UV-CURABLE COATINGS HAVING HIGH REFRACTIVE INDEX

Abstract

The present invention relates to coating compositions, comprising i) single or mixed metal oxide nanoparticles, wherein the volume average diameter (D.sub.v50) of the metal oxide nanoparticles is in the range of 1 to 20 nm; the nanoparticles comprise at least one volatile surface-modifying compound selected from alcohols, β-diketones, or salts thereof; carboxylic acids and β-ketoesters and Ge mixtures thereof, wherein the total amount of volatile surface-modifying compounds is at least 5% by weight, preferably at least 10% by weight based on the amount of metal oxide nano-particles, and ii) a solvent, coatings obtained therefrom and the use of the comositions for coating surface relief micro- and nanostructures (e.g. holograms), manufacturing of optical waveguides, solar panels, light outcoupling layers for display and lighting devices and anti-reflection coatings. Coatings obtained from the coating composition have a high refractive index and holograms are bright and visible from any angle, when the coating compositions are applied to them.

Claims

1.-15. (canceled)

16. A coating composition, comprising i) single or mixed metal oxide nanoparticles, wherein the volume average diameter (D.sub.v50) of the metal oxide nanoparticles is in the range of 1 to 20 nm; the nanoparticles comprise at least one volatile surface-modifying compound selected from alcohols; β-diketones, or salts thereof; carboxylic acids and p-ketoesters and mixtures thereof, wherein the total amount of volatile surface-modifying compounds is at least 5% by weight, based on the amount of metal oxide nanoparticles, and ii) a solvent; with the proviso that the coating composition comprises less than 1% w/w of water and does not comprise a binder.

17. The coating composition according to claim 16, wherein the metal oxide nanoparticles are titanium dioxide nanoparticles.

18. The coating composition according to claim 16, wherein the volatile surface-modifying compound is selected from ethanol and acetylacetone and mixtures thereof.

19. The coating composition according to claim 16, wherein the volume average diameter (D.sub.v50) of the metal oxide nanoparticles is in the range of 1 to 10 nm.

20. The coating composition according to claim 16, wherein the total amount of volatile surface-modifying compounds is in the range of from 15 to 50% by weight based on the amount of metal oxide nanoparticles.

21. The coating composition according to claim 16, wherein the solvent is selected from C.sub.2-C.sub.4 alcohols.

22. The coating composition according to claim 16, wherein the single, or mixed metal oxide nanoparticles are obtained by a process comprising the following steps: a) preparing a mixture, comprising a metal alkoxide of formula Ti(OR.sup.12).sub.4, metal halide of formula Ti(Hal).sub.4, wherein R.sup.12 is C.sub.1-C.sub.4alkyl; Hal is Cl; a solvent, a tertiary alcohol and optionally water, b1) heating the mixture to a temperature of from 80° C. to 180° C.; b2) separating the obtained TiO.sub.2 nanoparticles from the mixture; b3) resuspending the TiO.sub.2 nanoparticles in an C.sub.1-C.sub.4alcohol, or a mixture of C.sub.1-C.sub.4alcohols; b4) optionally treating the TiO.sub.2 nanoparticles with a β-diketone(s), or salts thereof, which are selected from compounds of formula Me(OR.sup.20).sub.x(L).sub.y, or mixtures thereof, wherein R.sup.20 is a C.sub.1-C.sub.8 alkyl group; ##STR00012## L is a group of formula R (VI), R.sup.21 and R.sup.22 are independently of each other a C.sub.1-C.sub.8 alkyl group; a phenyl group, which may optionally be substituted by one or more C.sub.1-C.sub.4 alkyl groups, or C.sub.1-C.sub.4 alkoxy groups; a C.sub.2-C.sub.5 heteroaryl group, which may optionally be substituted by one or more C.sub.1-C.sub.4 alkyl groups, or C.sub.1-C.sub.4 alkoxy groups; or a C.sub.1-C.sub.8 alkoxy group, R.sup.23 is a hydrogen atom, a fluorine atom, a chlorine atom, or a C.sub.1-C.sub.8 alkyl group, or R.sup.21 and R.sup.22 together form a cyclic or bicyclic ring, which may optionally be substituted by one or more C.sub.1-C.sub.4 alkyl groups; Me is selected from alkali and alkali earth metals, Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V), Ta (V), x is in the range from 0 to 4.9, y is in the range from 0.1 to 5, and the sum x+y equals to the oxidation state of metal; c1) treating the TiO.sub.2 nanoparticles with a base; c2) optionally treating the TiO.sub.2 nanoparticles with a β-diketone(s), or salt(s) thereof, c3) optionally treating the TiO.sub.2 nanoparticles with a compound of formula Me′(OR.sup.20′).sub.z, or mixtures thereof, wherein R.sup.20′ is a C.sub.1-C.sub.8 alkyl group; Me′ is selected from Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V) and Ta (V); and z equals to the oxidation state of metal; wherein the ratio of moles of hydroxy groups of tertiary alcohol to total moles of Ti is in the range 1:2 to 6:1; the base is selected from the group consisting of alkali metal alkoxides, the solvent is selected from 2-methyltetrahydrofurane, tetrahydropyrane, 1,4-dioxane, cyclopentylmethyl ether, di-n-propyl ether, di-isobutyl ether, di-tertbutyl ether, di-n-butyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, di(ethylene glycol) dimethyl ether, di(ethylene glycol) diethyl ether, di(ethylene glycol) di-n-propyl ether, di(ethylene gly-col) di-n-butyl ether, di(propylene glycol) dimethyl ether, di(propylene glycol) diethyl ether, tri(propylene glycol) dimethyl ether, tri(propylene glycol) diethyl ether, tri(ethylene glycol) dimethyl ether, tri(ethylene glycol) diethyl ether, tetra(ethylene glycol) dimethyl ether and tetra(ethylene glycol) diethyl ether and mixtures thereof; the tertiary alcohol is selected from tert-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol, 3-ethyl-3-pentanol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, 1-methylcyclopentanol, 1-ethylcyclopentanol, 1-methylcyclohexanol, 1-ethylcyclohexanol, 2,3-dimethyl-2,3-butanediol, 2,5-dimethyl-2,5-hexanediol, 2,6-dimethyl-2-heptanol, 3,5-dimethyl-3-heptanol, 3,6-dimethyl-3-heptanol, 2-methyl-3-buten-2-ol, 2-phenyl-2-propanol, 2-phenyl-2-butanol, 3-phenyl-3-pentanol, 2-methyl-1-phenyl-2-propanol, α-, β-, γ- or δ-terpineol, 4-(2-hydroxyisopropyl)-1-methylcyclohexanol (p-menthane-1,8-diol), terpinen-4-ol (4-carvomenthenol), and wherein in step b1) the alcohol R.sup.12OH is removed by distillation.

23. The coating composition according to claim 16, comprising i) titanium dioxide nanoparticles, wherein the volume average diameter (D.sub.v50) of the titanium dioxide nanoparticles is in the range of 1 to 10 nm; the nanoparticles comprise at least one volatile surface-modifying compound selected from ethanol and acetylacetone and mixtures thereof, wherein the total amount of volatile surface-modifying compounds is in the range of from 15 to 50% by weight based on the amount of metal oxide nanoparticles; and ii) a solvent which is selected from C.sub.2-C.sub.4 alcohols.

24. A coating having a refractive index of greater than 1.7, obtained from the coating composition according to claim 16.

25. A method for forming a coating having a high refractive index on a substrate comprising the steps of: a) providing a substrate; b) applying the coating composition according to claim 16 to the substrate by means of wet coating, or printing; c) removing the solvent; and d) exposing the dry coating to actinic radiation.

26. A security, or decorative element, comprising a substrate, which may contain indicia or other visible features in or on its surface, and on at least part of the said substrate surface, a coating according to claim 24.

27. A method for forming a surface relief micro- and nanostructure on a substrate comprising the steps of: a) forming a surface relief micro- and/or nanostructure on a discrete portion of the substrate; b) depositing the coating composition according to claim 16 on at least a portion of the surface relief micro- and/or nanostructure; c) removing the solvent; and d) curing the dry coating by exposing it to actinic radiation; or a method for forming a surface relief micro- and/or nanostructure on a substrate comprising the steps of a′) providing a sheet of base material, said sheet having an upper and lower surface; b′) depositing the coating composition according to claim 16 on at least a portion of the upper surface; c′) removing the solvent; d′) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition, such that said micro- and/or nanostructure is formed also in the base material, and e′) curing the coating composition by exposing it to actinic radiation; or a method for forming a surface relief micro- and/or nanostructure on a substrate comprising the steps of a″) providing a sheet of base material, said sheet having an upper and lower surface; b″) depositing the coating composition according to claim 16 on at least a portion of the upper surface; c″) removing the solvent; d″) curing the dry coating by exposing it to actinic radiation; and e″) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition, such that said micro- and/or nanostructure is formed also in the base material.

28. The method according to claim 27, wherein step a) comprises a1) applying a curable compound to at least a portion of the substrate; a2) contacting at least a portion of the curable compound with surface relief micro- and nanostructure forming means; and a3) curing the curable compound.

29. A method comprising providing the coating composition according to claim 16 and coating diffractive optical elements (DOEs), holograms, manufacturing of optical waveguides and solar panels, light outcoupling layers for display and lighting devices, high dielectric constant (high-k) gate oxides and interlayer high-k dielectrics, anti-reflection coatings, etch and CMP stop layers, optical thin film filters, optical diffractive gratings and hybrid thin film diffractive grating structures, high refractive index abrasion-resistant coatings, in protection and sealing (OLED), or organic solar cells.

30. A process for the preparation of the composition according to claim 16, comprising the following steps: a) preparing a mixture, comprising a metal oxide precursor compound(s), a solvent, a tertiary alcohol, or a secondary alcohol, wherein the tertiary alcohol and secondary alcohol eliminate water upon heating the mixture to a temperature of above 60° C., or mixtures, containing the tertiary alcohol(s) and/or the secondary alcohol(s), and optionally water, b1) heating the mixture to a temperature of above 60° C.; b2) separating the obtained metal oxide nanoparticles from the mixture; b3) resuspending the metal oxide nanoparticles in an alcohol, or a mixture of alcohols; b4) optionally treating the metal oxide nanoparticles with a volatile surface-modifying compound selected from β-diketones, carboxylic acids and p-ketoesters and mixtures thereof; or salts thereof, which are selected from compounds of formula Me(OR.sup.20).sub.x(L).sub.y, or mixtures thereof, wherein R.sup.20 is a C.sub.1-C.sub.8 alkyl group; ##STR00013## L.sup.− is a group of formula R (VI), R.sup.21 and R.sup.22 are independently of each other a C.sub.1-C.sub.8alkyl group; a phenyl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; a C.sub.2-C.sub.5heteroaryl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; or a C.sub.1-C.sub.8alkoxy group, R.sup.23 is a hydrogen atom, a fluorine atom, a chlorine atom, or a C.sub.1-C.sub.8alkyl group, or R.sup.21 and R.sup.22 together form a cyclic or bicyclic ring, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups; Me is selected from alkali and alkali earth metals, Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V), Ta (V), x is in the range from 0 to 4.9, y is in the range from 0.1 to 5, and the sum x+y equals to the oxidation state of metal; c1) treating the metal oxide nanoparticles with a base, c2) optionally treating the metal oxide nanoparticles with the volatile surface-modifying compound, or salts thereof, and c3) optionally treating the TiO.sub.2 nanoparticles with a compound of formula Me′(OR.sup.20′).sub.z, or mixtures thereof, wherein R.sup.20′ is a C.sub.1-C.sub.8 alkyl group; Me′ is selected from Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V) and Ta (V); and z equals to the oxidation state of metal; wherein the metal oxide precursor compound(s) is selected from the group consisting of metal alkoxides of formula Me(OR.sup.12).sub.x, metal halides of formula Me′(Hal).sub.x′ and metal alkoxyhalides of formula Me″(Hal′).sub.m(OR.sup.12′).sub.n and mixtures thereof, wherein Me, Me′ and Me″ are independently of each other titanium, tin, tantalum, niobium, hafnium, or zirconium; x represents the valence of the metal and is either 4 or 5, x′ represents the valence of the metal and is either 4 or 5; R.sup.12 and R.sup.12′ are independently of each other a C.sub.1-C.sub.8 alkyl group; Hal and Hal′ are independently of each other Cl, Br or I; m is an integer of 1 to 4; n is an integer of 1 to 4; m+n represents the valence of the metal and is either 4 or 5; the solvent comprises at least one ether group and is different from the tertiary alcohol and the secondary alcohol; the ratio of the sum of moles of hydroxy groups of tertiary alcohol(s) and secondary alcohol(s) to total moles of Me, Me′ and Me″ is in the range 1:2 to 6:1.

Description

EXAMPLES

[0315] Measurement of pH of Dispersions in Ethanol

[0316] The aliquots of nanoparticles dispersions in ethanol were mixed with deionized water (1:1 v/v) under vigorous stirring and pH was measured in the resulting mixture by means of pH meter.

[0317] Measurement of refractive indices of the coatings by ellipsometry

[0318] The nanoparticles-containing dispersions were coated onto silicon wafers to obtain coatings with thicknesses of at least 200 nm (thickness was measured with KLA Tencor Alpha-Step D-100 Stylus Profiler). The data was acquired in Reflectance mode at 65°, 70° and 75° angles, using Woollam M-2000-R19 ellipsometer, and the obtained data was fitted using the Cauchy model with WVase32 software.

[0319] Measurement of Particle Size Distribution by DLS

[0320] The measurements were performed using Malvern Zetasizer Nano ZS device with ca. 3% w/w dispersions of nanoparticles in a suitable solvent. Measurements in ethanol were performed in presence of acrylic acid (15% w/w of acrylic acid relative to particles weight was added). Measurements in water were performed in presence of 1 mM HCl. D10, D50 and D90 values are given for volume distributions.

[0321] Measurement of Solids Content

[0322] The solids content of powders and dispersions was determined using Mettler-Toledo HR-73 halogen moisture analyzer at 100° C.

[0323] Measurement of Total Amount of Volatile Surface-Modifying Compounds

[0324] The total amount of volatile surface-modifying compounds was determined in dispersions after neutralization step as weight loss in the range 200-600° C. relative to the residue at 600° C. in thermogravimetric analysis using TGA/DSC 3+ thermogravimetric analyser from Mettler-Toledo, with the proviso that the highest boiling solvent in the composition has a boiling point of below about 170° C. About 20 to 40 mg of dispersion sample was filled in a tared aluminum crucible, sealed immediately to avoid weight loss before experiment and weighed. The exact mass of sample was recorded. The aluminum crucible is put in the TGA oven at 30° C. The lid of the crucible is pierced at the time. Heating rate was 10° C./min, the measurements were done under nitrogen flow in the range from 30 to 600° C.

[0325] XRD Measurements

[0326] Powder samples were loaded on to a special flat plate Silicon sample holder, taking special care on producing a flat and smooth surface with the correct alignment to the sample holder zero-reference to avoid large systematic errors. The silicon sample holder was manufactured such that the it does not produce sharp diffraction features but only a weak and smooth background.

[0327] The sample on the sample holder was loaded in to a Panalytical ′XPert3 Powder equipped with a sealed Cu tube producing a characteristic X-ray lines Cu K.sub.α and Cu K.sub.β with wavelengths Δ.sub.1=1.54056 Å (Cu K.sub.α1), Δ.sub.2=1.54439 Å (Cu K.sub.α2), I.sub.2/I.sub.1=0.5 and Δ.sub.2=1.3922 Å (Cu K.sub.β). The contribution of the latter (Cu K.sub.β) was removed introducing a Nifilter on the incident beam of the diffractometer right after the Cu-tube.

[0328] Diffraction data was collected from 10 to 80 °2θ, using a step of 0.026 °2θ for a total time of 2 h and spinning the sample around its axis at a rate of 0.13 rate/s in order to increase the sampling statistic.

[0329] The analysis of the diffraction patterns in terms of crystallographic phase analysis and average domain size was performed using the Panalytical HighScore software (v 4.8+) and the Bruker Topas6 program, obtaining consistent results.

[0330] The volume weighted domain size of diffraction (Dv) was evaluated using the Scherrer equation (B. E. Warren, X-Ray Diffraction, Addison-Wesley Publishing Co., 1969) Dv=K Δ/[β cos(θ)], where K(˜1) is the shape factor, dependent on the shape and reciprocal space direction, λ the wavelength, β the integral breadth of the diffraction peak and θ the scattering half-angle. To ensure a correct determination of the Dv, the integral breadth β was amended of the instrumental contribution. To achieve this, the line-broadening of the powder reference material LaB.sub.6 was measured and evaluated according to the same procedure, as described above.

Example 1

[0331] Step 1. Synthesis of TiO.sub.2 Nanoparticles

[0332] All operations were carried out under dry nitrogen atmosphere. Di(propylene glycol) dimethyl ether (400 g) was placed in a 1 L double-wall reactor, equipped with a mechanical stirrer and a distillation head with a Liebig condenser. 2,5-Dimethyl-2,5-hexanediol (234 g) was added, followed by addition of tetraethyl orthotitanate (273.8 g). The mixture was heated to 65° C. over 30 min with stirring and was kept for 15 min at this temperature. Titanium tetrachloride (75.9 g) was added dropwise with stirring and the reaction mixture was heated to 130° C. over 2 h, during which time distillation has begun. The reaction mixture was stirred at 125-130° C. internal temperature (with constant jacket temperature) for 3 h, upon which time distillate was collected and the beige precipitate has formed. After that, the internal reaction temperature was increased to 150° C. over 2 h and stirring was continued for 5 h at this temperature. In total, 315 g distillate was collected.

[0333] The reaction mixture was cooled to 77° C., absolute ethanol (200 g) was added and stirring was continued for 5 h at 77° C. The mixture was cooled to 25° C., isopropanol (300 g) was added, the mixture was stirred for 30 min at 25° C. and filtered under vacuum through a paper filter (20 μm pore size). The product was washed on the filter with isopropanol (1000 g) and absolute ethanol (300 g) and dried on the filter for 1 min. The beige powder of TiO.sub.2 nanoparticles agglomerates was obtained (247 g). Solids content at 100° C. 61.7% w/w. XRD analysis showed anatase to be the predominant phase with crystalline domain size of 3.1±0.3 nm. D10(v)=2.1 nm, D50(v)=3.0 nm, D90(v)=4.8 nm (in 1 mM HCl in water).

[0334] Step 2. Neutralization/Re-Dispersion of TiO.sub.2 Nanoparticles

[0335] The powder, obtained in Step 1 (227 g), was resuspended in absolute ethanol (450 g). The temperature of the mixture was raised to 75° C., acetylacetone (5.6 g) was added and the pH of the mixture was brought to 4.5 via dropwise addition of 24% w/w potassium ethylate solution in absolute ethanol (98.6 g) with stirring at 75° C. Upon addition of potassium ethylate solution the turbidity of the mixture was strongly reduced due to the re-dispersion of TiO.sub.2 nanoparticles agglomerates. The mixture was cooled to 25° C. and filtered through the depth filter sheet (Seitz@ KS 50) under 2.5 Bar pressure to remove the formed potassium chloride along with the traces of non-re-dispersed TiO.sub.2 nanoparticles. The brownish filtrate, containing re-dispersed TiO.sub.2 nanoparticles, was collected (730 g). Solids content at 100° C. 18.1% w/w. D10(v)=2.0 nm, D50(v)=2.8 nm, D90(v)=4.2 nm (in presence of acrylic acid in ethanol). Total amount of volatile surface-modifying compounds as determined by thermogravimetric analysis (weight loss in the range 200-600° C. relative to the residue at 600° C.) was found to be 28%.

Application Example 1

[0336] a) Preparation of Coating Compositions

[0337] The dispersion, obtained in Example 1, was diluted to 10% w/w solid content with absolute ethanol.

[0338] b) Preparation of UV-Cured Coatings with High Refractive Index

[0339] The coating composition, prepared in Application Example 1a), was spin-coated onto polished silicon wafers. The coating was dried with an air-dryer at 80° C. for 10 seconds to evaporate solvent and the dry coating was cured using a medium pressure mercury UV lamp (total UV dose ca. 500 mJ/cm.sup.2) to obtain a cured coating. Thickness and refractive index at 589 nm wavelength of the cured coating were found to be 155 nm and 2.03, respectively.

Application Example 2

[0340] Evaluation of Chemical Fastness Properties and Mechanical Stability of Coatings

[0341] A PET foil (Melinex 506) was coated with a UV-curable varnish (Lumogen OVD 311, commercially available from BASF) using a wired hand-coater #1 and thus obtained coating was cured using a medium pressure mercury UV lamp (total UV dose ca. 350 mJ/cm.sup.2). The coating composition, prepared in Application Example 1a), was coated onto this substrate using a wired K Hand Coater #1 (6 μm wet coating thickness), dried with an air-dryer at 80° C. for 10 seconds and cured using a medium pressure mercury UV lamp (total UV dose ca. 500 mJ/cm.sup.2) to obtain a cured coating.

[0342] Chemical fastness was evaluated by immersing the coated foil (before and after UV-curing) into absolute ethanol, or 1-methoxy-2-propanol for 30 minutes at room temperature. After that, the foils were dried with an air-dryer at room temperature. The dry foils were assessed visually (reflectance color, caused by interference, was observed) using a grayscale note from 0 to 4 (0—coating completely disappeared, 1—major change; more than 50% damaged, 2—considerable change; less than 50% damaged, 3—minor changes, 4—coating unchanged), as compared to the untreated reference.

[0343] Mechanical stability of the coatings before and after UV-curing was evaluated by manually rubbing the coating once with a nylon glove and visually assessing the behavior using a note of 0 or 1 (0—white traces on the coating, 1—coating unchanged).

TABLE-US-00001 TABLE 1 Evaluation of chemical and mechanical stability of coatings before and after UV curing. 1-Methoxy-2- Acetic Mechanical Ethanol propanol Acid Stability Before UV curing 0 0 0 0 After UV curing 4 4 4 1

Example 2

[0344] Treatment of TiO.sub.2 dispersions with metal alcoholates [0345] a) The dispersion obtained in step 2 of Example 1 was diluted with 2-butanone to adjust the solid content to 10% w/w. Tetraethyl orthotitanate (27 mg, 0.12 mmol of Ti) was added to the thus obtained dispersion (4 g) with stirring and the mixture was stirred at 50° C. for 12 h under nitrogen. [0346] b) The dispersion, obtained in step 2 of Example 1 was diluted with 2-butanone to adjust the solid content to 10% w/w. A solution of zirconium tetra-1-propylate (70% w/w in 1-propanol, 56 mg of solution, 0.12 mmol Zr) was added to the thus obtained dispersion (4 g), with stirring and the mixture was stirred at 50° C. for 12 h under nitrogen. [0347] c) The dispersion, obtained in step 2 of Example 1 was diluted with 2-butanone to adjust the solid content to 10% w/w. Niobium pentaethylate (38 mg, 0.12 mmol of Nb) was added to the thus obtained dispersion (4 g) with stirring and the mixture was stirred at 50° C. for 12 h under nitrogen. [0348] d) The dispersion obtained in step 2 of Example 1 was diluted with 2-butanone to adjust the solid content to 10% w/w. Tantalum pentaethylate (49 mg, 0.12 mmol of Ta) was added to the thus obtained dispersion (4 g) with stirring and the mixture was stirred at 50° C. for 12 h under nitrogen.

Application Example 3

[0349] The compositions, obtained in Example 2, were coated and cured as described in Application Example 2. Chemical fastness and mechanical stability of the coatings was evaluated as described in Application Example 2. The results are summarized in Table 2.

TABLE-US-00002 TABLE 2 1-Methoxy-2- Acetic Mechanical Dispersion type Ethanol propanol Acid Stability Example 2 a) 0 0 0 0 Before UV curing Example 2 a) 4 4 4 1 After UV curing Example 2 b) 0 0 0 0 Before UV curing Example 2 b) 4 4 4 1 After UV curing Example 2 c) 0 0 0 0 Before UV curing Example 2 c) 4 4 4 1 After UV curing Example 2 d) 0 0 0 0 Before UV curing Example 2 d) 4 4 4 1 After UV curing

[0350] The present invention comprises the following embodiments: [0351] 1. A method for forming a coating having a high refractive index on a substrate comprising the steps of: [0352] a) providing a substrate, preferably carrying a surface relief nano- and/or microstructure; [0353] b) applying a coating composition to the substrate by means of wet coating, or printing; [0354] c) removing the solvent; and [0355] d) exposing the dry coating to actinic radiation, especially UV-light; or a method for forming a coating having a high refractive index on a substrate comprising the steps of [0356] a′) providing a sheet of base material, said sheet having an upper and lower surface; [0357] b) depositing a composition on at least a portion of the upper surface; [0358] c′) removing the solvent; [0359] d′) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition, such that said micro- and/or nanostructure is formed also in the base material, and [0360] e′) curing the coating composition by exposing it to actinic radiation, especially UV-light; or a method for forming a coating having a high refractive index on a substrate comprising the steps of [0361] a″) providing a sheet of base material, said sheet having an upper and lower surface; [0362] b″) depositing a coating composition on at least a portion of the upper surface; [0363] c″) removing the solvent; [0364] d″) curing the dry coating by exposing it to actinic radiation, especially UV-light; and [0365] e″) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition, such that said micro- and/or nanostructure is formed also in the base material; wherein [0366] the coating composition, comprising [0367] i) single or mixed metal oxide nanoparticles, wherein the volume average diameter (D.sub.v50) of the metal oxide nanoparticles is in the range of 1 to 20 nm; the nanoparticles comprise at least one volatile surface-modifying compound selected from alcohols, which are preferably selected from C.sub.1-C.sub.4alcohols; β-diketones, carboxylic acids and p-ketoesters and mixtures thereof, wherein the total amount of volatile surface-modifying compounds is at least 5% by weight, preferably at least 10% by weight based on the amount of metal oxide nanoparticles, and [0368] ii) a solvent. [0369] After exposing coating composition to actinic radiation, especially UV-light; the coating composition is cross-linked. [0370] 2. The method according to item (claim) 1, wherein the metal oxide nanoparticles are titanium dioxide nanoparticles. [0371] 3. The method according to item 1, or 2, wherein the volatile surface-modifying compound is selected from ethanol and acetylacetone and mixtures thereof. [0372] 4. The method according to any of item 1 to 3, wherein the volume average diameter (D.sub.v50) of the metal oxide nanoparticles is in the range of 1 to 10 nm, preferably 1 to 5 nm. [0373] 5. The method according to any of items 1 to 4, wherein the total amount of volatile surface-modifying compounds is in the range of from 15 to 50% by weight, especially from 20 to 40% by weight, very especially from 25 to 35% by weight based on the amount of metal oxide nanoparticles. [0374] 6. The method according to any of items 1 to 5, wherein the solvent is selected from C.sub.2-C.sub.4alcohols, especially ethanol, 1-propanol and isopropanol; ketones, especially acetone, 2-butanone, 2-pentanone, 3-pentanone, cyclopentanone and cyclohexanone; ether alcohols, especially 1-methoxy-2-propanol; mixtures thereof and their mixtures with esters, especially ethyl acetate, 1-propyl acetate, isopropyl acetate and butyl acetate. [0375] 7. The method according to any of items 1 to 6, wherein the single, or mixed metal oxide nanoparticles are obtained by a process comprising the following steps: [0376] a) preparing a mixture, comprising a metal alkoxide of formula Ti(OR.sup.12).sub.4 (Ia), metal halide of formula Ti(Hal).sub.4 (IIa), wherein R.sup.12 and R.sup.12′ are independently of each other C.sub.1-C.sub.4alkyl, preferably methyl, ethyl, n-propyl, iso-propyl and n-butyl; Hal is Cl; a solvent, a tertiary alcohol and optionally water, [0377] b1) heating the mixture to a temperature of from 80° C. to 180° C.; [0378] b2) separating the obtained TiO.sub.2 nanoparticles from the mixture; [0379] b3) resuspending the TiO.sub.2 nanoparticles in an C.sub.1-C.sub.4alcohol, or a mixture of C.sub.1-C.sub.4alcohols; [0380] b4) optionally treating the TiO.sub.2 nanoparticles with a β-diketone(s), or salts thereof, which are preferably selected from compounds of formula Me(OR.sup.20).sub.x(L).sub.y (V), or mixtures thereof, wherein [0381] R.sup.20 is a C.sub.1-C.sub.8 alkyl group, preferably, a C.sub.1-C.sub.4 alkyl group, such as, for example, methyl, ethyl, n-propyl, iso-propyl and n-butyl; [0382] L.sup.− is a group of formula

##STR00010## [0383] R.sup.21 and R.sup.22 are independently of each other a C.sub.1-C.sub.8alkyl group; a phenyl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; a C.sub.2-C.sub.5heteroaryl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; or a C.sub.1-C.sub.8alkoxy group, R.sup.23 is a hydrogen atom, a fluorine atom, a chlorine atom, or a C.sub.1-C.sub.8alkyl group, or [0384] R.sup.21 and R.sup.22 together form a cyclic or bicyclic ring, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups; [0385] Me is selected from alkali and alkali earth metals, Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V), Ta (V), preferably Zn (II), Ti (IV), Zr (IV), Hf (IV), Sn (IV), Nb (V) and Ta (V), more preferably Ti (IV), Zr (IV), Sn (IV), Nb (V) and Ta (V), [0386] x is in the range from 0 to 4.9, preferably 0 to 4.5, y is in the range from 0.1 to 5, preferably 0.5 to 5, and the sum x+y equals to the oxidation state of metal; [0387] c1) treating the TiO.sub.2 nanoparticles with a base; [0388] c2) optionally treating the TiO.sub.2 nanoparticles with a β-diketone(s), or salt(s) thereof; [0389] c3) optionally treating the TiO.sub.2 nanoparticles with a compound of formula Me′(OR.sup.20′).sub.z (VII), or mixtures thereof, wherein R.sup.20′ is a C.sub.1-C.sub.8alkyl group, preferably a C.sub.1-C.sub.4 alkyl group; [0390] Me′ is selected from Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V) and Ta (V), preferably Ti (IV), Zr (IV), Sn (IV), Nb (V) and Ta (V); and [0391] z equals to the oxidation state of metal; wherein [0392] the ratio of moles of hydroxy groups of tertiary alcohol to total moles of Ti is in the range 1:2 to 6:1, preferably 1:2 to 4:1, most preferably 1:2 to 3.5:1; [0393] the base is selected from the group consisting of alkali metal alkoxides, especially potassium ethylate; alkali metal hydroxides, especially potassium hydroxide; alkali metal salts of carboxylic acids, especially potassium acrylate and methacrylate and combinations thereof, [0394] the solvent is selected from 2-methyltetrahydrofurane, tetrahydropyrane, 1,4-dioxane, cyclopentylmethyl ether, di-n-propyl ether, di-isobutyl ether, di-tert-butyl ether, di-n-butyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, di(ethylene glycol) dimethyl ether, di(ethylene glycol) diethyl ether, di(ethylene glycol) di-n-propyl ether, di(ethylene gly-col) di-n-butyl ether, di(propylene glycol) dimethyl ether, di(propylene glycol) diethyl ether, tri(propylene glycol) dimethyl ether, tri(propylene glycol) diethyl ether, tri(ethylene glycol) dimethyl ether, tri(ethylene glycol) diethyl ether, tetra(ethylene glycol) dimethyl ether and tetra(ethylene glycol) diethyl ether and mixtures thereof; [0395] the tertiary alcohol is selected from tert-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol, 3-ethyl-3-pentanol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, 1-methylcyclopentanol, 1-ethylcyclopentanol, 1-methylcyclohexanol, 1-ethylcyclohexanol, 2,3-dimethyl-2,3-butanediol, 2,5-dimethyl-2,5-hexanediol, 2,6-dimethyl-2-heptanol, 3,5-dimethyl-3-heptanol, 3,6-dimethyl-3-heptanol, 2-methyl-3-buten-2-ol, 2-phenyl-2-propanol, 2-phenyl-2-butanol, 3-phenyl-3-pentanol, 2-methyl-1-phenyl-2-propanol, α-, β-, γ- or δ-terpineol, 4-(2-hydroxyisopropyl)-1-methylcyclohexanol (p-menthane-1,8-diol), terpinen-4-ol (4-carvomenthenol), and wherein in step b1) the alcohol R.sup.12OH is removed by distillation. [0396] 8. The method according to any of items 1 to 7, comprising [0397] i) titanium dioxide nanoparticles, wherein the volume average diameter (D.sub.v50) of the titanium dioxide nanoparticles is in the range of 1 to 10 nm, especially 1 to 5 nm; the nanoparticles comprise at least one volatile surface-modifying compound selected from ethanol and acetylacetone and mixtures thereof, wherein the total amount of volatile surface-modifying compounds is in the range of from 15 to 50% by weight, especially from 20 to 40% by weight, very especially from 25 to 35% by weight based on the amount of metal oxide nanoparticles; and [0398] ii) a solvent which is selected from C.sub.2-C.sub.4alcohols, especially ethanol, 1-propanol and isopropanol; ketones, especially acetone, 2-butanone, 2-pentanone, 3-pentanone, cyclopentanone and cyclohexanone; ether alcohols, especially 1-methoxy-2-propanol; mixtures thereof and their mixtures with esters, especially ethyl acetate, 1-propyl acetate, isopropyl acetate and butyl acetate. [0399] 9. The method according to any of items 1 to 8, wherein the coating composition comprises less than 1% w/w of water. [0400] 10. The method according to any of items 1 to 9, wherein the coating composition does not comprise a binder.

[0401] Preferably, the coating composition does not comprise an organic radical photoinitiator.

[0402] The pH of the coating composition is in the range of 3 to 10, preferably 3 to 7.

[0403] Preferably, the titanium dioxide nanoparticles are present in the anatase modification.

[0404] Preferably, the volatile surface-modifying compound is selected from a C.sub.1-C.sub.4alcohols, such as, for example, ethanol, 1-propanol and isopropanol; β-diketones and mixtures thereof. More preferably, the volatile surface-modifying compound is selected from ethanol and acetylacetone and mixtures thereof. [0405] 11. The method according to any of items 1 to 10, comprising the steps of: [0406] a) forming a surface relief micro- and/or nanostructure on a discrete portion of the substrate; [0407] b) applying a coating composition on at least a portion of the surface relief micro- and/or nanostructure by means of wet coating, or printing; [0408] c) removing the solvent; and [0409] d) curing the dry coating by exposing it to actinic radiation, especially UV-light. [0410] 12. The method according to item 11, wherein step a) comprises [0411] a1) applying a curable compound to at least a portion of the substrate; [0412] a2) contacting at least a portion of the curable compound with surface relief micro- and nanostructure forming means; and [0413] a3) curing the curable compound. [0414] 13. A security, or decorative element, comprising a substrate, which may contain indicia or other visible features in or on its surface, and on at least part of the said substrate surface a coating obtained according to the method according to any of items 1 to 12. [0415] 14. A process for the preparation of a coating composition, comprising the following steps: [0416] a) preparing a mixture, comprising a metal oxide precursor compound(s), a solvent, a tertiary alcohol, or a secondary alcohol, wherein the tertiary alcohol and secondary alcohol eliminate water upon heating the mixture to a temperature of above 60° C., or mixtures, containing the tertiary alcohol(s) and/or the secondary alcohol(s), and optionally water, [0417] b1) heating the mixture to a temperature of above 60° C., especially to a temperature of from 80 to 180° C.; [0418] b2) separating the obtained metal oxide nanoparticles from the mixture; [0419] b3) resuspending the metal oxide nanoparticles in an alcohol, or a mixture of alcohols; [0420] b4) optionally treating the metal oxide nanoparticles with a volatile surface-modifying compound selected from β-diketones, carboxylic acids and p-ketoesters and mixtures thereof; or salts thereof, which are preferably selected from compounds of formula Me(OR.sup.20).sub.x(L).sub.y (V), or mixtures thereof, wherein [0421] R.sup.20 is a C.sub.1-C.sub.8 alkyl group, preferably, a C.sub.1-C.sub.4 alkyl group, such as, for example, methyl, ethyl, n-propyl, iso-propyl and n-butyl;

##STR00011## [0422] L.sup.− is a group of formula [0423] R.sup.21 and R.sup.22 are independently of each other a C.sub.1-C.sub.8alkyl group; a phenyl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; a C.sub.2-C.sub.5heteroaryl group, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups, or C.sub.1-C.sub.4alkoxy groups; or a C.sub.1-C.sub.8alkoxy group, R.sup.23 is a hydrogen atom, a fluorine atom, a chlorine atom, or a C.sub.1-C.sub.8alkyl group, or [0424] R.sup.21 and R.sup.22 together form a cyclic or bicyclic ring, which may optionally be substituted by one or more C.sub.1-C.sub.4alkyl groups; [0425] Me is selected from alkali and alkali earth metals, Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V), Ta (V), preferably Zn (II), Ti (IV), Zr (IV), Hf (IV), Sn (IV), Nb (V) and Ta (V), more preferably Ti (IV), Zr (IV), Sn (IV), Nb (V) and Ta (V), [0426] x is in the range from 0 to 4.9, preferably 0 to 4.5, y is in the range from 0.1 to 5, preferably 0.5 to 5, and the sum x+y equals to the oxidation state of metal; [0427] c1) treating the metal oxide nanoparticles with a base, especially a base which is selected from the group consisting of alkali metal alkoxides, alkali metal hydroxides, alkali metal salts of carboxylic acids, tetraalkylammonium hydroxides, trialkylbenzylammonium hydroxides and combinations thereof, [0428] c2) optionally treating the metal oxide nanoparticles with the volatile surface-modifying compound, or salts thereof; and [0429] c3) optionally treating the TiO.sub.2 nanoparticles with a compound of formula [0430] Me′(OR.sup.20′).sub.z (VII), or mixtures thereof, wherein [0431] R.sup.20′ is a C.sub.1-C.sub.8alkyl group, preferably a C.sub.1-C.sub.4 alkyl group; [0432] Me′ is selected from Zn (II), In (III), Sc (III), Y (III), La (III), Ce (IV), Ti (III), Ti (IV), Zr (IV), Hf (IV), Sn (IV), V (IV), Nb (V) and Ta (V), preferably Ti (IV), Zr (IV), Sn (IV), Nb (V) and Ta (V); and [0433] z equals to the oxidation state of metal; wherein [0434] the metal oxide precursor compound(s) is selected from the group consisting of [0435] metal alkoxides of formula Me(OR.sup.12).sub.x (I), metal halides of formula Me′(Hal).sub.x′ (II) and [0436] metal alkoxyhalides of formula Me″(Hal′).sub.m(OR.sup.12′).sub.n (III) and mixtures thereof, wherein [0437] Me, Me′ and Me″ are independently of each other titanium, tin, tantalum, niobium, hafnium, or zirconium; [0438] x represents the valence of the metal and is either 4 or 5, [0439] x′ represents the valence of the metal and is either 4 or 5; [0440] R.sup.12 and R.sup.12′ are independently of each other a C.sub.1-C.sub.8alkyl group; [0441] Hal and Hal′ are independently of each other Cl, Br or I; [0442] m is an integer of 1 to 4; [0443] n is an integer of 1 to 4; [0444] m+n represents the valence of the metal and is either 4 or 5; [0445] the solvent comprises at least one ether group and is different from the tertiary alcohol and the secondary alcohol; [0446] the ratio of the sum of moles of hydroxy groups of tertiary alcohol(s) and secondary alcohol(s) to total moles of Me, Me′ and Me″ is in the range 1:2 to 6:1.