organic into inorganic transformation.

The tribopolymer B is characterized by being a non-adhesive, viscous layer consisting of polysiloxanes with a low degree of Si-O-Si crosslinking. The term "low degree of Si-O-Si cross- linking" refers to a micro-structure that is characterized by the presence of functional organic groups and Si-O-Si, as well as unreacted S1-OCH3 and Si-OH groups.

That means that the tribopolymer B is weakly cross-linked by hydrolysis and condensation reactions.

A further object of the present invention is directed to a method of reducing friction of a lubricating oil composition, the method comprising the steps of:

(a) adding a trialkoxysilane or a trialkoxysilane-based compound of general formula (I) to a lubricating oil composition;

(b) applying the resulting lubricating oil composition to tribologically stressed surfaces being inside engines, motors, manual transmissions, differential gears, automatic transmissions, continuously variable transmissions, axles, dual clutches, dedicated hybrid transmissions, hydraulic machinery, power steerings, shock absorbers, compressors, industrial gears, paper machines, machine tools, metal workings, and transformers; and

(b) forming a multilayer tribofilm. The trialkoxysilane or a trialkoxysilane-based compound of general formula (I), the lubricating oil composition and the multilayer tribofilm are characterized as outlined further above.

A further object of the present invention is directed to a method of reducing wear of a lubricating oil composition, the method comprising the steps of:

(b) forming a multilayer tribofilm.

The trialkoxysilane or a trialkoxysilane-based compound of general formula (I), the lubricating oil composition and the multilayer tribofilm are characterized as outlined further above.

A further object of the present invention is directed to a method of reducing friction and wear of a lubricating oil composition, the method comprising the steps of:

(b) forming a multilayer tribofilm.

The trialkoxysilane or a trialkoxysilane-based compound of general formula (I), the lubricating oil composition and the multilayer tribofilm are characterized as outlined further above. A further object of the present invention is directed to the use of trialkoxysilanes or trialkoxysilane- based compounds of general formula (I) to form a multilayer tribofilm. A further object of the present invention is directed to the multilayer tribofilm prepared from trialkoxysilanes or trialkoxysilane-based compounds of general formula (I), the tribofilm comprising the following layers:

(a) a tribofilm A and

(b) a tribopolymer B.

The trialkoxysilanes or trialkoxysilane-based compounds of general formula (I), tribofilm A and tribopolymer B are characterized as being outlined further above. A further object of the present invention is directed to an element, comprising at least two components movable with respect to one another, between the surfaces of which is present a multilayer tribofilm formed by a lubricating oil composition, characterized in that the multilayer tribofilm comprises:

(a) a tribofilm A and

(b) a tribopolymer B.

The trialkoxysilanes or trialkoxysilane-based compounds of general formula (I), tribofilm A and tribopolymer B are characterized as being outlined further above. The element can be inside engines, motors, manual transmissions, differential gears, automatic transmissions, continuously variable transmissions, axles, dual clutches, dedicated hybrid transmissions, hydraulic machinery, power steerings, shock absorbers, compressors, industrial gears, paper machines, machine tools, metal workings, and transformers. The invention is partly illustrated by the enclosed Figures.

Figure 1 : Standard Stribeck curves measured @ 136 minutes (30N, 100°C and 50% SRR).

Figure 2: ATR-FTIR spectra of MTM disc wear tracks tested with a composition comprising

2% by weight of Example 1 and of the pure precursor.

Figure 3: ATR-FTIR spectra of MTM disc wear tracks tested with a composition comprising

2% by weight of Example 6 and of the pure precursor.

The invention has been further illustrated by the following non-limiting examples. Experimental Part

The trialkoxysilanes or trialkoxysilane-based compounds according to the present invention were investigated as oil additives in terms of their tribological as well as their tribochemical behavior. The additives were dissolved in a group III base oil and tested in a ball-on-disc tribometer to evaluate the influences on friction and to track the film formation. In addition, the influence on wear is investigated by four-ball wear testing, respectively. The obtained reaction films on the discs from the friction tests were analyzed by Fourier-transform infrared spectroscopy (FTIR) for their chemical structure.

Tribological test methods

A Mini-Traction-Machine with spacer layer imaging method MTM-SLIM ball on disc tribometer, by PCS-lnstruments UK, was used to investigate the tribofilm formation and to record traction curves. During this test, a 19.05 mm diameter ball made of AISI 52100 (R_a = 0.02 pm, 800 - 920 HV) is loaded against a flat surface of a 48 mm diameter AISI 52100 steel disc (R_a = 0.01 pm, 720 - 780 HV) which is immersed in the oil sample. The disc and ball are separately driven and a fixed sliding to roll ratio of 50 % (SRR) was set. The tests were conducted under a constant load of 30 N (maximum Hertzian pressure P max^— 0.95 GPa) and temperature of 100°C (see Table 1 for summary).

The testing procedure was as follows: The total time was set to 150 minutes with rubbing steps running at 100 mm/s. In addition, 6 Stribeck curve measurements are recorded between the rubbing steps, with mean speeds starting from 2500 mm/s to 5 mm/s.

Table 1 : Test conditions and specimen properties for the friction measurements of MTM tests.

For evaluation, the area (integral) below the coefficient of friction curve determined for the examples according to the present invention is expressed as a ratio to the area for the reference oil. The reference oil used is an API group III oil Nexbase®3050 without addition of film-forming polymers.

At high sliding speeds, the coefficient of friction is typically very small since the high speed results in a large amount of oil being introduced from the areas of friction into the lubrication gap. With decreasing sliding speed, less and less oil is introduced into the lubrication gap, and the coefficients of friction rise. It is of particular interest to lower the coefficients of friction, especially also at low sliding speeds. Therefore, the reduction in friction at low sliding speeds (5-90 mm/s). The reduction in friction at low speeds is accordingly calculated as follows:

Reduction in friction (low speed) ^:

J⁹⁰ friction values reference oil J⁹° friction values , candidate oil

* 100%

J⁹⁰ friction values reference oil

The anti-wear effect was measured with a 4-ball wear tribometer following DIN 51350-3. In this setup, four identical steel balls made of AISI 52100 with hardness of 63 +/- 3 HRC and a diameter of 12.7 mm were used. The load was set to 300 N resulting in a Hertzian pressure of 2.12 GPa, the tests were run at room temperature, without any cooling or heating of the system. The total testing time was set to 60 minutes where the top ball rotated with 1450 rotations per minute. Afterwards the calotte diameters of all fixed three balls were measured with an optical microscope and the total average of two tests are presented. Surface characterization

All test specimen have been rinsed with benzene after testing. In addition, some parts of the wear tracks have been wiped clean with tissues and acetone for further characterization.

Documentation of wear tracks on the MTM discs have been obtained with a laser scanning digital optical microscope Olympus LEXT OLS4100. The calotte diameters of the 4 ball wear balls were determined with a METOCHECK V-7000 digital optical light microscope.

The obtained tribofilms were chemically analyzed by IR spectroscopy using an unused steel disc background with a FT-IR spectrometer Varian 670 FT-IR in attenuated total reflection-FTIR (AT- FTIR) mode. Materials and preparation of oil blends

Example 1 : vinyltrimethoxysiloxane oligomer with n > 2

The vinyltrimethoxysilane oligomer (also called vinylmethoxysiloxane oligomer) was prepared according to example 1 as disclosed in US 9,273,186 B2: 220 g of vinyltrimethoxysilane were charged to a reaction flask. 95 g of methanol were mixed with 21 g of water and 0.4 g of 20% strength hydrochloric acid, and the mixture was transferred to a dropping funnel. At a temperature of about 25°C, dropwise addition to the vinylsilane took place from the dropping funnel, slowly and with stirring. After the end of the addition, the oil bath was heated to 85°C, and so the methanol boiled under reflux. After a reaction time of around three hours, the methanol was distilled off at the stated oil bath temperature and at a reduced pressure of about 150 to 180 mbar. For further removal of methanol, the vacuum was set to below 1 mbar.

Example 2: vinylethoxysilane-/propylethoxysilane co-oligomer with (m+n) > 2

The co-oligomer of propyltriethoxysilane with vinyltriethoxysilane was prepared according to example 3 as disclosed in US 9,273,186 B2:

98 g of vinyltriethoxysilane and 100 g of propyltriethoxysilane were charged to a reaction flask. 87 g of ethanol were mixed with 13 g of water and 0.2 g of 20% strength hydrochloric acid, and the mixture was transferred to a dropping funnel. At a temperature of about 25°C, dropwise addition to the vinylsilane took place from the dropping funnel, slowly and with stirring. After the end of the addition, the oil bath was heated to 85°C, and so the ethanol boiled under reflux. After a reaction time of around three hours, the ethanol was distilled off at the stated oil bath temperature and at a reduced pressure of about 150 to 180 mbar. For further removal of ethanol, the vacuum was set to below 1 mbar.

Example 3: hexadecyltrimethoxysilane

Hexadecyltrimethoxysilane is inter alia available from Sigma Aldrich (CAS Number 16415-12-6). Example 4: propyltriethoxysilane oligomer with n > 2 (compare e.g. US 6,841 ,197)

Example 5: vinyltrimethoxysilane monomer

H₃CO— Si— OCH₃

OCH₃

Vinyltrimethoxysilane is inter alia available from Sigma Aldrich or Evonik Industries AG (CAS Number 2768-02-7).

Example 6: vinylethoxysilane oligomer with n > 2

The vinylethoxysilane oligomer (also called vinylethoxysiloxane oligomer) was prepared according to example 2 as disclosed in US 9,273,186 B2:

195 g of vinyltriethoxysilane were charged to a reaction flask. 93 g of ethanol were mixed with 14.8 g of water and 0.2 g of 20% strength hydrochloric acid and the mixture was transferred to a dropping funnel. At a temperature of about 25°C, dropwise addition to the vinylsilane took place from the dropping funnel, slowly and with stirring. After the end of the addition, the oil bath was heated to 85°C, and so the ethanol boiled under reflux. After a reaction time of around three hours, the ethanol was distilled off at the stated oil bath temperature and at a reduced pressure of about 150 to 180 mbar. For further removal of ethanol, the vacuum was set to below 1 mbar.

Table 2: Analytical results of siloxane oligomers prepared.

For further testing, all examples were mixed with the Group III base oil Nexbase® 3050 from Neste AG. The base oil has a dynamic viscosity of 5.89 mPas at 100°C. After addition of the

organosilane/organosiloxane the mixed blends were ultrasonically stirred for 10 min and then stored for a couple of days before use. Blends with 2% by weight of organosilane/-siloxane in Nexbase® 3050 and pure base oil were tested. The blends were colorless transparent liquids and the organosilane/-siloxane are fully soluble and are showing no signs of precipitation or hazing before and after the tests. Table 3: Coefficients of friction.

The results in Table 3 clearly show that the trialkoxysilanes and trialkoxysilane-based compounds according to the present invention lead to a distinct change in the coefficients of friction. Particularly in the region of low sliding speeds, which often occur in real applications and are therefore of particular interest, reductions in the coefficient of friction of more than 19% are achievable.

The results also show that trialkoxysilane-based compounds with vinyl- and methoxy- functionalization exhibit better friction characteristics than the corresponding ethoxy- and other alkyl-functionalized products.

The film-forming properties of said organosilane or organosiloxane according to the present invention and their influences in different lubrication regimes on friction are compared with base oil results and are shown in Figure 1. Figure 1 shows the results from the stribeck measurement at 136 min testing time, so after more than 120 min rubbing. No change of friction curves was observed after testing for 45 to 60 minutes, therefore it is expected that a state of equilibrium is adjusted.

It is evident from the friction curves that the organosilane and organosiloxane is interacting with the surface and influencing the friction. By comparing all Stribeck curves corresponding to the different examples, following trends are observable at low speeds: Oligomeric samples show a higher friction reduction than monomeric samples. Also methoxy functionalized

organosilane/organosiloxane lead to lower friction than ethoxy functionalized precursors, see Examples 1 and 6. Another advantage regarding the friction reduction can also be obtained by vinyl groups instead of alkyl groups. The increase at higher mean speeds can be attributed to the formation of a solid tribofilm, which often show a disadvantageous behavior (L.J. Taylor, H.A.

Spikes, Friction-Enhancing Properties of ZDDP Antiwear Additive: Part I - Friction and Morphology ofZDDP Reaction Films, Tribilogy Transactions 2003, 46(3), 303-309). The following Table 4 shows the wear results obtained from the 4-ball wear tests, comparing the antiwear performance of the examples according to the present invention with the base oil.

Table 4: Results of anti-wear testing.

The wear results follow the same trend as the friction reduction of the examples according to the present invention. While the monomeric Example 5 leads only to a slight decrease of wear compared to pure base oil, the oligomeric Example 1 reduces the wear significantly by around 43% in calotte diameter.

The wear tracks on MTM discs are further investigated by FTIR. It was found that the obtained wear tracks from organosilane/siloxane containing oil mixtures are covered by polymeric residues, which is the tribopolymer as described further above in the specification of the present invention. It can also be found that the amount of tribopolymer on the wear track is higher when oligomeric precursors are used. However, these polymers are weakly bonded to the wear track, since they can easily wiped off with an acetone soaked tissue. The cleaned wear tracks appear different than the wear track obtained with base oil. The following FTIR analysis proves the formation of an adhesive tribofilm layer.

As a conclusion it was found that a multi-layered film structure is obtained with a non-adhesive tribopolymer and an adhesive tribofilm layer.

The ATR-FTIR spectra of the reaction films are analyzed and compared with the spectra of the pure precursors; the results are presented in Figures 2 and 3.

The main IR-absorption sites of Example 1 are associated to C-H (2950-2850 cm ¹), Si-O- CH₃ (2840, 1 190, 1 100-1080 cm ¹), Si-CH=CH₂ (1600, 1410, 1275, 1010, 967 cm ¹) and Si-O-Si (1000 to 1 150 cm^-1) as assigned according to the literature (U.b.B.A. Philip J. Launer, Infrared analysis of organosilicon compounds, Spectra-Structure Correlations, Reprinted from Silicon Compounds: Silanes & Silicones, 2013 Gelest, Inc Morrisville, PA.).

The gelation and film formation of the used organosilanes/siloxanes occurs via hydrolysis and condensation reactions with water leading to Si-O-Si cross-linking and release of alcohol:

The spectra of the multi-layered film represent the tribopolymer and show strong similarities with the spectra of the pure precursor. The main differences are a broadening of the Si-O-Si band, the vanishing of bands corresponding to Si-0-CH3 at 1 190 cm^-1, and the appearance of bands corresponding to Si-OH at 890 and 3620 cm^-1.

The broadening of the Si-O-Si band is a clear indication for the formation of a polysiloxane network and is in agreement with the hydrolysis and condensation reactions. However, it is interesting to note, that the Si-OH groups have not reacted to full conversion to Si-O-Si during the long test and severe conditions.

It can be concluded that the deposits on the uncleaned wear tracks of Example 1 tests are a mixture of polysiloxane polymers, the "tribopolymer", with residues of methanol and degraded and non-degraded base oil.

The cleaned wear track, the "tribofilm", formed from the test with Example 1 shows nearly no signals except that of a broad Si-O-Si band, which is shifted to higher wave numbers when compared with the tribopolymer or pure precursor. Therefore, it can be deduced that a tribofilm derived from Example 1 deposited on the wear track. The tribofilm shows hardly any bands which can be assigned to organic groups and C-H vibrations. This may have two reasons, on the one hand the intensity is low, due to the low tribofilm thickness, but on the other hand it is known that polyorganosiloxanes undergo an organic-into-inorganic transformation resulting in SiOC-glasses and ceramics at elevated temperatures.

The transformation is described in two steps: During the first transformation/decomposition step, the polymeric gels experience redistribution reactions which lead to the formation and release of low molecular weight organosilanes and siloxanes from the network. This defragmentation is related with a strong shrinkage and occurs already at around 20CTC. The second decomposition step starts around 500°C and is the so called ceramization process, which involves the homolytic cleavage of Si-C and C-H bonds. The steel surface favors the decomposition at lower temperatures than it would occur in the bulk fluid, but also the low film thickness favors the transformation process.

Generally, the asymmetric stretching vibration of Si-O-Si around 1060 cm-1 is linked to networking and densification, while the same stretching vibration at 1 120 cm-1 is correlated with higher bonding angles due to porosity, disorder and stress. That means that the strong band shift suggests a porous and disordered structure. This is in accordance with the first decomposition process, leading to porous films due to, the defragmentation and evaporation of fragmented species. However, also the absence of C-H vibrations indicate the start of a ceramization process.

The chemical reactions of the monomeric Example 5 follow the same pattern as the oligomeric Example 1. The only differences of the IR analysis are the low intensities from vibrations which can be related to OH-groups and to organic groups of the tribopolymer. This finding can be explained by the lower amount of tribopolymer on the wear track and therefore lower intensities. Conclusions:

The trialkoxysilane-based compounds according to the present invention form tribopolymer and tribofilms during mineral oil lubricated tribological tests, leading to a multilayered film architecture. The viscous polymer layer shows only weak adhesion on top of an adhesive tribofilm on e.g. steel surfaces.

While the tribopolymers are weakly cross-linked by hydrolysis and condensation reactions, the tribofilm is a strongly cross-linked film resulting from severe frictional decomposition of the trialkoxysilane-based compounds according to the present invention. The lack of organic groups in the film suggests a partly ceramization of the polymers to a kind of SiOC-glass or -ceramic.

The formed tribofilms provide in addition a strong corrosion protection.

The formed tribopolymers show a viscous behavior and are able to form thick layers at low speeds. If the boundary films are thick enough to overcome the tribofilm roughness a significant friction reduction is achieved, which is only the case for the oligomer derived tribopolymers.

Claims

1 Use of tria!koxysilanes or trialkoxysilane-based compounds of general formula (I)

wherein

R¹ independently of each other denote CH3,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0, as film forming additives in lubricating oil compositions.

2 The use according to claim 1 , characterized in that the trialkoxysilanes or trialkoxysilane- based compounds of general formula (I) form a multilayer tribofilm.

3. The use according to claim 2, characterized in that the multilayer tribofilm comprises the following layers:

(a) a tribofilm A and

(b) a tribopolymer B.

4. The use according to claim 2 or 3, wherein the tribofilm A is characterized by being an adhesive, rough layer consisting of polysiloxanes with a high degree of Si-O-Si crosslinking.

5. The use according to any one of claims 2 to 4, wherein the tribopolymer B is characterized by being a non-adhesive, viscous layer consisting of polysiloxanes with a low degree of Si-O-Si crosslinking.

6. Lubricating oil composition, comprising:

(A) 80 to 99.5% by weight of a base oil;

(B) 0.5 to 5% by weight of a trialkoxysilane or a trialkoxysilane-based compound of general formula (I)

wherein

R¹ independently of each other denote CH3,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0; and

(C) 0 to 15% by weight of one or more further additives.

7. Lubricating oil composition according to claim 6, characterized in that the base oil is selected from the group consisting of API Group I, II, III, IV, V oils and mixtures thereof; preferably of API group III oils and mixtures thereof.

8. Lubricating oil composition according to claim 6 or 7, characterized in that the base oil is selected from the group consisting of API Group III oils and mixtures thereof.

9. The lubricating oil composition according to any one of claims 6 to 8, characterized in that component (C) is selected from the group consisting of VI improvers, dispersants, defoamers, detergents, antioxidants, pour point depressants, antiwear additives, extreme pressure additives, friction modifiers, anticorrosion additives, dyes and mixtures thereof.

10. Method of reducing wear and/or friction of a lubricating oil composition, the method comprising the steps of:

(a) adding a trialkoxysilane or a trialkoxysilane-based compound of general formula (I)

wherein

R¹ independently of each other denote Chb,

R² independently of each other denote CH3, R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0,

to a lubricating oil composition;

(b) forming a multilayer tribofilm.

11. Multilayer tribofilm, comprising:

(a) a tribofilm A, characterized by being an adhesive, rough layer consisting of

polysiloxanes with a high degree of Si-O-Si crosslinking; and

(b) a tribopolymer B, characterized by being a non-adhesive, viscous layer consisting of polysiloxanes with a low degree of Si-O-Si crosslinking, the polysiloxane being derived from a trialkoxysilane or a trialkoxysilane-based compound of general formula (I)

(I),

wherein

R¹ independently of each other denote Ch ,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0.

12. Element comprising at least two components movable with respect to one another, between the surfaces of which is present a multilayer tribofilm formed by a lubricating oil composition, characterized in that the multilayer tribofilm comprises: (a) a tribofilm A, characterized by being an adhesive, rough layer consisting of polysiloxanes with a high degree of Si-O-Si crosslinking and

(b) a tribopolymer B, characterized by being a non-adhesive, viscous layer consisting of polysiloxanes with a low degree of Si-O-Si crosslinking,

the polysiloxane being derived from a trialkoxysilane or a trialkoxysilane-based compound of general formula (I)

wherein

R¹ independently of each other denote Chb,

R² independently of each other denote CH3,

R³ independently of each other denote vinyl,

m denotes an integer of > 1 , preferably > 2, and

n denotes an integer of > 0,

with the proviso that (m+n) is > 2, especially m > 2 with n = 0.

13. Element according to claim 12, characterized in that the element is inside engines, motors, manual transmissions, differential gears, automatic transmissions, continuously variable transmissions, axles, dual clutches, dedicated hybrid transmissions, hydraulic machinery, power steerings, shock absorbers, compressors, industrial gears, paper machines, machine tools, metal workings, and transformers.