Coating composition using high viscous polysilazane

Abstract

The present invention relates to a coating composition, comprising (i) a mixture of organic polysilazanes having together an adjusted viscosity of a specific target viscosity; (ii) an acrylic based adhesion promoter in a concentration of more than 1 wt.% and less than 10 wt.% based on the solid content of the coating composition and (iii) a radical starter and (iv) metal particles or a dispersion of metal particles and a method of preparing said coating composition and a method for preparing a coated article by using said coating composition.

Description

[0001] Coating Composition using high viscous polysilazane

[0002] Field of the Invention

[0003] The present invention relates to a coating composition, comprising (i) a mixture of organic polysilazanes having together an adjusted viscosity of a specific target viscosity; (ii) an acrylic based adhesion promoter in a concentration of more than 1 wt.% and less than 10 wt.% based on the solid content of the coating composition and (iii) a radical starter and (iv) metal particles or a dispersion of metal particles and a method of preparing said coating composition and a method for preparing a coated article by using said coating composition.

[0004] Background of the Invention

[0005] Polymers with silazane repeating units -[SiR2-NR’-] are typically referred to as polysilazanes (PSZ). If all substituents R and R’ are hydrogen, the material is called perhydropolysilazane (PHPS) and, if at least one of R and R’ is an organic moiety, the material is called organopolysilazane (OPSZ). PHPS and OPSZ are used for a variety of functional coatings to impart certain properties to surfaces. Hence, silazanes are widely used for functional coatings for various applications. A basic information on polysilazanes and their structures is described in Brewer S.D. et al, J. Am. Chem. Soc, 1948, 70, 3888, EP0303498 B1 and WO0136427 A1 which are incorporated by reference.

[0006] Polysilazanes are resins that are used for the preparation of functional coatings for different types of application. They can be crosslinked by hydrolysis, for example, by reaction with moisture from the air. This leads to an increasing molecular weight and to a solidification or curing of the material. For this reason, the terms “curing” and “crosslinking” and the corresponding verbs “cure” and “crosslink” are interchangeably used as synonyms in the present application when referred to silazane based polymers such as e.g. polysilazanes.

[0007] Coating compositions comprising polymers with silazane repeating units are particularly suitable for the preparation of functional coatings on various base material substrates to provide improved physical and chemical surface properties such as, in particular, improved mechanical resistance and durability (including improved surface hardness, improved scratch resistance, improved abrasion resistance and / or improved smoothness); improved wetting and adhesion properties (including hydro- and oleophobicity, easy-to- clean effect and / or anti-graffiti effect); improved chemical resistance (including improved corrosion resistance (e.g. against solvents, acidic and alkaline media and corrosive gases) and / or improved anti-oxidation effect); improved optical effects (improved light fastness); and improved physical barrier or sealing effects. Typically, polysilazanes are liquid polymers which become solid at molecular weights of ca. > 10,000 g / mol. In most applications, liquid polymers of moderate molecular weights, typically in the range from 200 to 10,000 g / mol, are used. For preparing solid coatings with the above-mentioned properties from such liquid polymers, a curing step is required which is carried out after applying the material on a substrate, either as a pure material or as a formulation. The curing should be as fast as possible. As long as the coating is liquid, some defect formation of the film can occur. For example, the liquid coating may reflow and cause some film thickness variations or the wet film is sensitive to absorption of airborn impurities (e.g. dust) which stick on the wet surface. Another disadvantage is a delay time in the processing of the substrates until the coating is dry. In general, it is possible to speed up the drying time by applying higher temperatures, radiation or an atmosphere with higher humidity. Polysilazanes may act as preceramic polymers.

[0008] 3 D Steel metal construction is still widely used in automotive industry due to their unique tensile strength. A special way to reinforce safety critical body components is to heat steel parts to high temperature >900°C while pressing them into shape. In this stage the uncoated metal coils are vulnerable to oxidation. As known in the art, said method is called “hot stamping” method or is variously also referred to as heat-stamping, hot- pressing, hot press forming, high-temperature stamping, or die-quenching. In the hot stamping method, the steel material to be formed is initially heated to a high temperature, and then the steel sheet that has been softened by the heating is stamped and then cooled. Thus it is possible to form a steel sheet into a complicated shape with good dimensional accuracy.

[0009] Organic protection layers are not suitable for this process since such high temperature would burn off the coating layer.

[0010] In the application field of steel protective coating compositions, said compositions shall be designed for protecting the steel sheet from unwanted oxidation and oxide formation that occurs during the metallurgical process of heat-stamping.

[0011] However, even in case the steel sheet is heated in a non-oxidizing atmosphere in a heating furnace, the sheet retains the possibility of contacting the atmosphere while further processing which likely result in the oxidation formation of iron oxides on the surface of the steel sheet. The removal of such oxides or oxide films on the the surface of the steel is cost intensive, time and labor intensive. A further disadvantage arise once the steel sheet shall be coated with a paint. The resulting painted surface film might have poor adhesion to the steel sheet. US5459114 A describes a repetition of a process of impregnating a metal fiber or ceramic fiber preform or porous ceramic with a mixture of polysilazane-type polymers with a number average molecular weight of 200 to 3,000 and a viscosity adjusted to 100 Pas.

[0012] JP3414489 B2 describes the use of acrylic resins together with perhydropolysilazane and solvent-soluble fluororesins.

[0013] US6296805 B1 describes a hot-rolled steel sheet coated with an aluminum-based or aluminum alloy coating, manufactured for example by dipping the steel sheet in an aluminum bath containing either from 8% to 11% silicon and 2% to 4% iron, or from 2% to 4% iron, or even in an aluminum bath preferably containing from 9% to 10% silicon and 2% to 3.5% iron. It is reported that the presence of the coating at the time of thermal treatment of the castings makes it possible to prevent any decarbonization of the base metal as well as any oxidation.

[0014] Xiao et al, Ceramics International 2014, 40, 745-752 describes crack-free ceramic coatings using polysilazane as a preceramic polymer and aluminum powder as an active filler. The coating compositions contained 10 to 40 vol% Al powder, 0.5 wt.% Dicumyl peroxide, a polysilazane of formula -[SiNH(CHaCH2)]x-[NHSiNH(CH3)]y- having a molecular weight of 600 to 1 ,000 and xylene as solvent.

[0015] LIS2011041913 A1 describes formulations containing perhydropolysilazane, a solvent and a catalyst and may comprise a substance influencing the viscosity of the formulation.

[0016] US20220267617 A1 describes an oxidation-protective coating composition for steel sheets comprising the chemical components of: an aromatic organic solvent; at least one source of aluminum; a silazane; an organic synthesis catalyst; or an organophosphorus compound.

[0017] US20230124254 A1 and US20230212425 A1 describe an oxidative-protective coating composition for steel sheets comprising an aromatic organic solvent, at least one source of aluminum, a silazane, and an organic synthesis catalyst.

[0018] Nevertheless adjustment of viscosity within the desired application window is challenging since polysilazanes (PSZ) often react with viscosity modifying additives. Typically, viscosities in the range of 70 to 200 mPas are used as target measured by using e.g. an Anton Paar Modular Compact Rheometer MCR 92 equipped with a measuring Cone CP50-1 , at a shear rate of 1000s'1, and at a temperature of 25°C. Although, various types of coating compositions are known, there is still a need for adjustment of coating formulation parameters like viscosity and curing speed within the desired application window.

[0019] Hence, it is an object of the present invention to overcome the disadvantages in the prior art and to provide new coating compositions for preparing functional surface coatings preferably for coating compositions applicable for coil coating which allow adjustment of the desired film thickness of the surface coating and / or allow high adhesion of the coating to the surface and / or allow the reduction of time for curing at conditions to fulfill the requirement of “dry-to-touch” especially for coated metal coils.

[0020] According to industry standards, the maximum total coating thickness is 30 to 50 pm. There is also a need for coating compositions having the ability to cure following application to steel within the period of time that a coil steel processing line allows before initiating the next step in the coil steel’s processing procedure. Generally said time period is 15 to 90 seconds.

[0021] The object of the present invention is furthermore to manufacture a coating composition fulfilling application targets for coil coatings.

[0022] Consequently, it is an object of the present application to provide such a solution.

[0023] Summary of the Invention

[0024] The present inventors have now found that the above object may be attained by the coating composition of the present application and the method of its manufacturing described herein.

[0025] The coating composition according to the invention is preferably a steel protective coating composition.

[0026] It has been found that a coating composition containing a mixture of polysilazanes as preceramic polymers having an adjusted viscosity of a specific target viscosity together with an acrylic based adhesion promoter in the presence of a radical starter and metal particles is able to allow forming of a desired film thickness of a surface coating while being curable at a specific time at a specific peak metal temperature while having high adhesion.

[0027] It has been found that a coating composition containing a mixture of polysilazanes having an adjusted viscosity of a specific target viscosity together with an acrylic based adhesion promoter in the presence of a radical starter and metal particles is able to fulfill all application targets known in the art and expected for coil coatings. Furthermore, the coating compositions allow an easy application by user-friendly coating methods so that functional surface coatings may be obtained in an efficient and easy manner under targeted conditions.

[0028] The invention therefore relates to a coating composition, comprising

[0029] (i) a mixture of organic polysilazanes having together an adjusted viscosity of a specific target viscosity;

[0030] (ii) an acrylic based adhesion promoter in a concentration of more than 1 wt.% and less than 10 wt.% based on the solid content of the coating composition;

[0031] (iii) a radical starter and

[0032] (iv) metal particles or a dispersion of metal particles.

[0033] The invention furthermore relates to a method for preparing a coating composition as described before or preferably described in the following, wherein the method comprising the following steps:

[0034] (a) providing a first amount of an organic polysilazane (Polysilazane A) having a viscosity of less than the target viscosity;

[0035] (b) providing a second amount of an organic polysilazane (Polysilazane B) having a viscosity of more than the target viscosity;

[0036] (c) mixing Polysilazane A from (a) and Polysilazane B from (b) in a ratio resulting in an adjusted viscosity of the target viscosity;

[0037] (d) adding to the mixture from (c) an acrylic based adhesion promoter in a concentration so that the solid content of the acrylic resin in the coating composition is more than 1 wt.% and less than 10 wt.%, a radical starter and metal particles or a dispersion of metal particles and optionally one or more solvents or an organic synthesis catalyst under stirring until a homogenous dispersion is achieved.

[0038] The invention furthermore relates to a method for preparing a coated article, wherein the method comprises the following steps:

[0039] (a) applying a coating composition as described before or preferably described below or manufactured as described before or preferably described below to a surface of an article; and

[0040] (b) curing said coating composition to obtain a coated article.

[0041] The invention furthermore relates to a coated article, obtainable by the method as described before or preferably described below.

[0042] Preferred embodiments of the invention are described in the dependent claims. Detailed Description of the Invention

[0043] For the purposes of the present invention, the term "composition" is also used synonymously alongside the term "preparation" or “formulation”.

[0044] The coating composition may include or comprise, essentially consist of or consist of the necessary or optional constituents mentioned above and / or below. All compounds or components which can be used in the coating composition are either known and commercially available or can be synthesised by known processes.

[0045] The term “viscosity” as used herein is the dynamic viscosity measured in mPas. The dynamic viscosity is determined by using an Anton Paar Modular Compact Rheometer MCR 92 with a Cone-Plate System equipped with a measuring Cone CP50-1 in accordance with the ASTM D7042 standard, "Standard Test Method for Dynamic Viscosity and Density of Liquids” by Stabinger Viscometer from ASTM International.

[0046] The term “functional coating” as used herein refers to coatings which impart one or more specific properties to a surface. Generally, coatings are needed to protect surfaces or impart specific effects to surfaces. A preferred surface is a metal panel, metal sheet or coil.

[0047] The term “cure” means conversion to a crosslinked polymer network or a ceramic binder network (for example, through catalysis).

[0048] The term “polymer” includes, but is not limited to, homopolymers, copolymers, for example, block, random, and alternating copolymers, terpolymers, quaterpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible configurational isomers of the material. These configurations include, but are not limited to isotactic, syndiotactic, and atactic symmetries. A polymer is a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units (i.e. repeating units) derived, actually or conceptually, from molecules of low relative mass (i.e. monomers). Typically, the number of repeating units is higher than 10, preferably higher than 20, in polymers. If the number of repeating units is less than 10, the polymers may also be referred to as oligomers.

[0049] The term “monomer” as used herein, refers to a molecule which can undergo polymerization thereby contributing constitutional units (repeating units) to the essential structure of a polymer. The term “homopolymer” as used herein, stands for a polymer derived from one species of (real, implicit or hypothetical) monomer.

[0050] The term “copolymer” as used herein, generally means any polymer derived from more than one species of monomer, wherein the polymer contains more than one species of corresponding repeating unit. In one embodiment the copolymer is the reaction product of two or more species of monomer and thus comprises two or more species of corresponding repeating unit. It is preferred that the copolymer comprises two, three, four, five or six species of repeating unit. Copolymers that are obtained by copolymerization of three monomer species can also be referred to as terpolymers. Copolymers that are obtained by copolymerization of four monomer species can also be referred to as quaterpolymers. Copolymers may be present as block, random, and / or alternating copolymers.

[0051] The term “block copolymer” as used herein, stands for a copolymer, wherein adjacent blocks are constitutionally different, i.e. adjacent blocks comprise repeating units derived from different species of monomer or from the same species of monomer but with a different composition or sequence distribution of repeating units.

[0052] Further, the term “random copolymer” as used herein, refers to a polymer formed of macromolecules in which the probability of finding a given repeating unit at any given site in the chain is independent of the nature of the adjacent repeating units. Usually, in a random copolymer, the sequence distribution of repeating units follows Bernoullian statistics.

[0053] The term “alternating copolymer” as used herein, stands for a copolymer consisting of macromolecules comprising two species of repeating units in alternating sequence.

[0054] The term “polysilazane” as used herein, refers to a polymer in which silicon and nitrogen atoms alternate to form the basic backbone. Since each silicon atom is bound to at least one nitrogen atom and each nitrogen atom to at least one silicon atom, both chains and rings of the general formula -[SiR’R”-NR’”-]m(silazane repeating unit) occur, wherein R’ to R’” may be hydrogen atoms, organic substituents or heteroorganic substituents; and m is an integer. If at least one substituent R’ to R’” is an organic or heteroorganic substituent, the polymer is designated as organopolysilazane.

[0055] The term “Polysilazane A” as used herein, refers to one or more polysilazane polymers having a lower viscosity as a target viscosity having a cyclic, linear, branched and / or cross-linked form or any mixture thereof. The term “Polysilazane B” as used herein, refers to one or more polysilazane polymers having a higher viscosity as a target viscosity having a cyclic, linear, branched and / or cross-linked form or any mixture thereof.

[0056] In a preferred embodiment of the invention, the mixture of organic polysilazanes having an adjusted viscosity of a specific target viscosity consists of of one or more polysilazane polymers having a lower viscosity as the target viscosity (Polysilazane A) and one or more polysilazane polymers having a higher viscosity as the target viscosity (Polysilazane B). The ratio of Polysilazane A to Polysilazane B allows the design of a mixture having a specific target viscosity.

[0057] The adjusted viscosity of said mixture enables the manufacture of the coating composition according to the invention without the need of adding further viscosity adjusting agents such as thickening agents or further organic solvents.

[0058] In a preferred embodiment of the invention, both Polysilazane A and Polysilazane B are oligomers or polymers containing a repeating unit M1 represented by Formula (1), -[SiR1R2-NR3-] (1), wherein R1is an alkenyl group having 2 to 30 carbon atoms; and R2is selected from hydrogen, an alkyl group having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1, and R3is selected from hydrogen, an alkyl having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms, an aryl group having 2 to 30 carbon atoms or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1; and where Polysilazane A and Polysilazane B may have a cyclic, linear, branched and / or cross-linked form or any mixture thereof.

[0059] In a preferred embodiment, R1is an alkenyl group having 2 to 20 carbon atoms, more preferably 2 to 10 carbon atoms, most preferably 2 or 3 carbon atoms. In a particularly preferred embodiment, R1is ethenyl. However while curing said substituent R1is also able to participate in the transformation to form a coating on the surface of an article.

[0060] For the purposes of the present invention, an alkenyl group having 2 to 20 carbon atoms is a straight-chain or branched alkenyl group having 2 to 20 C atoms and is preferably ethenyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl.

[0061] In a preferred embodiment, R2is hydrogen, an alkyl group having 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, most preferably 1 to 6 carbon atoms, or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1. In a particularly preferred embodiment, R2is methyl or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1.

[0062] For the purposes of the present invention, an alkyl group having 1 to 20 carbon atoms is a straight-chain or branched alkyl group having 1 to 20 C atoms and is preferably methyl, ethyl, n-propyl, i-propyl, cyclopropyl, n-butyl, i-butyl, s-butyl, t-butyl, cyclobutyl, 2-methyl- butyl, n-pentyl, s-pentyl, t-pentyl, 2-pentyl, neopentyl, cyclopentyl, n-hexyl, s-hexyl, t-hexyl, 2-hexyl, 3-hexyl, neohexyl, 2-methylpentyl, n-heptyl, 2-heptyl, 3-heptyl, 4-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, 2-(2,6-dimethyl)octyl, 3-(3,7-dimethyl)octyl, 1 ,1-dimethyl-n- hex-1-yl-, 1 , 1 -dimethyl-n-hept-1 -yl-, 1 ,1-dimethyl-n-oct-1-yl-, n-undecyl, n-dodecyl, 1 ,1- dimethyl-n-dec-1-yl-, n-tridecyl, n-tetradecyl, 1 ,1-dimethyl-n-dodec-1-yl-, n-heptadecyl, n- hexadecyl, 1 ,1-dimethyl-n-tetradec-1-yl-, n-octadecyl, n-nonadecyl, 1 ,1-dimethyl-n- hexadec-1-yl-, 1 ,1-dimethyl-n-octadec-1-yl-, 1 , 1 -diethyl-n-hex-1 -yl-, 1 , 1 -diethyl-n-hept-1 - yl-, 1 , 1 -diethyl-n-oct-1 -yl-, 1 , 1 -diethyl-n-dec-1 -yl-, 1 ,1-diethyl-n-dodec-1-yl-, 1 , 1 -diethyl-n- tetradec-1-yl-, 1 ,1-diethyl-n-hexadec-1-yl-, 1 ,1-diethyl-n-octadec-1-yl- and n-eicosyl.

[0063] In a preferred embodiment, R3is hydrogen, an alkyl group having 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, most preferably 1 to 6 carbon atoms, or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1.

[0064] In a particularly preferred embodiment, R3is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2. In a particularly preferred embodiment, R3is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same to M1 or M2.

[0065] In a preferred embodiment of the invention, both Polysilazane A and Polysilazane B are oligomers or polymers containing beside of the repeating unit M1 a repeating unit M2 represented by Formula (2), -[SiR4R5-NR6-] (2), wherein R4and R5are the same or different from each other and independently selected from hydrogen, an alkyl group having 1 to 30 carbon atoms or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2; and R6is selected from hydrogen, an alkyl group having 1 to 30 carbon atoms atoms or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2; and where Polysilazane A and Polysilazane B may have a cyclic, linear, branched and / or cross-linked form or any mixture thereof. In a preferred embodiment, R4is hydrogen, an alkyl group having 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, most preferably 1 to 6 carbon atoms, or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2. In a particularly preferred embodiment, R4is methyl or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2.

[0066] In a preferred embodiment, R5is hydrogen, an alkyl group having 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, most preferably 1 to 6 carbon atoms, or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2. In a particularly preferred embodiment, R5is hydrogen or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2.

[0067] In a particularly preferred embodiment, R6is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2. In a particularly preferred embodiment, R6is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same to M1 or M2.

[0068] It is preferred that Polysilazane A and Polysilazane B comprise a repeating unit M1 and a repeating unit M2, wherein M1 and M2 have a meaning as described before or having substituents as preferably described before.

[0069] It is preferred that Polysilazane A consists of repeating units M1 and M2, wherein M1 and M2 have a meaning as described before or having substituents as preferably described before.

[0070] It is preferred that Polysilazane B consists of repeating units M1 and M2, wherein M1 and M2 have a meaning as described before or having substituents as preferably described before.

[0071] Preferably, Polysilazane A and Polysilazane B are copolymers such as random copolymers or block copolymers or copolymers containing at least one random sequence section and at least one block sequence section. More preferably, each Polysilazane A and Polysilazane B is a random copolymer or a block copolymer.

[0072] The Polysilazane A and Polysilazane B have a molecular weight distribution. Preferably, the silazane-containing polymers used in the present invention have a mass average molecular weight Mw, as determined by GPC, of at least 1 ,000 g / mol, more preferably of at least 1 ,200 g / mol, even more preferably of at least 1 ,500 g / mol. Preferably, the mass average molecular weight Mwof the silazane-containing polymers is less than 100,000 g / mol. More preferably, the molecular weight Mwof the silazane-containing polymers is in the range from 1 ,500 to 50,000 g / mol.

[0073] Suitable organic polysilazanes to be used according to the invention as described before are commercially available e.g. Durazane 1800 (Merck Electronics KGaA, Germany) or IOTA OPSZ 1800 (IOTA Corporation Ltd.). Suitable organic polysilazanes to be used according to the invention are easily manufactured based on manufacturing processes as described in the prior art e.g. WO0136427 A1 or Zhang Z. et al, Journal of Materials Chemistry 2012, 22, 5300-5303.

[0074] In a preferred embodiment, Polysilazane A is Durazane 1800 or IOTA OPSZ 1800.

[0075] In a further preferred embodiment, Polysilazane B is manufactured by using Polysilazane A as described before or preferably described before or below.

[0076] A rather easy way of increasing the viscosity of Polysilazane A as described before or preferably described before is to heat the pure liquid Polysilazane A at temperatures in the range of 50-140°C for a time of minutes to hours preferably under nitrogen atmosphere to avoid hydrolysis by moisture from the air resulting in Polysilazane B. Details are described in the experimental section regarding temperature, time of reaction and viscosity of product. The viscosity of Polysilazane B is easily steered by the temperature and time of reaction.

[0077] Manufacture of the mixture of organic polysilazanes having together an adjusted viscosity of a specific target viscosity will be described below and details are described in the experimental section. The target viscosity is easily steered by the ratio of Polysilazane A and Polysilazane B. The target viscosity depends on the desired application of the coating composition according to the invention and is preferably in the range of 70 to 200 mPas.

[0078] In a preferred embodiment of the invention, the coating composition according to the invention comprises 60 wt.% to 80 wt.% of the mixture of organic polysilazanes having together an adjusted viscosity of the specific target viscosity based on the total weight of the coating composition, preferably 65 to 75 wt.%, most preferably 68 to 73 wt.% based on the total weight of the coating composition.

[0079] It is optional that the coating composition of the present invention comprises one or more solvents. Suitable solvents are organic solvents such as, for example, aliphatic and / or aromatic hydrocarbons, which may be halogenated, such as 1-chloro-4- (trifluoromethyl)benzene, esters such as ethyl acetate, n-butyl acetate or tert-butyl acetate, ketones such as acetone or methyl ethyl ketone, ethers such as tetrahydrofuran or dibutyl ether, and also mono- or polyalkylene glycol dialkyl ethers (glymes), or mixtures thereof. The organic solvent, if present, is preferably present in the coating composition in an amount from about 10 wt.% to 25 wt.% based on the total weight of the coating composition. In a preferred embodiment of the coating composition no further organic solvent is used beside of the organic solvents used in dispersions containing the metal particles or the acrylic resin of the acrylic resin based adhesion promoter or in solutions of the radical starter. The kind of solvents useful for manufacture of such dispersions or solutions is described before.

[0080] It is possible to further accelerate the curing of the coating composition by the addition of one or more organic synthesis catalysts.

[0081] Suitable organic synthesis catalysts are known in the art and may be selected from Lewis acids such as boron-, aluminum-, titanium-, tin- or zinc-alkyls, aryls or carboxylates, Brbnsted acids such as carboxylic acids, bases such as primary, secondary or tertiary amines or phosphazenes, or metal salts such as Pd, Pt, Al, B, Sn or Zn salts of carboxylates, acetylacetonates or alkoxylates or N-heterocyclic compounds such as 1- methylpiperazine, 1 -methylpiperidine, 4,4’-trimethylenedipiperidine, 4,4’-trimethylene-(1- methylpiperidine), diazabicyclo-(2,2,2)octane, cis-2,6-dimethylpiperazine, 1,8- diazabicyclo[5.5.0]undec-7-ene.

[0082] The organic synthesis catalyst, if present, is preferably present in the coating composition in an amount of from about 0.5 wt.% to 5 wt.% based on the total weight of the composition.

[0083] The invention therefore relates further to the coating composition as described before or preferably described below wherein the composition further comprises one or more solvents or an organic synthesis catalyst.

[0084] The coating composition according to the invention comprises beside of the mixture of organic polysilazanes Polysilazane A and Polysilazane B having together an adjusted viscosity of a specific target viscosity as described before or preferably described before and optionally an organic solvent or an organic synthesis catalyst an acrylic based adhesion promoter in a concentration of more than 1 wt.% and less than 10 wt.% based on the solid content of the coating composition.

[0085] Adhesion promoters are chemicals that act at the interface between an organic polymer and an inorganic substrate to enhance adhesion between the two materials. An adhesion promoter, in its optimal sense, will act effectively at the organic-inorganic interface to chemically and physically wed these dissimilar materials into a strong cohesive bond structure. Adhesion pomoters are chemical materials that contain dual functionality in the molecular structure.

[0086] Critical to performance of the coating composition according to the invention is the concentration of the acrylic based adhesion promoter. Concentration < 1 ,0 wt.% do not show significant improvement of adhesion properties. Concentration > 10 wt.% lead to delamination effects, sometimes also during coil curing process at elevated temperature (100°C to 300°C) because of high acrylic concentration which is not heat stable at such high temperatures. By using a concentration as defined before in formulation delivers best adhesion properties without burn effects of the acrylic backbone. The organic polysilazane content is high enough to built in acrylic resin and protect from disintegration of acrylic backbone.

[0087] To perform optimum application a concentration of 4.5 to 5.2 wt.%, preferably 5,0 wt.%, of acrylic adhesion promoter is preferred based on the solid content of the coating composition.

[0088] The acrylic based adhesion promoter may be a solution or dispersion containing an acrylic resin.

[0089] The invention therefore relates to a coating composition as described before or preferably described below wherein the acrylic based adhesion promoter is a solution or a dispersion containing an acrylic resin.

[0090] Suitable acrylic resins and solutions or dispersions containing these as acrylic based adhesion promoters are known in the art and are commercially available. Suitable solvents for acrylic resins are organic solvents such as, for example, aliphatic and / or aromatic hydrocarbons, which may be halogenated.

[0091] Examples of such an acrylic resin include an acrylic acid ester (examples of alcohol residues include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, phenyl group, benzyl group, phenylethyl group), methacrylic acid esters (alcohol residues are the same as above), hydroxy-containing monomers such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropylmethacrylate, acrylamide, methacrylamide, N-methylmethacrylamide, N-phenylacrylamide, N,N-diethylaminoethyl acrylate, N,N-diethylaminoethyl methacrylate, glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether. Said acrylate or methacrylate polymers may contain polymerized monomers containing carboxyl groups or salts thereof, anhydrides or other monomers such as vinyl isocyanate, allyl isocyanate, styrene, vinyl methyl ether, vinyl ethyl ether, vinyl trisalkoxysilane, alkyl maleic acid monoester, alkyl fumalic acid monoester, acrylonitrile, methacrylonitrile, alkyl itaconic acid monoester, vinylidene chloride, vinyl acetate, vinyl chloride. It is preferable that the acrylic based adhesion promoter contains 50 mol percent or more of components of acrylic monomer such as acrylic acid derivatives and particularly those containing components of methyl acrylate and n-butyl acrylate. The acrylic based adhesion promoter may contain low amounts of methacrylic acid derivatives however it is preferred that concentration of such derivatives is below 0.2 % based on the total of the acrylic based adhesion promoter.

[0092] In a preferred embodiment of the invention, the acrylic based adhesion promoter is commercially available. Commercially available acrylic based resins are marketed under the brand name of PARALOID™ from The Dow Chemical Company (DOW), Degalan® from Rohm, Synthalat® from Synthopol, and Elvacite® from Lucite International.

[0093] In a particular preferred embodiment of the invention, PARALOID™ B-48N (art. no. 226913z) is used in amounts as described before or preferably described before.

[0094] The coating composition according to the invention comprises beside of the mixture of organic polysilazanes Polysilazane A and Polysilazane B having together an adjusted viscosity of a specific target viscosity as described before or preferably described before, the acrylic based adhesion promoter as described before or preferably described before and optionally an organic solvent or an organic synthesis catalyst a radical starter.

[0095] In the coating composition according to the invention, as described above or as preferably described, a free-radical initiator is also added in order that curing can take place.

[0096] The term initiator, radical starter, free-radical starter or free-radical initiator is used synonymously.

[0097] Suitable radical starter are not limited and may be selected from initiators such as acetyl peroxide, stearoyl peroxide, 2,2'-azo-bis-(isobutyronitrile), tert-butyl peroxypivalate, 2,2‘- azo-bis-(2-cyanobutane nitrile), benzoyl peroxide, dicumyl peroxide (Luperox DCP),

[0098] Luperox TBEC, 2,2'-(diazene-1,2-diyl)bis(2,4-dimethylpentane nitrile), decanoyl peroxide, di(tert-butylcyclohexyl) peroxydicarbonate, lauroyl peroxide, tert-butyl peroxydiethylacetate, tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2- ethylhexanoate, VA-086, 1 ,1’-azodi(hexahydrobenzonitrile), 1,1-bis(tert-butylperoxy)- 3,3,5-trimethylcyclohexane, peroxy dicarbonate, and also photochemical initiators, such as Darocur 2273, Darocur 3331 , Darocur 1164, Irgacur 1850, Darocur 1700, Darocur 1173, Irgacur 184, Darocur 1116, Darocur 2959, benzoin methyl ether, benzoin ethyl ether, Irgacur 651, 2,4,6-(trimethylbenzoyl)diphenylphosphine oxide, bis(2,4,6- trimethylbenzoyl)phenylphosphine oxide, bis(2,6-dichlorobenzoyl)-4- propylphenylphosphine oxide.

[0099] In a preferred embodiment of the invention, the radical starter is a peroxide which may be used as such or in solution.

[0100] The invention therefore relates further to a coating composition as described before or as preferably described before wherein the radical starter is a peroxide.

[0101] In a preferred embodiment of the invention, the radical starter is commercially available and is selected form dicumyl peroxide (e.g. Luperox DCP), 1 , 1 -bis(tert- amylperoxy)cyclohexan (solution; 80 % in mineral spirits; Luperox 531 M80; Sigma Aldrich) or 1,1-bis(tert-butylperoxy)cyclohexan (solution; 80 % in mineral spirits; Luperox 331 M80; Sigma Aldrich).

[0102] The radical starter is preferably present in the coating composition in an amount of from about 0.1 wt.% to 1 wt.%, preferably 0.2 wt.% to 0.3 wt.% based on the total weight of the composition. Said amount based on the total weight of the composition shall be used also once the radical starter is used in a solution with an organic solvent.

[0103] The coating composition according to the invention comprises beside of the mixture of organic polysilazanes Polysilazane A and Polysilazane B having together an adjusted viscosity of a specific target viscosity as described before or preferably described before, the acrylic based adhesion promoter as described before or preferably described before, the radical starter as described before or preferably described before and optionally an organic solvent or an organic synthesis catalyst metal particles or a dispersion of metal particles.

[0104] The metal particles are preferably metal particles of aluminum, zinc, copper or mixtures thereof, particularly preferably metal particles of aluminum.

[0105] Said metal particles may exhibit any shape or form and are provided, for example, in a platelet shape, a spherical, spheroidal, lenticular or in an irregular granular shape. A platelet shape is preferred to ensure a thin, smooth and homogeneous surface structure of the surface coating of an article as described below.

[0106] Said metal particles when used as a substrate may be further coated with one or more passivation layers or inorganic coating which does not decompose at temperatures up to 500°C. A preferred inorganic coating is silicon dioxide. The size of the flaky metal particle / substrate is not critical per se. In general, all types of flaky metal particles / substrates, which are available in the market, in particular those of the materials as described above, may be used according to the present invention. Preferably, the flaky metal particles or substrates exhibit an average particle size of from 2 to 100 pm and an average thickness of from 0.005 to 5 pm, more preferably an average particle size of from 5 to 50 pm and an average thickness of from 0.01 to 2 pm, and particularly preferably an average size of from 5 to 30 pm and an average thickness of from 0.02 to 2 pm.

[0107] In a preferred embodiment, the coating composition as described before or preferably described comprises 15 wt.% to 40 wt.% of metal particles as described before or 15 wt.% to 40 wt.% of a dispersion of metal particles as described before based on the total weight of the coating composition and where the dispersion of metal particles has a content of 5 wt.% to 45 wt.% of an organic solvent based on the total weight of the dispersion.

[0108] In a particular preferred embodiment, the coating composition as described before or preferably described comprises 20 wt.% to 35 wt.% or 25 wt.% to 30 wt.%, of metal particles as described before or 20 wt.% to 35 wt.% or 25 wt.% to 30 wt.% of a dispersion of metal particles as described before based on the total weight of the coating composition and where the dispersion of metal particles has a content of 5 wt.% to 45 wt.% of an organic solvent based on the total weight of the dispersion.

[0109] In a preferred embodiment of the invention, the metal particle or dispersion of metal particles is commercially available and is preferably selected from STAPA Metallic R 507 R (art. no. 057307G70, Eckart GmbH), Hydrolan® 501 (art. no. 005332HV0, Eckart GmbH), Toyal 1100 NA (TOYO Aluminum K.K.) or Toyal 1260 M-S (TOYO Aluminum K.K.) or aluminum powder from Beijing Dekedaojin Co. Ltd.

[0110] Moreover, the coating composition according to the present invention may comprise one or more additives, preferably selected from the list consisting of additives influencing evaporation behavior, additives influencing film formation, cross-linking agents, dispersants, fillers, rheological modifiers (e.g. thickeners) or surfactants (e.g. wetting and leveling agents). Such kind of additives are all optional and it is preferred that no other additives are present in the coating composition according to the invention.

[0111] It is to be understood that combination of the above-mentioned preferred, more preferred, particularly preferred and most preferred embodiments relating to the coating composition and definitions of its components is possible in any desired way. The invention relates further to a method for preparing a coating composition as described before, wherein the method comprising the following steps:

[0112] (a) providing a first amount of an organic polysilazane (Polysilazane A) having a viscosity of less than the target viscosity as described before or preferably described before;

[0113] (b) providing a second amount of an organic polysilazane (Polysilazane B) having a viscosity of more than the target viscosity as described before or preferably described before;

[0114] (c) mixing Polysilazane A from (a) and Polysilazane B from (b) in a ratio resulting in an adjusted viscosity of the target viscosity;

[0115] (d) adding to the mixture from (c) an acrylic based adhesion promoter in a concentration so that the solid content of the acrylic resin in the coating composition is more than 1 wt.% and less than 10 wt.% as described before or preferably described before, a radical starter as described before or preferably described before and metal particles or a dispersion of metal particles as described before or preferably described before and optionally one or more solvents or an organic synthesis catalyst under stirring until a homogenous dispersion is achieved.

[0116] In a preferred embodiment of the invention, step (b) is carried out by heating a second amount of Polysilazane A under stirring to a temperature of 50°C to 140°C for two to eight hours under an inert gas atmosphere and cooling down to ambient temperature thus providing the second amount of Polysilazane B. A preferred temperature for said step (b) is 80 °C to 120°C. The reaction time is preferably three to six hours.

[0117] To have perfect control of the viscosity, the two materials Polysilazane A and Polysilazane B with higher and lower viscosity as the target viscosity are mixed in proper ratios. By careful selection of the mixing ratio, it is possible to produce the mixture of organic polysilazanes having together an adjusted viscosity within a narrow viscosity range as needed to prepare a coating composition according to the invention with reproducible properties and performance.

[0118] It is preferred that the mixing of the polysilazane of step (a) and the polysilazane of step (b) takes place at room temperature, preferably at a temperature in the range from 20 to 25°C.

[0119] The description of all preferred embodiments apply to the method of preparing the coating composition in the same way.

[0120] The coating compositions of the present invention are storage-stable formulations, which can be applied by conventional coating methods such as e.g. coil coating, spray coating, dip coating, spin coating, slit or slot coating or other coating methods and which can be easily cured to functional surface coatings especially at high temperatures up to 450°C.

[0121] If the coating composition according to the invention has some time to settle, it is preferable to redisperse the metal particles before using in a coating application, which is reached in an easy manner as known in the art e.g. by slowly shaking the formulation until a homogenous dispersion is again achieved. The obtainable coated article according to the invention can be further treated by high temperatures e.g. in a hot-stamping method.

[0122] The present invention further relates to a method for preparing a coated article, wherein the method comprises the following steps:

[0123] (a) applying a coating composition according to the present invention to a surface of an article; and

[0124] (b) curing said coating composition to obtain a coated article.

[0125] It is preferred that the coating composition is applied in step (a) by an application method suitable for applying liquid compositions to a surface of an article. Such methods include, for example, coil coating, spray coating, dip coating, spin coating, flow coating, roll-to-roll coating, slit or slot coating or other coating methods. Coil coating, dip coating and roll-to- roll coating are particularly preferred. Coil coating is most preferred.

[0126] The coating composition of the invention may be applied to the surface of various articles such as, for example, plastic products, glass products, ceramics products or metal products. It is preferred that the surface of the article is made of metal preferably of steel.

[0127] Typically, the coating composition is applied in step (a) as a layer in a thickness of 1 pm to 50 pm, preferably 5 pm to 20 pmm, particularly preferably 7 pm to 15 pm, to the surface of the article. In a preferred embodiment, the coating composition is applied as a thin layer having a thickness of 8 pm to 12 pm.

[0128] The curing of the coating in step (b) may be carried out under various conditions, preferably by thermal curing up to 450°C substrate temperature, preferably under a temperature of 100°C to 300 °C. The curing is carried out in the absence of moisture, preferably under inert gas atmosphere, typically under nitrogen atmosphere.

[0129] Preferably, the curing time for step (b) is from 10 seconds to 5 minutes, more preferably from 15 seconds to 1 minute, still more preferably from 20 seconds to 40 seconds, depending on the coating composition and coating thickness. After curing in step (b), the silazane-containing polymers are crosslinked to form a coating on the surface of the article. Without wishing to be bound to any theory, it is assumed that during the curing Si-N and Si-H bonds of the silazane-containing polymers are converted to Si-0 bonds and an addition of Si-H to Si-CH=CH2 leads to Si-CH2-CH2-Si cross-linking triggered by the radical initiator.

[0130] The coating obtained by the above method forms a rigid and dense functional coating which is excellent in adhesion to the surface which is preferably a surface of a metal sheet or metal panel made from steel, wherein said steel surface can be pretreated with nonpermanent passivation layers, such as phosphatization layers.

[0131] The coating composition according to the present invention may be applied in so called “direct” or “indirect” hot forming / stamping processes. In an indirect process of hot stamping, a flat substrate as described before or preferably described before coated with the protective coating composition according to the invention is sequentially pre-stamped, heated and then hot stamped. In a direct process, the coated flat substrate as described before or preferably described before is first heated and then hot stamped.

[0132] Moreover, a coated article is provided, which is obtainable or obtained by the above- mentioned preparation method. A preferred coated article is a coated coil prior to heatstamping it.

[0133] The present invention is further illustrated by the examples following hereinafter which shall in no way be construed as limiting. The skilled person will acknowledge that various modifications, additions and alternations may be made to the invention without departing from the spirit and scope of the present invention.

[0134] Examples

[0135] Durazane 1800, commercially available from Merck Electronics KGaA, Germany, art. no. 214049, is a low viscous clear and colorless to slightly yellow liquid with a density of 0,95- 1,05 g / cm3(at a temperature of 25°C) and a viscosity of 10 to 40 mPas according to the specification. Since it is a material of oligomeric to polymeric nature the viscosity may vary from batch to batch in the specified viscosity range.

[0136] Example 1 : Increasing viscosity for Durazane 1800 by thermal treatment

[0137] An amount of 300ml of pure Durazane 1800 having a viscosity of 37 mPas [by using e.g. an Anton Paar Modular Compact Rheometer MCR 92 equipped with a measuring Cone CP50-1 , at a shear rate of 1000s'1, and at a temperature of 25°C] are filled into a 500ml three necked glass flask equipped with a Nitrogen inlet, a thermometer measuring the inside temperature and a mechanical stirrer placed in an oil bath on a heating device. Under stirring the flask is purging with Nitrogen for 1h. The complete reaction is done under Nitrogen atmosphere. Then the temperature is increased to the value shown in Table 1 and kept for the time as shown in Table 1. After cooling down to ambient temperature the viscosity of the heat treated Durazane is again measured and the result is shown in Table 1.

[0138] The dynamic viscosity is determined using an Anton Paar Modular Compact Rheometer MCR 92 Cone-Plate System equipped with a measuring Cone CP50-1 in accordance with the ASTM D7042 standard, "Standard Test Method for Dynamic Viscosity and Density of Liquids” by Stabinger Viscometer from ASTM International.

[0139] Table 1 :

[0140] Example 2: Adjusting viscosity by mixing of high and low viscous polysilazanes derived from commercially available Durazane 1800, art. no. 214049, Merck Electronics KGaA, Germany

[0141] Durazane 1800 material A (Material A) and Durazane 1800 material B (Material B) either commercially available or synthesized according to Example 1 with viscosities and in amounts as specified in Table 2 are filled into a 500ml three necked glass flask equipped with a Nitrogen inlet and a mechanical stirrer. Under Nitrogen atmosphere the mixture is stirred for 4h at a temperature of 25°C. Then the viscosity of the Durazane 1800 mixture is measured and results are shown in Table 2. Table 2: Viscosities are given in mPas and are measured as described; amounts are given in g

[0142] As can be seen in Table 2, by mixing Durazane materials with a higher and a lower viscosity it is possible to produce a material having an adjusted viscosity. As for 2.1 to 2.4, said adjusted viscosity only have a deviation of less than + / - 5 mPas by using different starting materials.

[0143] Example 3: Manufacturing of coating compositions and application to steel panels by a coil coating process

[0144] The material of Experiment No. 2.3 is used as the mixture of organic polysilazanes having an adjusted viscosity to prepare the following coating compositions. Further ingredients used are commercially available and are the following:

[0145] Hydrolan® 501 (Eckart GmbH, art. no. 005332HV0);

[0146] Luperox DCP (dicumyl peroxide, 4% in butanone, Sigma Aldrich, art. no. 329541); Paraloid™ B-48N (acrylic resin, 20% in n-butyl acetate, Dow Chemicals, art. no. 226913z);

[0147] Toyal 1100 NA (aluminum flakes, TOYO Aluminum K.K.);

[0148] Toyal 1260 M-S (aluminum flakes, TOYO Aluminum K.K).

[0149] Comparative Example 3a:

[0150] The coating composition is produced by stirring viscosity adjusted organic polysilazane (Tab. 1 material 2.3) while slowly adding a dispersion of aluminum particles (Hydrolan 501). After 5 min stirring time the formulation is ready to use without degassing or similar actions. Details of said coating composition are given in Table 3.

[0151] The formulation is applied to a steel panel by a coil coating process. This is performed with a ZAA 2300 automated film coater (Zehntner GmbH) with formed Lab Rod No 38 from RDS (RD Specialities); 60 mm / s. The used substrate is a steel panel having 15,25 mm x 77 mm x 0,2 mm. Results are summarized in Table 4.

[0152] Comparative Example 3b:

[0153] The coating composition of comparative Example 3a is reproduced and the radical starter Dicumyl peroxide (Luperox ) is added after 5 minutes while stirring. An additional stirring time of 5 minutes resulted in the formulation ready to use. Details of said coating composition are given in Table 3.

[0154] The formulation is then applied to a steel panel as described in comparative Example 3a. Results are summarized in Table 4.

[0155] Example 3c:

[0156] The coating composition of comparative Example 3b is reproduced and the adhesion promoter (Paraloid™) is added after 10 minutes while stirring. An additional stirring time of 5 minutes resulted in the formulation ready to use. Details of said coating composition are given in Table 3.

[0157] The formulation is then applied to a steel panel as described in comparative Example 3a. Results are summarized in Table 4.

[0158] Example 4:

[0159] The coating composition of Example 3c is reproduced but instead of Hydrolan 501 aluminum flakes, the Toyal 1100 NA aluminum flakes are used. Details of said coating composition are given in Table 3.

[0160] The formulation is then applied to a steel panel as described in comparative Example 3a. Results are summarized in Table 4.

[0161] Example 5:

[0162] The coating composition of Example 3c is reproduced but instead of Hydrolan 501 aluminum flakes, the Toyal 1260 M-S aluminum flakes are used. Details of said coating composition are given in Table 3.

[0163] The formulation is then applied to a steel panel as described in comparative Example 3a. Results are summarized in Table 4. Table 3:

[0164] Table 4:

[0165] As can be seen according to Table 4, the coating composition according to the invention enables a lower layer thickness, an improved or equal curing and very good adhesion properties.

[0166] Figure 1 gives an illustration about T-bends and the set-up of the bending. The T-bend test is measured according to DIN EN 13523-7: Coil coated metals, resistance to cracking on bending.

[0167] This part of EN 13523 describes the method for determining the resistance to cracking of an organic coating on a metallic substrate when it is bent through 135° to 180°. The adhesion strength can also be evaluated. Both folding and bending methods are taken into account. Within this experiment folding method with clamping jaws is used.

[0168] The coated steel panel of examples 3a to 3c, 4 and 5 (specimen) is bent within two clamping jaws with the coating on the outside of the bending point.

[0169] The specimen is clamped between the clamping jaws to ensure uniform bending.

[0170] The resistance of the coating is measured at room temperature. The bending point is examined with a magnifying glass immediately after bending. As a result, the smallest radius around which the specimen can be bent without cracking is specified in T or rounded up to the nearest half T. In Figure 1, the symbols and numbers denote the following:

[0171] A clamping jaws

[0172] B specimen

[0173] C thumb

[0174] 1 direction of movement

[0175] 2 T-bends

[0176] 3 correct

[0177] 4 incorrect

[0178] 5 Rolling direction of the tape (not used in examples)

Claims

Patent Claims1. A coating composition, comprising(i) a mixture of organic polysilazanes having together an adjusted viscosity of a specific target viscosity;(ii) an acrylic based adhesion promoter in a concentration of more than 1 wt.% and less than 10 wt.% based on the solid content of the coating composition;(iii) a radical starter and(iv) metal particles or a dispersion of metal particles.

2. The coating composition according to claim 1 , wherein the mixture consists of one or more polysilazane polymers having a lower viscosity as the target viscosity (Polysilazane A) and one or more polysilazane polymers having a higher viscosity as the target viscosity (Polysilazane B).

3. The coating composition according to claim 2, wherein both Polysilazane A and B are oligomers or polymers containing a repeating unit M1 represented by Formula (1),-[SiR1R2-NR3-] (1), wherein R1is an alkenyl group having 2 to 30 carbon atoms; and R2is selected from hydrogen, an alkyl group having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 , and R3is selected from hydrogen, an alkyl having 1 to 30 carbon atoms, an alkenyl group having 2 to 30 carbon atoms, an aryl group having 2 to 30 carbon atoms or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 ; and where Polysilazane A and Polysilazane B may have a cyclic, linear, branched and / or cross-linked form or any mixture thereof.

4. The coating composition according to claim 2 or 3, wherein both Polysilazane A and Polysilazane B are oligomers or polymers containing beside of the repeating unit M1 a repeating unit M2 represented by Formula (2),-[SiR4R5-NR6-] (2), wherein R4and R5are the same or different from each other and independently selected from hydrogen, an alkyl group having 1 to 30 carbon atoms or a bond to a N atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2;and R6is selected from hydrogen, an alkyl group having 1 to 30 carbon atoms atoms or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2; and where Polysilazane A and Polysilazane B may have a cyclic, linear, branched and / or cross-linked form or any mixture thereof.

5. The coating composition according to one or more of claims 2 to 4, wherein in repeating unit M1 of Formula (1) of both Polysilazane A and Polysilazane B, R3is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M1 or M2 and wherein in repeating unit M2 of Formula (2), R6is hydrogen or a bond to a Si atom of a further repeating unit where the further repeating unit might be the same or different to M2 or M1 ; and where Polysilazane A and Polysilazane B may have a cyclic, linear, branched and / or cross-linked form or any mixture thereof.

6. The coating composition according to one or more of claims 1 to 5 wherein the composition further comprises one or more solvents or an organic synthesis catalyst.

7. The coating composition according to one or more of claims 1 to 6 wherein the acrylic based adhesion promoter is a solution or dispersion containing an acrylic resin.

8. The coating composition according to one or more of claims 1 to 7 wherein the radical starter is a peroxide.

9. The coating composition according to one or more of claims 1 to 8 wherein the metal particles are metal particles of aluminum, zinc, copper or mixtures thereof.

10. The coating composition according to one or more of claims 1 to 9 wherein it comprises 60 wt.% to 80 wt.% of the mixture of organic polysilazanes having together an adjusted viscosity of the specific target viscosity based on the total weight of the coating composition.

11. The coating composition according to one or more of claims 1 to 10 wherein it comprises 15 wt.% to 40 wt.% of metal particles or 15 wt.% to 40 wt.% of a dispersion of metal particles based on the total weight of the coating composition and where the dispersion of metal particles has a content of 5 wt.% to 45 wt.% of an organic solvent based on the total weight of the dispersion.

12. A method for preparing a coating composition according to one or more of claims 1 to 11 , wherein the method comprising the following steps:(a) providing a first amount of an organic polysilazane (Polysilazane A) having a viscosity of less than the target viscosity;(b) providing a second amount of an organic polysilazane (Polysilazane B) having a viscosity of more than the target viscosity;(c) mixing Polysilazane A from (a) and Polysilazane B from (b) in a ratio resulting in an adjusted viscosity of the target viscosity;(d) adding to the mixture from (c) an acrylic based adhesion promoter in a concentration so that the solid content of the acrylic resin in the coating composition is more than 1 wt.% and less than 10 wt.%, a radical starter and metal particles or a dispersion of metal particles and optionally one or more solvents or an organic synthesis catalyst under stirring until a homogenous dispersion is achieved.

13. A method for preparing a coating composition according to claim 12, wherein step (b) is carried out by heating a second amount of Polysilazane A under stirring to a temperature of 50°C to 140°C for two to eight hours under an inert gas atmosphere and cooling down to ambient temperature thus providing the second amount of Polysilazane B.

14. A method for preparing a coated article, wherein the method comprises the following steps:(a) applying a coating composition according to one orm ore of claims 1 to 11 to a surface of an article; and(b) curing said coating composition to obtain a coated article.

15. A coated article, obtainable by the method according to claim 12 or 13.

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